摘要:

J. MATER. CHEM., 1994,4(lo), 1559-1569 Crystal Structures and Electrical Properties of the Radical Salts of the Unsymmetrical Donor EOTT [4,5-Ethylenedithio-4’,5’-(2-oxatrimethylenedithio)tetrathiafuIvalene] Akiko Tateno,a Takashi Udagawa,a Toshio Naito,” Hayao Kobayashi,*” Akiko Kobayashib and Takashi Nogami“ a Department of Chemistry, Faculty of Science, Toho University, Funabashi, Chiba 274, Japan Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 773, Japan Department of Applied Physics and Chemistry, The University of Electro-communications, Chofu, Tokyo 782, Japan The crystal structures and/or electrical properties of EOTT2X [X =AuCI,, AuBr,, Aul,, Au(CN),, Ag(CN),, ICI,, IE3r2, 12Br, I,, Br, and MnCI,, FeCI,, CoCI,, NiCI,] are reported.Concerning the linear anion salts seven (X=AuBr,, Aul,, ICI,, 12Br, I,, Br3) of them were found to be isostructural. Although they apparently have columnar structures, there are many sulfur-sulfur short contacts, which lead to a two-dimensional network in the donor sheet. Their Fermi surfaces were calculated to be closed and elliptical by the tight-binding band approximation. EOTT,FeCI, is disordered at the anion site and has a dimeric columnar structure along the a axis. The Aul, salt is metallic down to low temperaturres. The IBr, salt also exhibited decreased resistivity with an inflexion at ca. 110 K and a sample-dependent minimum around 25 K. The metal-insulator (M-I) transitions occurred in AuBr,, ICI, and Br3 salts at temperatures >lo0 K.The other salts [I,, I2Br,AuCI,, Au(CN),, Ag(CN),, FeCI,, NiCI,, MnCI, and CoCI,] are semiconductors. EOTT,ICI, is polymorphic: the cw-type is isostructural with the other linear anion salts with an M-l transition around 145 K. The P-type is semiconducting and has a dimeric structure. The M-l transition of a-EOTT,ICI, might be explained by a structural transition to a similar insulating state to the P-type. Since the first example was found in 1979,’ organic super- using graphite-monochromatized Mo-Kcr radiation. All the conductors have been investigated extensively., Among them reflections were measured by the w28 scan technic. No the most popular and successful donor system is that of absorption corrections were made.The data for the 1,Br salt BEDT-TTF [bis(ethylenedithio)tetrathiafulvalene], which has were recorded on an AFC-5R diffractometer and absorption yielded the largest number of organic metals and superconduc- corrections were made. Crystal structures were refineti by the tors. For this reason, studies of donor molecules of this class block-diagonal least-squares method. Hydrogen atom were have generally concentrated on this molecule or similar sym- found on the differenfe map or located at the calculated metrical derivatives. However, a study of unsymmetrical positions with B =4.0 A,; they were not included in I he final donors remains equally important for further understanding refinement. Atomic scattering factors were taken froin ref. 5. of the relation between the molecular and/or crystal structure Calculations were performed using the programs UN I CS 111, and properties, since such a study could enhance the knowl- and SHELXS’ on a Hitac 680H computer at the Ccbmputer edge of the physics and chemistry of organic superconductors.2 Centre of the University of Tokyo. The details are summarized EOTT [4,5-ethylenedithio-4,5’-(2-oxatrimethyleneditho)tetra-in Table 1.thiafulvalene] is an unsymmetrical donor where a terminal In respect of the lattice constants of the linear anion salts ethylene group of BEDT-TTF is replaced by oxatrimethylene of EOTT, the coordinations reported previously3 are the same group. This donor molecule was first synthesized by Nakan~,~ as those in this work except for EOTT,I,Br.The relation but only preliminary results of the electrical behaviour of between the lattice vector (a’, b’, c’) of EOTT,I,Br arid those some radical ion salts (PF,, AsF,, SbF,, IBr,, I,, AuBr, and (a,b, c) of the other EOTT linear anion salts is AuI, salts) have been rep~rted.~ As for the crystal structures a’=b* b’= -c; c’= -aof the radical salts, only those of the AuI, and IBr, salts have been rep~rted.~ There remains much to be established and The temperature dependences of the resistivities were meas- further and systematic study is required to obtain the infor- ured by the four-probe method. mation which this system could offer. In this paper we report full details of the crystal structures and electrical properties Results and Discussion of some EOTT radical salts.Crystal Structures of the Linear Anion Saltst Experimental All linear anion salts of EOTT except for P-type ICl,, AuCl,, EOTT was synthesized by a standard cross-coupling method Au(CN), and Ag(CN), salts are isostructural. First wc discuss in P(OEt),., Single crystals of the radical salts were prepared some general features. by electrochemical crystallization with the tetrabutylam-The unit cell is shown in Fig. 1. The stoichiometr: is 2: 1. monium salts (40-60 mg, ca. 0.1 mmol dm-3) as the support- The unit cell contains two EOTT donor molecules and an ing electrolytes in chlorobenzene, THF or 1,1,2-trichloro-anion, which is on the inversion centre. One donor molecule ethane (20 cm3) using a constant current (0.1-0.6 yA) for a and a half of the anion are crystallographically independent.~~few weeks at room temperature. Most of the intensity data of X-ray reflections were collected Crystallographic data have been deposited with the Cimbridge using a Rigaku automated four-circle AFC-6 diffractometer Crystallographic Data Centre; see Information for Authors, Issue 1. Table 1 Structure determination summary EOTT,AuI, EOTT2TBr2 EOTT,AuBr2 CZ-EOTT~ICI~ EOTT21, EOTT,12Br P-EOTT21C12 EOTT,FeCl, Crystal data empirical formula S16°2C20H161Br2 S1602C20H16AuBr2 S16°2C20H161C12 16°2C20H 16I3 S16°2C20H1612Br S16°2C20H16TC12 16°2C20H 16Fec12 formula weight, M 1088.1 1 1158.18 999.21 1182.12 1135.11 999.21 998.94 crystal colour, habit black needle black plate black needle black plate black plate black plate black plate black plate crystal size/mm 0.35 x 0.23 x 0.01 0.41 x 0.25 x 0.01 0.55 x 0.01 x 0.01 0.44 x 0.26 x 0.01 0.38 x 0.22 x 0.01 0.51 x 0.24 x 0.01 0.27 x 0.13 x 0.01 0.35 x 0.13 x 0.01 crystal system triclinic triclinic triclinic triclinic triclinic triclinic triclinic triclinic sp!ce group Pi pi Pi pi pi pi pi pi a/+ 4.778( 1) 4.743( 1) 4.707( 2) 4.726( 3) 4.784( 1) 11.740(1) 13.556(2) 9.384(4) blh 11.656( 5) 11.631( 5) 11.684( 5) 11.584( 5) 11.762(7) 16.223 (2) 14.825( 3) 15.263( 7) CIA 16.254( 5) 16.108( 6) 16.304( 6) 15.797( 13) 16.27(2) 4.7771 (4) 8.736( 5) 6.716(3) aldegrees 107.53( 3) 106.82( 3) 110.13( 3) 105.13( 5) 107.38( 7) 94.00( 1) 94.79(3) 99.62(4) Pldegrees 93.92( 2) 93.30( 2) 95.54( 3) 92.38(8) 93.83( 6) 96.44( 1) 97.42( 3) 90.62( 4) yldegrees 83.72( 3) 83.97( 3) 83.40( 4) 84.1 6( 5) 83.57(4) 72.54(1) 102.21 (1) 75.05( 3) VIA, 857.4( 7) 845.5( 5) 834.7 (6) 830.4( 9) 867( 1) 861.8 (2) 1690.5( 10) 9 15.7( 7) Z 1 1 1 1 1 1 2 1 DcalcIg cm-, 2.425 2.137 2.304 1.998 2.263 2.187 1.964 1.181 F( 100) 593 53 1 557 495 567 549 990 502 Data collection scan speed, wldegrees min 8 8 8 8 8 6 8 8 scan width, Au 1.04+0.5 tan 0 8 +0.5 tan 0 3.39+0.5 tan 0 2.38 +0.5 tan 0 1.25+0.5 tan 0 1.10+0.3 tan 0 1.62+0.5 tan 0 1.30+0.5 tan 0 20ma,/degrees 55.0 50.0 45.0 50.0 50.0 55.0 50.0 50.0 index ranges -66h66 -5<h,<5 -56h65 -56hG5 -5Gh65 OGh615 -166h616 -11<h<11 -15GkG15 -126kG 12 -13Gk613 -146k6 14 -21 Gk621 -176kG 17 -186k618 OG1621 061617 061619 OdlG 19 -66166 061610 OG168 reflections collected 3629 2352 3172 3294 4269 6646 3650 independent reflections 2938 764 1979 2519 3342 4154 1966 IF,I >30 (F,) IF01>50 (Fo) IF,I >4a (F,) lFol>30 (F,) Fol> 3a (Fo) Fol> 30 (Fo) IF01 >3a (Fo) refinement weighting scheme lFol <30.0: w-~= IF, I <25.0: w -'= IF,[ < 10.0: w-l= 1 Fo I <25.0: w -= IFo[<30.0: W-' = IFo\<30.0: w-~= lFol <25.0: W-I = IF, I <20.0: w -= 20.0 +0.01IF,(2 10.0+ O.O1IF0I2 10.0+0.01IF, 12 10.0 +0.011F012 20.0+0.01~F0~2 20.0+0.01lFo12 12.0 +0.01IFo12 10.0 +0.0051F012 4 = pol2 20.0: w -=IFo]2 30.0: W-' = IF01225.0: w-l= IFo1210.0: w-l=q lF,I 225.0: w-l= lFol 230.0: w-l= IFo/230.0: w-' = IF',,[ 225.0: w-~ a~(F,)+O.Ol(F,~~a2(F0)+O.O1lF012 o~(Fo)+0.001~F,~-02(F,)+0.01~Fo~2 o'(F,) +0.001IF0/? a?(F,) +0.01IF, 12 a-(F,)+0.01IF, 12 02(F0)t0.005/F0(2 z no.of parameters 191 191 141 191 191 191 371 218 5 mfinal R and R, 0.068, 0.094 0.046, 0.059 0.070, 0.079 0.055, 0.065 0.036, 0.031 0.046, 0.055 0.065, 0.099 0.084, 0.095 .w goodness of fit 0.38 0.18 1.99 0.15 0.08 0.10 0.62 0.68 A --WWP d r P J. MATER. CHEM., 1994, VOL. 4 n n Fig. 1 Unit cell of the EOTT linear anion (AuBr,, AuI,, EICI,, IBr,, I,Br, I, and Br,) salts According to a previous rep~rt,~ the positions of Au1,- and 1Br2- were not ordered in either salt: 20% of the anions were not located on the inversion centre. We, however, could not find evidence of this disorder on the difference map.Only the 1,Br anion, (I-I-Br)- ,exhibited orientational disorder. The atomic parameters are listed in Table 2. The bond lengths and non-bonding distances of EOTT molecules in the salts are tabulated with the numbering scheme of EOTT molecule in Table 3. C( l)-C(6), C(2)-C(3) and C(7)-C(8) exhibit double- bond character and their bonding distances range from 1.331(15) to 1.372(9) A. All the C(l)-C(6) lengths in the salts in question fall in the range 1.342(8)-1.372(9) A, These values are larger than a typical double bond (e.g. 1.33 A in ethylene), indicating delocalization of the n-electrons. The lengths of tbe bonds, C(2)-C(3) and C(7)-C(8) [1.331( 15)-1.353( 10) A] are between those of ethylene and C(l)-C(6) (Table3).In this discussion, the AuBr, salt is not included because the standard deviations are too large and the temperature factors of the carbon atoms are isotropic. The length of the C(4)-C(5) bond in the terminal ethylene group [1.337( 19)-1.431( 16) A], is shqrter than a typical single bond, e.g. that of ethane (1.54 A). Note, however, that the standard deviation is rela- tively large due to the large thermal moticn, suggested by the fairly large temperature factors, 5.37-8.39 A,. The temperature factors of the atoms in the oxatrimethylene groups [C(9)-O-C(lO)] are small. As for the ethylene group, it clearly deviates from the molecular plane in the opposite direction from the ethylene group. For C-S bonds, the bond distances are longe! in both terminal alkyl groups [1.731( 19)-1.870(22) 41 and shorter in the fulvalene moiety [1.721(11)-1.761(11) A].S(l).--S(2) is longer than S(5)-..S(6). The S( 1)-C( 1)-S(2) angle is larger than S(5)-C(6)-S(6). S(3)..-S(4) (the diameter of the six-membered ring) is longer than S(7).--S(8) (the diameter of the seven-membered ring). Therefore the OPT [bis( 2- oxatrimethylene) tetrathiafulvalene] unit in the EOTT mol- ecule is narrower than the rest of the molecule, i.e. the BEDT- TTF unit. S(l).-.S(3), S(l)...S(5) and S(5)...S(7) are longer than S( 2). .-S(4), S(2)..-S(6) and S(6).-.S(8), respectively. EOTT molecules stack regularly along the shortest axis (aaxis) and form one-dimensional columns (Fig. 2), consistent with the previous rep~rt.~ The intracolumn distances between neighbouring EOTT molecular planes, where the plane is defined by the six central atoms of the ethylene unit in the fulvalene moiety IC( l), S(l), 5(2), C(6), S(5J S(6)], are cs follows: I,, 3.63 A; IzBr, 3.62 A; AuI,, 3.62 A; IBrz, 3.62 A; IC12, 3.61 A.These distances reflect the unit-cell volumes and the sizes of the anions. The unjt-cell volumes dtcrease in the foljowing order: I, (867.5 A,) >12Br (861.8 A3)>AuI, (857.4 A3)>IBr, (845.5 A,) >ICl, (830.4 A3), while the lengths of the anions decreaseo in the order: 013 (2.901 A) >1,Br (2.878 A) >AuI, (2.747 A) >TBr, (2.700 A) >ICl, (2.554 A). Although all the distances above are shorter than the 2um of the van der Waals radii of sulfur atoms (S-S: 3.7CI A), the overlap integrals are small, as shown in Fig. 2.EOTT mol- ecules stack with slipping not only along the long axis of the molecule but also along the short axis (Fig. 3). Consequently, most of the short contacts between sulfur atoms, which haye shorter distances than the van der Waals distances (3.70 A) have been found between columns rather than within a coluomn; notably the distances along p2 and b, in Fig. 2 (ca. 3.5 A) are shorter than the rest (Table 4). These structural features result in a two-dimensional network of the chalcogen atoms in the EOTT sheet. The X-ray photographs i'evealed Fig. 2 Molecular arrangement between highest occupied molecular orbitals of EOTT molecules in the linear anion (AuBr,, AuI,, a-ICI,, IBr,, 12Br, I, and Br,) salts.Overlap integrals (x lo3): a P1 P2 bl b2 EOTT,AuI, 4.78 -7.18 -7.60 14.69 13.62 EOTTJBr, 5.38 -6.82 -7.51 15.09 14.86 EOTT,AuBr, 4.35 -6.85 -6.90 15.25 13.97 a-EOTT2IC12 5.21 -7.27 -7.32 15.04 16.06 EOTT,I, 4.78 -6.54 -7.23 14.61 12.54 EOTT,I,Br 4.23 -6.49 -7.28 14.58 12.69 iJ Fig. 3 Overlapping mode of EOTT molecules shared amongi AuBr,, Ad,, a-ICI,, IBr,, I,Br, I, and Br, salts J. MATER. CHEM., 1994, VOL. 4 Table 2 Atomic coordinates of EOTT salts of AuI,, AuBr,, IBr,, a-ICl,, I, and 1,Br EOTT,AuI, EOTT,AuBr, atom 0.0 0.0 0.0 4.78 0.50 0.0 0.0 0.0 5 36 0.50 0.1628 0.21 52 -0.0078 4.92 1.oo 0.1066 0.1902 -0.0087 7 66 1.oo -0.0389 0.3173 0.4133 2.52 1.oo -0.0463 0.3151 0.4103 3 71 1.oo 0.2406 0.1240 0.4780 2.69 1.00 0.2449 0.1228 0.4738 3 18 1.oo 0.2587 0.2441 0.2506 3.60 1.oo 0.2417 0.2327 0.2452 3 95 1.oo 0.5949 0.0148 0.3301 3.62 1.oo 0.5786 0.0030 0.3201 4 38 1.oo -0.3972 0.4168 0.5887 2.20 1.oo -0.3866 0.4243 0.5918 3 30 1.oo -0.1184 0.2190 0.6479 2.41 1.oo -0.0974 0.2265 0.6474 3 04 1.oo -0.7824 0.5058 0.7354 2.52 1.oo -0.7472 0.5204 0.7444 3 89 1.oo -0.4584 0.2730 0.8032 3.01 1.oo -0.4027 0.2888 0.8074 4 73 1.oo -0.6189 0.5032 0.8997 3.18 1.oo -0.5631 0.5222 0.907 1 5 28 1.oo -0.0055 0.2473 0.4946 2.27 1.oo -0.0090 0.2508 0.4962 3 56 1.oo 0.21 78 0.21 80 0.3491 2.16 1.oo 0.2079 0.2090 0.3405 147 1.oo 0.3407 0.1277 0.3778 2.19 1.oo 0.3296 0.1247 0.3693 2 54 1.oo 0.5181 0.1338 0.201 5 6.9 1 1.oo 0.4866 0.1206 0.1917 5 31 1.oo 0.5460 0.0267 0.2177 8.39 1.00 0.4989 0.0014 0.2002 5 63 1.oo -0.1598 0.2890 0.5667 1.81 1.oo -0.1343 0.2938 0.5694 1'5 1.oo -0.51 19 0.4017 0.6853 1.84 1.oo -0.4840 0.4 170 0.6864 2 72 1.oo -0.3796 0.31 10 0.7109 1.99 1.00 -0.3464 0.3242 0.7141 2 23 1.oo -0.6205 0.5735 0.8416 3.11 1.oo -0.5817 0.5915 0.8548 3 73 1.oo -0.3970 0.4095 0.8879 3.16 1.oo -0.3394 0.4294 0.8969 4 53 1.oo 0.7217 0.1747 0.2215 4.00 1.oo 0.49 18 0.1188 0.1327 4.00 1.oo 0.3552 -0.0153 0.1917 4.00 1.oo 0.7225 -0.0271 0.1830 4.00 1.oo -0.7262 0.661 8 0.8716 4.00 1.00 -0.3962 0.6200 0.8339 4.00 1.oo -0.2065 0.4200 0.8736 4.00 1.oo -0.3585 0.3865 0.9477 4.00 1.oo EOTT,IBr, a-EOTT,ICl, 0.0 0.0 0.0 3.82 0.50 0.0 0.0 0.0 4.51 0.50 0.1567 0.2151 -0.0032 5.15 1.oo 0.1366 0.2084 0.0019 6.25 1.oo -0.0380 0.3210 0.4133 2.81 1.00 -0.0333 0.3264 0.4 144 2.78 1.00 0.2435 0.1238 0.4760 3.04 1.00 0.2451 0.1260 0.4776 3.03 1.oo 0.2637 0.2546 0.2492 3.82 1.oo 0.2761 0.2655 0.2485 3.85 1.oo 0.6025 0.0206 0.3249 3.78 1.oo 0.6123 0.0288 0.3257 3.74 1.oo -0.3956 0.4 169 0.5913 2.60 1.oo -0.3973 0.4151 0.5941 2.65 1.oo -0.1172 0.2148 0.6485 2.84 1.oo -0.1 235 0.2096 0.651 7 2.82 1.oo -0.7826 0.5021 0.7398 2.86 1.00 -0.7935 0.4939 0.7434 2.85 1.oo -0.4592 0.2643 0.8052 3.40 1.oo -0.4789 0.2515 0.8088 3.48 1.oo -0.6258 0.4913 0.9034 3.83 1.oo -0.6539 0.4731 0.9086 3.77 1.oo -0.001 1 0.2488 0.4945 2.40 1.oo -0.0012 0.2512 0.4972 2.46 1.oo 0.2183 0.2236 0.3472 2.53 1.oo 0.2273 0.23 1 1 0.348 1 2.20 1.oo 0.3471 0.1327 0.3764 2.55 1.oo 0.3547 0.1401 0.3768 2.54 1.oo 0.5321 0.1448 0.1965 5.37 1.oo 0.5322 0.1516 0.1954 7.32 1.oo 0.541 1 0.0293 0.2120 6.47 1.oo 0.5570 0.0439 0.2126 6.57 1.oo -0.1560 0.2887 0.5684 2.24 1.oo -0.1586 0.2858 0.5702 2.41 1.oo -0.5070 0.3996 0.6883 2.37 1.oo -0.5 152 0.3927 0.6916 2.24 1.oo -0.3813 0.3059 0.7138 2.36 1.oo -0.3918 0.2986 0.7173 2.54 1.oo -0.6259 0.5655 0.8473 3.47 1.oo -0.6450 0.5498 0.8540 3.37 1.oo -0.3996 0.397 0.8927 3.69 1.oo -0.4285 0.3824 0.8986 3.71 1.oo 0.7378 0.1800 0.21 89 4.00 1.oo 0.7437 0.1874 0.2145 4.00 1.oo 0.5114 0.1341 0.1275 4.00 1.OO 0.5076 0.1404 0.1261 4.00 1.oo 0.3335 -0.0044 0.1889 4.00 1.00 0.3577 0.0058 0.1876 4.00 1.00 0.7000 -0.0306 0.1715 4.00 1.oo 0.7276 0.0108 0.1726 4.00 1.oo -0.7397 0.65 18 0.8775 4.00 1.oo -0.7596 0.6366 0.8856 4.00 1.oo -0.4066 0.5807 0.8401 4.00 1.oo -0.4230 0.5666 0.8492 4.00 1.oo H( 101) H( 102) -0.21 17 -0.3475 0.4272 0.3774 0.8715 0.9530 4.00 4.00 1.oo 1.oo H( 101) H( 102) -0.2298 -0.3976 0.4189 0.3545 0.8885 0.9598 4.00 4.00 1.oo 1.oo J.MATER. CHEM., 1994, VOL. 4 Table 2 (continued) EOTTJ, EOTT,I,Br 0.0 0.0 0.0 3.36 0.50 1 0.0 0.0 0.0 3.92 0.50 0.1697 0.2260 0.007 1 4.13 1.oo IBr 0.2248 0.0067 -0.1682 3.93 1.oo -0.0384 0.3 173 0.4135 2.98 1.oo S(1) 0.2171 0.3524 0.1 198 3.11 1.oo 0.2386 0.1227 0.4769 3.07 1.oo S(2) 0.4149 0.41 19 0.3982 2.92 1.oo 0.2612 0.2480 0.25 17 3.87 1.00 S(3) 0.1231 0.5232 -0.2394 3.38 1.oo 0.5942 0.0165 0.3278 3.90 1.oo S(4) 0.3180 0.5864 0.0388 3.21 1.00 -0.3993 0.4144 0.5872 2.68 1.oo S(5) 0.2723 0.1969 0.460 1 3.72 1.oo -0.1210 0.2174 0.6474 2.83 1.oo S(6) 0.5030 0.2655 0.7849 3.10 1.oo -0.7860 0.5031 0.7333 2.83 1.oo S(7) 0.01 73 0.6727 -0.5955 4.20 1.oo -0.4605 0.2734 0.8025 3.36 1.oo S(8) 0.2488 0.7486 -0.2612 4.14 1.oo -0.6224 0.5020 0.8978 3.66 1.oo 0 0.5001 0.1013 0.6226 4.02 1.oo -0.0059 0.2478 0.4945 2.46 1.oo C(1) 0.2868 0.4336 0.1576 2.59 1.oo 0.2 169 0.2194 0.3493 2.67 1.oo C(2) 0.2477 0.5053 0.0072 2.65 1.oo 0.3413 0.1294 0.3777 2.53 1.oo C(3) 0.3094 0.2888 0.3833 2.69 1.oo 0.5243 0.1371 0.201 7 5.50 1.oo C(4) 0.3997 0.3 159 0.5125 2.57 1.oo 0.5300 0.0230 0.2158 6.57 1.oo C(5) 0.1 309 0.6221 -0.3423 2.90 1.oo -0.1586 0.2873 0.5659 2.45 1.oo C(6) 0.2193 0.6512 -0.2171 2.76 1.oo -0.5126 0.3997 0.6829 2.31 1.oo (77) 0.4090 0.1114 0.3998 3.82 1 .oo -0.3850 0.3096 0.7102 2.40 1.oo C(8) 0.5695 0.1592 0.6278 3.70 1.oo -0.6248 0.5700 0.8400 3.18 1.oo (39) 0.0245 0.7839 -0.5347 6.76 1.oo -0.4002 0.4 102 0.8874 3.39 1.oo C( 10) 0.1390 0.7986 -0.5283 5.43 1.oo 0.7289 0.1697 0.2251 4.00 1.oo H(41) 0.4434 0.1248 0.21 18 4.00 1.oo 0.5078 0.1257 0.1327 4.00 1.oo H(42) 0.3875 0.0506 0.3616 4.00 1.oo 0.3 183 -0.0093 0.1937 4.00 1.oo H(51) 0.6555 0.1297 0.741 1 4.00 1.oo 0.6802 -0.0402 0.1746 4.00 1.oo H(52) 0.5837 0.1663 0.4104 4.00 1.oo -0.4064 0.5816 0.8316 4.00 1.oo H(9 1 ) -0.0065 0.8069 -0.3226 4.00 1.oo -0.7337 0.6567 0.8684 4.00 1.oo H( 92) -0.0372 0.825 1 -0.6855 4.00 1.oo H( 101) -0.21 14 0.4439 0.8739 4.00 1.00 H(101) 0.1729 0.7742 -0.7328 4.00 1.oo H( 102) -0.3625 0.3889 0.9482 4.00 1.oo H(102) 0.1278 0.8677 -0.5142 4.00 1.oo ~~ ~ Atomic multidicitv.Table 3 Bond lengths, non-bonding distances (A)and atomic numbering scheme of the EOTT molecule in ((0 EOTT,IBr,.(h) the other linear anion (AuI,, IBr2, AuBr,, cx-IC1, and 13) salts AuI, salt IBr, salt AuBr, salt cr-IC1, salt I, salt 1,Hr salt 1.361( 14) 1.372(9) 1.296(44) 1.346( 11) 1.347(8) 1.?842(8) 1.340( 16) 1.353( 10) 1.287(48) 1.333( 12) 1.343(9) 1.2 31(9) 1.331( 15) 1.344( 10) 1.386(48) 1.332( 12) 1.338(8) 1.?40(9) 1.343( 3 1) 1.431( 16) 1.473(67) 1.337( 19) 1.425( 14) 1.431(14) 1.423( 17) 1.418( 10) 1.356( 52) 1.395( 12) 1.404(8) 1.4 13(9) 1.415(16) 1.41 2( 10) 1.400( 44) 1.401( 11) 1.408(8) 1.402(8) 1.740( 13) 1.739( 8) 1.789 (44) 1.745( 9) 1.737( 7) 1.711(7) 1.721( 11) 1.727( 6) 1.759( 36) 1.734( 8) 1.739(5) 1.;38( 6) 1.752( 10) 1.748 (6) 1.793 (27) 1.749( 7) 1.745(5) 1.7 53(5) 1.744( 12) 1.749(8) 1.693 (34) 1.750( 9) 1.750(7) 1.748 (7) 1.742( 12) 1.738(8) 1.795(37) 1.747( 9) 1.744( 7) 1.742(7) 1.724( 10) 1.736(6) 1.742( 30) 1.737( 7) 1.735( 5) 1.;42( 6) 1.731( 19) 1.768( 9) 1.692( 37) 1.758( 13) 1.755(8) 139(8) 1.870( 22) 1.856( 11) 1.940( 46) 1.844( 13) 1.851(9) l.834( 10) 1.732( 10) 1.739( 6) 1.776(28) 1.748(8) 1.743(6) 1.738 (6) 1.739( 12) 1.737(7) 1.698( 34) 1.734( 9) 1.747( 7) 1.7 37(7) 1.761( 11) 1.751(7) 1.682( 36) 1.749(9) 1.744( 6) 1.7 50(6) 1.736(9) 1.744( 6) 1.728(29) 1.746( 7) 1.748(5) 1.741(5) 1.726(10) 1.740( 6) 1.7 16( 29) 1.741(8) 1.738(5) 1.740 ( 5) 1.762( 12) 1.746( 7) 1.758(37) 1.750(9) 1.748(7) 1.736( 6) 1.826( 11) 1.823( 7) 1.840(33) 1.834( 9) 1.835 (6) 1.814(6) 1.800( 11) 1.818( 7) 1.8 15 (34) 1.815(9) 1.81 7 (6) 1.8 16(6) 1564 J.MATER. CHEM., 1994, VOL. 4 Table 3 (continued) AuI, salt 1Br, salt AuBr, salt a-IC1, salt I, salt 1,Br salt duH(101) S(8) s(6) 2.922( 5) 2.933( 3) 2.935( 15) 2.932( 4) 2.933 (3) 2.931 (2) 3.500( 6) 3.501 (3) 3.481( 16) 3.496( 4) 3.502( 3) 3.495(3) 2.917( 4) 2.929( 3) 2.916( 14) 2.928( 4) 2.93 1 (3) 2.926(2) 3.410(5) 3.423( 3) 3.405 ( 16) 3.42 1 (4) 3.41 1 (3) 3.408( 2) 2.938( 5) 2.942( 3) 2.938( 14) 2.940( 4) 3.9 3 4 (4) 2.937( 2) 3.263 (4) 2.263( 3) 3.286( 13) 3.255(4) 3.249 (4) 3.250(2) 2.916(5) 2.935( 3) 2.923( 13) 2.928(4) 2.932( 4) 2.935( 2) 3.192( 4) 3.200( 3) 3.186( 13) 3.194(4) 3.203( 4) 3.200(2) 2.973(4) 2.971 (3) 2.963( 14) 2.966( 4) 2.972( 4) 2.972(2) 2.964( 4) 2.960( 3) 2.928( 14) 2.961 (4) 2.959(4) 2.964( 2) 2.381 ( 19) 2.387( 11) 2.369( 57) 2.356( 14) 2.363(9) 2.374( 10) Table 4 Distances of short contacts between sulfur atoms (G3.70 A)in the linear anion salts a P1 P2 bl b, EOTT,AuI, S(5)-S(7): 3.688(4) S(4)-S(6) : 3.575( 5) S(1)-S(7): 3.627(5) S(4)-S(8): 3.504(4) none S(7)-S(5): 3.688(4) S(6)-S(4): 3.575(5) S(7)-S(1):3.627(5) S(8)-S(4): 3.504(4) S(3)-S(7): 3.456(5) S(7)-S(3): 3.456(5) EOTT,IBr, S(5)-S(7): 3.686(3) S(4)-S(6) :3.576( 3) S(1)-S(7): 3.656(3) S(4)-S(8): 3.48?(3) none S(7)-S( 5): 3.686(3) S(6)- S(4) :3.576( 3) S(7)-S(1):3.656(3) S(8)-S(4): 3.483 3) S(3)-S(7): 3.413(3) S(7)-S(3) :3.413( 3) EOTT,AuBr, S(5)-S(7) :3.73( 1) S(4)-S(6): 3.56(1) S(1)-S(7): 3.67(2) S(4)-S(8):3.47( 1) none S(7)-S(5): 3.73( 1) S(6)-S(4): 3.56( 1) S(7)-S( 1): 3.67(2) S(8)-S(4): 3.47( 1) S(3)-S(7):3.46(1) S(7)-S(3):3.46(1) LY-EOTT~ICI, S(5)-S(7):3.659(4) S(4)-S(6) :3.561 (4) S(l)-S(7):3.662(4) S(4)-S(8): 3.487(4) none S(7)-S( 5): 3.659(4) S(6)-S(4): 3.561(4) S(7)-S(1): 3.662(4) S(S)-S(4): 3.48?(4) S(3)-S(7): 3.395(4) S(3)-S(7) : 3.395(4) EOTT21, S(5)-S( 7): 3.960( 3) S(4)-S(6): 3.614(3) S(l)-S(7): 3.636(4) S(4)-S(8): 3.53914) none S(7)-S( 5):3.690( 3) S(6)-S(4): 3.614(3) S(7)-S(1): 3.636(4) S(8)-S(4): 3.539(4) S(3)-S(7): 3.457(3) S(7)-S(3): 3.457(3) EOTT,I,Br S(2)-S(6):2.679 (2) S(1)-S(7): 3.605(2) S(4)-S(6) :3.640( 3) S(5)-S(7) : 3.527(2) none S(6)-S(2) :2.679( 2) S(7)-S( 1):3.605(2) S(6)-S(4) :3.640( 3) S(7)-S(5):3.527( 2) S(6)-S(8): 3.450(2) S(8)-S(6): 3.450(2) Table 5 Atomic coordinates of EOTT,FeCI, 0.0694 0.0142 -0.0670 5.09 0.50 0.4732 0.2677 0.8399 3.86 1.oo -0.0950 0.1240 0.0105 6.53 0.50 0.5161 0.0955 0.6218 6.98 1.oo 0.2695 0.0585 0.0105 5.19 0.50 0.6690 0.4266 0.5189 2.96 1.oo 0.0177 0.01 16 -0.3884 6.09 0.50 0.7331 0.4958 0.4949 2.54 1.oo 0.1134 --0.1168 -0.1063 6.96 0.50 0.5632 0.2853 0.46 14 2.80 1.oo 0.7841 0.5115 0.2623 3.31 1.00 0.5550 0.3146 0.6632 2.52 1.oo 0.7665 0.5736 0.7000 3.09 1.oo 0.8721 0.5985 0.3584 3.36 1.oo 0.6406 0.3476 0.3184 3.36 1.oo 0.8650 0.6259 0.5561 3.65 1.oo 0.6234 0.4062 0.7568 3.19 1.00 0.5773 0.1005 0.4317 6.69 1.oo 0.9597 0.6351 0.1737 4.80 1.oo 0.5700 0.1458 0.78 13 6.05 1.oo 0.9358 0.71 17 0.6866 4.38 1.oo 0.9431 0.7553 0.2969 10.15 1.oo 0.4876 0.1999 0.3289 4.26 1.oo 0.9303 0.7854 0.501 1 9.89 1.oo J. MATER.CHEM., 1994, VOL. 4 that the crystal structures of the AuCI,, Au(CN)~ and Ag(CN), salts are different from the above-mentioned struc- ture. More study is required to obtain further structural information on them. Crystal Structure of the FeCl, Salt The crystal data are listed in Table 1. The unit cell includes two EOTT molecules. The FeCl, anion has positional disorder and is located slightly away from the inversion centre (Table 5). One Fe and four C1 atoms are crystallographically independent, each with occupation probabilities of 50%.EOTT molecules form a dimeric columnar structure along the a axis (Fig. 4), while the linear anion salts have regularly stacking columns, as described above. In the column, EOTT molecules stack in alternating orientations as shown in Fig. 5. The ethylene group is sandwiched between the neighbouring oxatrimethylene groups. Both carbon atoms of the ethylene group deviate from the molecular plane, while in the linear anion salts one carbon atom does and the other does not. The ethylene group has comformational disorder with a large thermal motion. EOTT molecules stack with slipping only along the long axis of the molecule. T,he inter- and intra- dimer distances are 3.875 and 3.481 A, respectively. The average of these values is nearly equal to theointermolecular distances of the linear anion salts (3.61-3.63 A).These struc- tural differences between FeCl, and the linear anion salts are reflected in the calculated overlap integrals of the highest occupied molecular orbitals (HOMOS) of EOTT; the intra- dimer overlap integral (21.49 x lop3) is largest, while the interdimer integral (15.00 x lop3)is comparable to b, or b2 in Fig. 2 and the others are small or negligible (Fig. 6). This dimeric form and anisotropic intermolecular interaction result in the semiconductive behaviour of this salt, which contrasts with the electrical properties of the linear anion salts (see below). Electrical Properties of Linear and Tetrahedral Anion Salts The temperature dependences of the resistivities are shown in Fig.7. The salts of AuI,, AuBr,, IBr,, IC1, (a-type, denoted ct-IC12 below) and Br, showed metallic resistivity around room temperature. Of all the linear anion salts, only the AuI, salt retained its metallic behaviour to low temperatures. The resistivity of EOTT,IBr, first decreased continuously with temperature to a minimum around 25 K and then climbed slowly at lower temperatures. The EOTT salts of AuBr,, a-IC1, and Br, exhibited clear M-I transitions at 145 K (a-ICl,) or ca. 210K (AuBr, and Br,). The other linear anion salts are semiconducting over the entire temperature range studied. Their resistivities at room temperature (PRT) are as follows: a-EOTT,ICl,, PRT =5 R cm; P-EOTT,ICl,, PRT =1800 R cm; EOTT,I,Br, PRT =0.3 R cm; EOTT,I,, PRT =1R cm; EOTT,Br,, PRT =0.01 R cm; EOTT,AuBr,, PRT =0.02 R cm; EOTT21Br,, pRT=0.02 R cm; EOTT,AuI,, PRT =0.006 R cm; EOTT/Au(CN),, pRT= 1400R cm; EOTT/Ag(CN),, PRT= 360 R cm; and EOTT/AuCl,, PRT= 590 R cm.In order to correlate their structure and electrical properties this series of salts was classified into three groups: the salts with a longer anion (i.e.with a large unit-cell volume; 13)are semiconductive from room temperature, while the salts with medium-length anion (AuI,, IBr,) are metallic down to low temperatures; salts with shorter (AuBr,, IC1,) anions exhibited M-I trans-itions. The activation energy (E,) was 0.13 eV for the I3 salt. EOTT,I,Br had a significant change in E, around 135 K; E, =0.03 eV at higher temperatures and 0.07 eV at tempera- tures lower than 135 K.For the other linear anion salts, the E, values are 0.12eV [AuCl,, Au(CN),] and 0.15eV [Ag(CN),]. All the tetrahedral anion salts were semi-conductors and their E, values are 0.06 eV (FeCl,), 0.13 eV (NiCl,), 0.11 eV (MnC1,) and 0.17 eV (CoCl,). The resistivity at room temperature of the tetrahedral anion salts are as follows: EOTT/NiCl,, PRT =60 R cm; EOTT/CoCl,, pRT= 35 R cm; EOTT/MnCl,, PRT =4 B cm; and EOTT/FeCl,, PRT =55 R cm. Crystal Structures and Electrical Properties of a-and p-EOTTJCI, EOTT,ICl, has two different phases. One is a-type, which showed an M-I transition around 145 K (Fig. 8) and is isostructural with the other linear anion salts, as remarked above. For the sake of clarity we refer to the insulating state Fig.4 Unit cell of EOTT2FeC14 3.481 A Fig. 5 Stacking mode of EDOTT molecules in EOTT2FeC1,. T$e interdimer distance is 3.875 A and the intradimer distance is 3.481 A Fig. 6 Molecular arrangement between highest occupied molecular orbitals of EOTT molecules in EOTT,FeCI,. Overlap integrals (x lo3):pl, 3.32;p2, 2.15; a,, 15.00;a2,21.49; cl, 0.15; and c:,, 0.15. of a-type as the a'-type. The other type is the B-type, which is a semicondiictor having pRT= 1800i2 cm and E, =0.13 eV. The crystal data of the P-type are displayed in Table 1. The unit cell contains four EOTT molecules and two ICl, anions (Fig. 9). Two of the donor molecules and one of the anions are crystallographically independent.The IC1, anion is almost 0 100 200 300 J. MATER. CHEM., 1994, VOL. 4 linear; the C1( l)-I-C1(2) angle is 178.2( 1)'. Th: bond distances are I-C1( l), 2.540( 4) A and I-C1( 2), 2.541(5)A. The distances of the central CaC bonds of EOTT molecules are 1.346( 17)-&381( 12) A for molecule A and 1.332( 13)- 1.339(14)A for molecule B (Table 6). Two carbon atoms of one terminal ethylene group in molecule B has large tempera- r 4--2-0-100 200 300 T/K Fig. 7 Temperature dependences of resistivities of (a) linear anion salts with metallic behaviour: 0,Br,; +, AuBr,; 0,ICl,; 0,IB,; A, AuI, salts; (b)linear anion salts with semiconducting behaviour: 0, I,; 0,Au(CN),; H, Ag(CN),; A, AuC1,; V, 1,Br salts; (c) tetrahedral anion salts: 0, [(CH,),N],[CoCl,] +KCl; H, [(C2H,)4N]2[NiCl,] +KCl; 0, [(C,H,),N],[MnCl,] +NH,Cl; A, [(C,H,),N][FeCl,].(Since the stoichiometries of the tetrahedral anion salts are not yet known, each crystal is distinguished by electrochemical synthetic conditions.) I I 1 I 1 0 100 200 300 TIK Fig. 8 Temperature EOTT21C12 dependences of resistivities for x- and p- Fig. 9 Unit cell of fi-EOTT,TCl, J. MATER. CHEM., 1994, VOL. 4 Table 6 Atomic coordinates of P-EOTT,ICl, 0.2110 0.4962 0.0010 4.31 1.oo 0.2041 0.5416 -0.2735 5.32 1.oo 0.2234 0.4542 0.2778 6.35 1.oo 0.2769 -0.0792 -0.8582 3.03 1.oo 0.0867 -0.0277 -0.8149 3.46 1.00 0.4015 0.0967 -0.6127 2.86 1.oo 0.2099 0.1515 -0.5823 2.96 1.oo 0.1823 -0.2445 -1.0816 4.14 1.oo -0.0462 -0.1837 -1.0223 5.76 1.oo 0.5290 0.2499 -0.3861 3.26 1.oo 0.3031 0.3 149 -0.3495 3.04 1.oo -0.2352 -0.1065 -0.8391 2.88 1.oo -0.4279 -0.0503 -0.8173 2.87 1.00 -0.1124 0.0676 -0.6002 3.14 1.oo -0.3032 0.1286 -0.5704 3.00 1.oo -0.3208 -0.2556 -1.0991 3.21 1.00 -0.5469 -0.1856 -1.0775 3.42 1.oo 0.0209 0.2277 -0.4016 4.84 1.oo -0.2007 0.3015 -0.3666 5.57 1.oo 0.4948 0.4241 -0.3423 3.83 1.oo -0.5170 -0.3586 -1.1580 4.84 1.oo 0.2176 -0.0036 -0.7668 2.62 1.oo 0.27 12 0.07 15 -0.6630 2.12 1.oo 0.1656 -0.1498 -0.9648 2.81 1.oo 0.0768 -0.1277 -0.9426 3.79 1.oo 0.4095 0.1975 -0.4876 2.52 1.00 0.3194 0.2227 -0.475 1 2.18 1.oo 0.0549 -0.3132 -1.1279 5.30 1.oo -0.0259 -0.2589 -1.1684 4.92 1.oo 0.5460 0.3698 -0.4343 3.66 1.oo 0.3910 0.4144 -0.4078 3.74 1.oo -0.2949 -0.0254 -0.7572 2.73 1.oo -0.2439 0.0473 -0.6574 2.84 1.oo -0.3426 -0.1690 -0.9703 2.27 1.oo -0.4300 -0.1423 -0.9588 2.55 1.oo -0.1026 0.1726 -0.4832 3.00 1.oo -0.1892 0.2005 -0.4715 3.09 1.oo -0.4189 -0.3579 -1.0827 3.81 1.oo -0.5721 -0.308 1 -1.0695 4.82 1.oo 0.0134 0.3423 -0.3678 12.64 1.oo -0.079 1 0.3786 -0.3609 9.34 1.oo ture factors of 9.34 and 12.64 A2 (Table 7 and supplementary data).Both carbon atoms in this group deviate from the molecular plane in the same direction (Fig. 10) like those of EOTT,FeC14 (Fig. 5). The EOTT donor molecules form col- umns along the shortest axis (Fig. 11) as in the cr-type. In this P-type salt, EOTT molecules stack dimerically, while a uni- form stack was observed in the a-type.EOTT molecules stack with two kinds of overlap modes (Fig. 10): in one mode the EOTT molecules overlap with slipping along both axes of the EOTT molecule, as with the a-type; in the other mode the donors overlap with slipping only along the long axis and undergo ring-over-bond type overlap as is the cFse with EOTT,FeC14. Thf interplanar distances are 3.789 A for the former and 3.498 A for the latter. In short, the crystal structure of B-IC12 is the same as that of a-TCl, except for the dimeriz- ation. In addition to this structural similarity the lattice energies of both types of crystal must be very similar since they crystallized under the same conditions. In order to clarify the relationship between the structures of a’-ICl, and p-ICl,, we examined the variation with temperature of the lattice constant for a-ICl, and took oscillation photographs around 95 K and room temperature. Fig.12 shows that the a and b axes of the a-type, which correspond to the c and a axes of the a-type, shortened almost monotonically with temperature with a small anomaly around 145K, at which the M-I transition occurred. On the contrary the length of c axis, which penetrates the conduction sheets, decreased down to 3.498A B A P Fig. 10 (a) Intra- and (b) inter-dimer overlapping modes of EOTT molecvles in P-EOTT21C1,. The interplanar distances are 3.498 and 3.789 A, respectively. Fig. 11 Molecular arrangement between highest occupied molecular orbitals of EOTT molecules in P-EOTT21C1,. Overlap integrals (x lo3):pl, 11.29; p2, 1.98; p3, 7.81; p4, 3.51; a,, 5.87; a2,2.95; a3, 5.88; a4, 2.95; cl, -0.88; c2, 22.46; c3, -0.86; and c4, 22.45. 145 K, then increased sharply at lower temperatures.In oscillation photographs of the a axis, which is the stacking axis, new Bragg reflections were found around 95 K (€;ig. 13). This means that the length of the a axis in the r-tjpe had already doubled at around 95 K. Therefore the leFgth of the stacking axis (a axis) of a’-ICl, (4.690 x 2 =9.38 A at 92 K) corresponds roughly to that (c axis) of the P-type (8.736 A at room temperature). Also, the activation energy of the P-type (0.13eV) agreed with that of the a’-type (O13eV).Unfortunately, the crystals shattered when they were cooled owing to the drastic transition and the crystal structure of the a‘-type could not be determined. However, from these results we conjectured that the crystal structure of the fi-typtt might be a state close, if not identical, to that of the a’-type, though it was not a metastable state of the @’-type or vice tiema. A preliminary X-ray study on the AuBr, salt at low temperatures also indicated the same situation. J. MATER. CHEV., 1994, VOL. 4 Table7 Bond lengths (A)and atomic numbering schemes of the EOTT molecule in P-EOTT,ICl, A B 1.381( 12) 1.346( 17) 1.365( 14) 1.714( 11) 1.723( 10) 1.716(9) 1.738( 19) 1.744(9) 1.738( 11) 1.750( 12) 1.723( 10) 1.752( 10) 1.735(9) 1.735(8) 1.749( 10) 1.786( 12) 1.712( 16) 1.835( 11) 1.842( 11) 1.515(21) 1.430( 15) 1.422( 13) C( ll)=C( 12) C( 15)=C( 16) C(13)=C( 14) C( 12)-S( 13) C( 12)-S( 14) C( 11)-S( 11) C(11)-S( 12) C(15)-S( 13) C(15)-S(17) C(16)-S( 14) C( 16)-S( 18) C( 13)-S(l1) C( 13)--S( 15) C(14)-S( 12) C(14)-S( 16) C( 19)--S( 17) C(20)-S( 18) C( 17)-S( 15) C( 18)-S( 16) C(19)-C(20) C(17)-O( 1) C( 18)-O( 1) 1.332( 13) 1.337( 16) 1.339( 14) 1.741(10) 1.7 5 8( 12) 1.740( 12) 1.763( 10) 1.760( 10) 1.730( 10) 1.754( 9) 1.736( 11) 1.765(8) 1.735( 10) 1.756( 10) 1.741 (9) 1.726( 19) 1.788( 10) 1.823( 11) 1.784( 13) 1.472( 30) 1.403( 14) 1.415( 18) -4.75 11.60 -15.90 Fig.12 Temperature dependence of the lattice dimensions of a-EOTT,ICl, Band Structures We calculated overlap integrals and band structures by the extended Hiickel method.In the previous rep~rt,~ only s and p orbitals for chalcogen atoms had been taken into consider- ation. However, re-examination of the band parameters in the calculation,8 which is compatible with the optical study of BEDT-TTF complexe~,~ has indicated that the d orbitals of the sulfur atoms play an essential role in the electronic band structure. Therefore we took them into consideration as well. The resultant overlap integrals were different from what was expected from the observed short contacts between chalcogen Fig. 13 Oscillation photographs of the a axis of r-EOTT,ICl, (a) at room temperature and (b)at 95 K b‘ I Fig. 14 Calculated Fermi surface of the linear anion (AuBr2, AuI,, a-ICl,, IBr2, 1,Br and 13) salts rx v r z vi Fig.15 Calculated electronic band structure of ~-EOTT,ICl, J. MATER. CHEM., 1994, VOL. 4 atoms. In the linear anion salts, b, and b, are larger than the other contacts and a is small (Fig. 2). The calculated Fermi surface is isotropic and closed, which implies a two-dimensional electronic structure (Fig. 14). Compared with the isostructural EOST [4,5-ethylenedithio-4,5'-(2-oxatrimethyl-eneditho)diselenadithiafulvalene] salts," where EOST is replaced at two of the four inner S atoms of EOTT by Se atoms, the EOTT salts exhibit M-I transitions at much higher temperatures. This is because the EOTT salts are thought to have narrower bandwidths and, consequently, less stable metallic states than the corresponding EOST salts." For the purpose of seizing the mechanism of the M-I transition of x-EOTT,ICl,, we have to know the band struc- ture of the %'-phase also.Unfortunately, a low-temperature structural analysis of the salt could not be carried out. Thus we calculated the band structure of the P-type instead, since we expected that it approximated the band structure of the a-type. As depicted in Fig. 11, the overlap integrals of the stacking direction [C( l), C(2), C(3), C(4)] clearly confirm the occurrence of dimerization; c1 and c, are much smaller than c2 and c4. The calculated band structure is shown in Fig. 15. The Fermi level lies within the energy gap; in other words, the Fermi surface has completely disappeared.This result can be reconciled with the measured electrical behav- iour. Therefore, we could discuss the M-I transition of a-EOTT,ICl, by picturing an imaginary transition from the a-phase to the P-phase; the transition can be attributed to the dimerization mentioned above. From this picture the mechanism of the transitions of the other isostructural linear anion salts might be deduced. Conclusions We have synthesized the salts of an unsymmetrical donor EOTT with various anions. With respect to the trihalides the salts with AuBr,, AuI,, a-ICl,, IBr,, I, and Br, are isostructu- ral with each other and have columnar structures along the shortest crystal axes without any disorder. A strong intermol- ecular interaction was found between the donor columns, and all these salts have two-dimensional network structures based on the sulfur-sulfur short contacts.The temperature depen- dences of the conductivities are different despite the isostruc- tural nature of the salts. Tight-binding band calculations indicated that the Fermi surfaces for the above EOTT salts were closed and two-dimensional. There is another phase of EOTT,ICl, (P-type), which exhibited semiconducting behaviour from room tempcrature. The crystal structure of the 13-type is similar to that of the a-type except for the dimerization in the EOTT columns in the former salt. To infer the cause of the M-I transition of x-type, we calculated the band structure of the b-type. Thc result agreed with the observed insulating behaviour.Consequently the M-I transition of a-EOTT,ICl, might be explained mainly by the dimerization along the donor columns. This M-I transition might occur with some of the other lineair anion salts in question. EOTT,FeCl, is a semiconductor with a dimerized crystal structure. References 1 D. Jerome, A. Mazaud, M. Ribault and K. Bechgaard, J. Phys. Lett. (Paris), 1980,41, L95. 2 Reviews: J. M. Williams, J. R. Ferraro, R. J. Thorn, K. D. Carlson, U. Geiser, H. H. Wang, A. M. Kini and M-H. Whangbo, The Physics and Chemistry of Organic Superconductors (Including Fullerenes), Prentice Hall, New Jersey, 1992; M. R. Brycc, Chem. Sac. Rev., 1991, 20, 355; (c)G. Saito, Phosphorous, Sulfur' Silicon, 1992, 67, 345; T. Ishiguro and K. Yamaji, Organic Superconductors, Springer-Verlag, Berlin, 1989. 3 H. Nakano, Ph.D. Thesis, Osaka University, 1991. 4 H. Nakano, K. Yamada, T. Nogami, Y. Shirota, A. Miyamoto and H. Kobayashi, Chem. Lett., 1990,2129. 5 International Tables for X-ray Crystallography, Kynoc h Press, Birmingham, 1974, vol. IV. 6 T. Sakurai and K. Kobayashi, Rep. Inst. Phys. Chem. Rts., 1979, 55, 69; C. K. Johnson, ORTEP, Report ORNL-3794, Oak Ridge National Laboratory, Tennessee, 1965. 7 G. M. Sheldrick, SHELX-86, Program for crystal structure deter- mination, University of Gottingen, Germany, 1986. 8 A. Kobayashi et al., to be published. 9 M. Tamura, K. Yakushi, H. Kuroda, A. Kobayashi, R. Kato and H. Kobayashi, J. Phys. SOC. Jpn., 1988,57,3239. 10 T. Naito, A. Tateno, T. Udagawa, H. Kobayashi, R. Kato, A. Kobayashi and T. Nogami., J. Chem. Soc., Faradaj Trans., 1994,90, 763. Paper 4102314E; Received 19th April, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401559

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( lo), 1571-1577 Synthesis and Second-harmonic Generation Properties of 2-(4=Nitroanilino)=1,3,5=Triazine Derivatives Hisatomo Yonehara,* Wen-Bing Kang, Tatsuo Kawara and Chyongjin Pac Ka wamura Institute of Chemical Research, 637 Sakado, Sakura, Chiba 285, Japan The synthesis and non-linear optical properties of a series of 2-(4-nitroanilino)-l,3,5-triazine compounds are degscribed. The triazines show various activities in powder second-harmonic generation (SHG) depending on the structures and have absorption maxima at d350 nm, shorter by 30-50 nm than those of the parent nitroanilines. 2-(4-Nitroanilino)-4,6-diphenyl-l,3,5-triazine affords different crystalline materials depending on the recrystallization solvent. A crystal formed by recrystallization from toluene reveals a high powder SHG activity comparable with that of 2-methyl- 4-nitroaniline, while recrystallization from N,N-dimethylformamide gives an SHG-inactive crystal in which solvent molecules are incorporated in a 1: 1 ratio by hydrogen bonding.The crystal structure of the latter was determined. Conjugated n-electron systems with intramolecular charge transfer have been recognized as potential candidates for second-harmonic generation (SHG) arising from the creation of large optical dipole moments. 4-Nitroaniline (4-NA) and 4-dimethylamino-4'-nitrostilbene (DANS) are typical com-pounds of high second-order hyperpolarizability (b).However, their net SHG effects in the crystalline state are negligible as a consequence of their centrosymmetric crystal structures.Intense experimental and theoretical effort has been devoted to chemical modifications of the basic molecular structures of 4-NA and DANS in order to develop non-centrosymmetric crystals without a significant decrease in fl.' For instance, 2-methyl-4-nitroaniline (MNA) and 3-methyl-4-methoxy-4'- nitrostilbene (MMONS)2 exhibit high SHG effects in the crystalline state because of the creation of non-centrosymmetric crystal structures by the introduction of the additional methyl substituent. However, these compounds have the serious disadvantages of relatively low melting points and long cut-off wavelengths, which would prevent their application in practical SHG devices for the generation of blue-green light.SHG-active organic compounds should meet the additional requirements of short cut-off absorption wave-lengths and high melting points. From this viewpoint, we have performed semiempirical molecular orbital calculations (MOPAC and CNDOjS-CI) for molecular design of key compounds. Consequently, it is predicted that the modifi- cation of the 4-NA and 4-amino-4'-nitro-trans-stilbene (4-ANS) chromophores with triazine groups may produce compounds with short cut-off wavelengths while retaining large non-linear activities. Another important benefit of the modification with triazine groups would come from the expectation of high melting points associated with thermal stabilities, since triazine compounds have been used as possible intermediates for the preparation of thermally stable organic polymer^.^ This paper deals with the preparation and SHG properties of a series of 2-(4-nitroanilino)-1,3,5-triazines and (4'-nitrostilbenylamino)-1,3,5-triazines.A parti- cularly interesting finding is that recrystallization of 2-(4-nitroanilino)-4,6-diphenyl-1,3,5-triazinefrom toluene gives a crystalline compound possessing a high powder SHG activity comparable with that of 2-methyl-4-nitroaniline, a high melting point (268 "C) and a relatively short absorption maximum.Experimental General The structure determination of the compounds was carried out by IR spectra (KBr disks) on a Jasco A-202 IR spec-trometer, 'H NMR spectra on a JEOL JNM-GSX-400 spec- trometer for CDCl, or ['H6] dimethyl sulfoxide (DM SO-d,) solution with tetramethylsilane (TMS) as the internal stan- dard, FD mass spectra on a Shimadzu 9100-MK mass spectrometer and UV-VIS spectra on a Hitachi U-3500 spectrophotometer.Reflectance spectra of powder hamples were recorded using an integrating-sphere attachment. Melting points were measured on a Yanagimoto micro melting-point apparatus and are uncorrected. Measurement of Second-harmonic Generation The SHG properties of the compounds were measured by a powder method4 using a Q-switched Nd : YAG laser SL803, SPECRTON Laser Systems Ltd.) operating at 1.064 iim fun- damental wavelength with 10 ns pulse duration and 10 Hz repetition freq~ency.~ Theoretical second-order hyperpc )lariza- bilities of these compounds were calculated by MIOPAC (PM3, ver 5.0),6,7and the calculations of absorption maxima of some triazine derivatives were carried out using CNDO/S-CI.6,7 Preparation of Triazine Compounds The triazine compounds were prepared according to eqn.( 1)-( 6). Commercially available 2,4,6-trichloro-1,3,5-1 riazine (C1,-TRAZ), 4-nitroaniline, 2-methyl-4-nitroanilirie, 2-chloro-4-nitroaniline, 3-nitroaniline, 4-nitrophenol and 4-nitrothiophenol were used as received. 4-ANS,' 2-chloro-4,6-diphenyl- 1,3,5-triazine (ClPh,-TRAZ)9 and 2-chloro-4,6-dimethoxy-1,3,5-triazine[Cl( MeO),-TNAZ] lo were prepared according to the literature methods (TRAZ, 1,3,5-triazine group). 2,4-Dichloro-6-arylamino-1,3,5-triazinesand 2,4-Dichlor0-6-phenoxy (or 6-thiophenoxy)-l,3,5-triazine(la-f, 6a) C1,-TRAZ (1.84 g, 10mmol) was added to a solution of 4-NA, MNA, 2-chloro-4-nitroaniline, 4-nitrothiophenol or %nitro- aniline (10mmol) in acetone (20 ml) with stirring at 0 "C,to a solution of 4-nitrophenol (10 mmol) in acetone (20 ml) at room temperature, and to a solution of 4-ANS (10 mmol) in benzene (20ml) under reflux.Continuous stirring of the solutions for 0.5-2 h resulted in the gradual formatiion of crystals. After addition of 5 wt.% aqueous potassium hydro- gencarbonate (20 g) to the reaction mixture, the resultant crystals were collected and washed with water and then with methanol to give la (%!YO), lb (86%), lc (56%), Id (81%), le (86%), If (goo/,) and 6a (62%).J. MATER. CHEM., 1994, VOL. 4 CI I d 1 a-f NO2 R X la &NO2 H NH lb &NO2 CH3 NH IC &NO2 CI NH Id &NO2 H 0 1. &NO2 H S 11 %NO2 H NH CI CI Nu I I NAN + Nu NAN CI ANANH9N02 NuANA NHpN02 R' d 1 2 3 l;R Nu 2a H MeO 2b H Et2N 2c H MeCOCHC02Et 2d CHB MeO 38-MeO 3b -OH CI CI (3) MeANA NHeNO, 20 OMe OMe I 1.-4a 2-Chloro-4-methoxy (or 2,4-dimethoxy) -6-arylamino-1,3,5- triazines (2a, 2d,3a) and 2,4-Dimethoxy-6-[4-( 4'-nitrophenyl- ethenyl)anilino]- 1,3,5-triazine (6c) For the preparation of 2a, 2d and 3a, la or lb (5 mmol) was treated with a 28 wt.% methanolic solution of sodium methox- ide (5 mmol in the cases of 2a and 2d; 10 mmol in the case of 3a) at room temperature for 3 h.Similarly, 6c was obtained from the reaction of 6a (5 mmol) with a 28 wt.% methanol solution of sodium methoxide (10 mmol) at room temperature for 8 h. The resultant precipitates were filtered off and washed with methanol to give 2a (ca. loo%), 2d (79%), 3a (900/) and 6c (62%). 2-Chloro-4-diethylamino-6-(4-nitroani1ino)- 1,3,5-triazine (2b) A solution of diethylamine (0.365 g, 5 mmol) in 10ml toluene was added to a suspension of la (1.43 g, 5 mmol) in 50 ml toluene, and the mixture was refluxed under stirring for 3 h and then cooled to room temperature. To the reaction mixture 4b was added 5 wt.% aqueous potassium hydrogencarbonate (20g) and the resultant crystals were collected and washed with water and then with toluene.White crystals of 2b were obtained in 62% yield. 2-Chloro-4-( l-ethoxybutane-1,3-dione-2-yl)-6-(4-nitroani1ino)-1,3,5-triazine (2c) To a suspension of NaH (0.44 g, 11mmol) in DMF (30 ml) was added ethyl acetoacetate (1.4 g, 11 mmol) in DMF (5 ml) under stirring at room temperature. After evolution of hydro- gen gas had ceased, la (2.86 g, 10 mmol) was added to the reaction mixture. After stirring the mixture at room tempera- ture for 3 h, usual work-up procedures gave 2c (38%). 2-Chloro-4-methyl-6-( 4-nitroanilino)- 1,3,5-tria:ine (2e) C1,Me-TRAZ was prepared from a reaction of C1,-TRAZ with methyl magnesium iodide in ether. A mixture of C1,Me-TRAZ (0.33 g, 2 mmol) and 4-nitroaniline (0.27 g, 2 mmol) in J.MATER. CHEM., 1994, VOL. 4 5a 4-ANS 25 ml acetone was refluxed for 4 h and then cooled to room temperature. After addition of 0.67 wt.Yo aqueous potassium hydrogencarbonate (30 ml) to the reaction mixture, the result- ant precipitates were collected and washed with water to give 2e (94%). 2,4-Dihydroxy-6-(aryZamino)-1,3,5-triazines (3b, 6b) A solution of la or 6a (2mmol) in a mixed solvent of DMSO (10 ml) and H20 (2 ml) was stirred at 50 "C for 2 h in the case of la or at 100"C for 5 min in the case of 6a, and then aqueous potassium hydrogencarbonate was added to the solution. The resultant crystals were collected and washed with water to give 3b as pale-yellow crystals in almost quantitative yield or were subjected to column chromatogra- phy (Kieselgel 60, methylene dichloride as eluent) to give 6b in 62% yield.2,4-Dimethoxy-6-(4-formylphenoxy)-l,3,5-triazine (4a) and 2,4- dimetho.vy-6-( 4-dicyanovinylphenox y)- 1,3,5-triazine (4b) A 20 ml solution of potassium hydroxide (10 mol) and 4-hydroxybenzaldehyde (10mmol) in methanol was added to a solution of C1( MeO),-TRAZ" ( 10 mmol) in 30 ml methanol and the mixture was stirred at room temperature for 2.5 h. The resultant white precipitates were collected and washed with methanol to give 4a (77%). A mixture of 4a (3.8 mmol), malononitrile (1.2mmol) and a few drops of piperidine in 40 ml methanol was stirred at room temperature for 2.5 h. The resultant white precipitates were collected and washed with methanol to give 4b (50%).2-(4-Nitroanilino)-4,6-diphenyE-1,3,5triazine (5a) A mixture of ClPh,-TRAZ (55 mmol) and 4-NA (165 mmol) in 150 ml DMF was heated at 120 "C for 8 h and then cooled to room temperature. After addition of 200ml ethanol and 100 ml water to the reaction mixture, the resultant precipitates were collected, washed with ethanol and then recrystallized from toluene to give 5a-1 (900/) as pale-yellow crystals or from DMF to give 5a-2 (90%) as colourless crystals. 2,4- Dimethyl-6- [4-( 4'-nitrophen ylet heny 1)anilinol- 1,3,5-triazine (6d) To a mixture of 0.33 ml diethyl ether solution of methyl magnesium iodide (1.1 mmol) and 20 ml DMF was added 6a (0.52 mmol), and the mixture was stirred at room temperature 6a 6b-f Y Z 6b OH OH 6c Me0 Me0 6d Me Me be ANS CI 6f 2-Me-4-N02-C,H3 CI for 10 min.After the usual work-up procedures, column chromatography of the product (Kieselgel 60, methylene dichloride as eluent) gave 6d in 85% yield. 2-Chloro-4,6-his[4-( 4'-nitrophen ylethenyl )aniline]-1,3,S-triazine (6e) and 2-Chloro-4-( 2-methyl-4-nitroanilino)-6-[4-(4'-nitrophenylethenyl)anilino]-l,3,5-triazine(6f) To a solution of 4-ANS or MNA (0.54 mmol) in 25 rnl DMF at -40 "C was added a 0.4 ml hexane solution of 15% n-butyllithium (0.60 mmol) under stirring. After 1 h, 6a (0.52 mmol) was added to the DMF solution. The mixture was stirred at -40°C for 1h, gradually warmed to room temperature and then poured into ice-water. The resultant precipitates were filtered and purified by column chromatogra- phy (Kieselgel 60, methylene dichloride as eluent) to give 6e (45%) or 6f (60%).X-Ray Structural Investigation of 2-( 4-Nitroanilino)-4,0- diphenyl-1,3,5-triazine (5a) Samples of (5a) used in XRD studies were prepared by recrystallization from DMF or toluene. Powder difkaction patterns were obtained with a Rigaku RAD-I1 diffract ometer. Scans were taken with a 28 step size of 0.02", using Cu-Ka radiation. A single crystal of 5a-2 was obtained by slow evaporation of the solvent from a DMF solution of 5a at room temperature. A colourless single crystal of C21H15N502-C3H7N0having approximate crystal dimensions of 0.5 mm x 0.2 mm x 0.5 mm was mounted on glass fibre. All measurements weice made on a Rigaku AFC5R diffractometer with gsaphite-monochromated Mo-Ka radiation.Experimental details for the crystal-structure determination were essentially the same as those reported previo~sly.~,~ The final cycle of full-matrix least-squares refinement was based on 2075 observed reflec- tions [I >3.0 a(l)]and 355 variable parameters and converged with unweighted and weighted agreement factors of R' =0.052 and R, =0.057 [w=4FO2/o2(Fo2)].? t Tables of atomic coordinates, bond lengths and bond angles have been deposited with the Cambridge Crystallographic Data Centre; see Information for Authors, in issue 1. Table 1 Calculated values of p and the longest absorption maxima (icalc)of some selected triazine compounds compared with those of 4-NA, MNA and 4-ANS flcalc/10-30esu compound PM3 CNDO/S-CI" ;*talc/nmb la 7.1 - - Ib 5.7 - - 3a 7.9 13.4 369.0 3b 4.6 11.2 353.2 5a 10.2 23.2 270.2 6a 20.7 ~ - 4-NA 6.3 15.8 393.0 MNA 6.11 17.7 393.9 4-ANS 37.6 - - "Ref.6 and 7. hicalccalculated by CNDO/S-CI. Results and Discussion Molecular-orbital (MO) Calculation of Hyperporlarizability (p) of Triazine Derivatives Table 1 lists the calculated /3 values (Pcalc)and the calculated longest absorption maxima (Acalc) of several triazine derivatives compared with those of 4-NA and MNA. The CNDO/S-C16,7 calculations of Pcalcand Acalc were performed for 3a, 3b and 5a, which have no chlorine substituents, because no param- eters for chlorine atom are given in the semiempirical MO method.The Pcalc values (PM3) of la, 3a and 5a are comparable with or greater than those of 4-NA and MNA, suggesting that the intrinsic hyperpolarizability of the 4-NA chromophore might be kept in the derivatization with the triazine groups. Similarly, the Pcalcvalues of lb, 3b and 6a are relatively high, again predicting that the triazine derivatives of 4-NA, MNA and 4-ANS would be potentially active in SHG. Another interesting result in the theoretical calculations is that the Acalc values are shorter by more than 24 nm than those of 4-NA and MNA. These results imply that the triazine derivatization of the chromophores should lead to significant blue shifts of the absorption maxima which are certainly beneficial to the construction of SHG-active materials for the generation of blue-green light.In particular, note that 5a shows much shorter icalcand considerably greater Pcalcthan MNA does. Synthesis and Second-harmonic Generation of Triazine Derivatives The dichlorotriazine compounds la-f and 6a were con-veniently synthesized by condensation reactions of C1,-TRAZ with the corresponding nitroarylamines, 4-nitrophenol and 4-nitrothiophenol [eqn. (l)] and were further converted to J. MATER. CHEM., 1994, VOL. 4 2a-d, 3a-b and 6b-f with ease [eqn. (2) and (6)]. Similarly, the monomethyl compound 2e was obtained by the derivatiz- ation of 4-NA with C1,Me-TRAZ, which had been prepared by the methylation of C1,-TRAZ with methyl magnesium iodide [eqn.(3)]. For comparison with the nitro compound 3a, the (dicyan0)ethenylphenoxytriazine4b was also prepared according to eqn. (4). Another important compound is the diphenyltriazine 5a, which was synthesized by the phenylation of C1,-TRAZ with phenyl magnesium bromide followed by the condensation reaction with 4-NA [eqn. (5)].Tables 2-6 summarize the observed SHG activities in powder, absorption maxima (,Imax)and melting points of the triazine compounds together with 'H NMR and IR data. The powder SHG activity of la is relatively high, as expected from the theoretical calculations. A1though the /Icalc value of la is comparable with that of MNA, the observed activity in the powder SHG is lower by a factor of 2.8 than that of MNA, presumably implying a non-centrosymmetric crystalline structure of la with partially cancelled molecular dipole moments.While the Pcalc values of 3a and 3b are significantly high, 3a and 3b are SHG-inactive. Therefore, it can be predicted that the dipole moments of 3a and 3b would be cancelled in centrosymmetric crystal structures. These discrepancies between Dcalc values and observed powder SHG activities have frequently been observed, probably because of the limitation of the prediction using simple MO calculation^.^,^ It is of significance to the SHG properties of the triazine compounds to note that the dichlorotriazine derivative of 4-NA (la) is the most active material of the dichlorotriazines (la-f and 6a), while no net powder SHG activity was observed with lb, the dichlorotriazine of MNA.This is in sharp contrast to the well known fact that 4-NA is totally inactive in powder SHG while MNA is active. Presumably, the dichlorotriazine derivatization of 4-NA should cause a change from the centrosymmetric crystal structure of 4-NA to a non-centrosymmetric one in la. On the other hand, an opposite change in crystal structure should occur in the triazine derivat- ization of MNA. Similarly, lc, which has a chlorine substituent at the 2 position of the 4-NA chromophore, shows a relatively low SHG activity, comparable with that of urea. In the case of the 3-NA derivative (If), the SHG activity is half that of la, as is the relationship of the reported values between 4-NA'l and 3-NA.12 The 4-nitrophenoxy dichlorotriazine (Id) is four times more active in powder SHG than urea, while the thiophenol analogue (le) is much less active.Although the chloromethoxytriazine of 4-NA (2c)shows an SHG activity ca. 10 times greater than urea, other similar compounds (2a, b, d, e, 3a and 3b) are almost inactive in powder SHG. Presumably, the formation of intermolecular hydrogen bonding would stabilize centrosymmetric crystal structures. Another interesting compound is 4b, which pos- Table 2 Physical and SHG properties of the triazine compounds, la-f &ax powder SHG (ethanol) mp 'H NMR compound efficiency" /nm 1°C (DMSO-d,) la 20 323 310 7.92 [2 H, d, arom.], 8.28 [2 H, d, arom.], 11.63 Ib lc 7 10-3 1 304 309 239 200 2.37 [3 H, s, CH,], 7.70 [2 H, d, arom.], 8.11 -c1 H, s, NHI [2 H, d, arom.], 10.97 [1 H, s, NH] Id le If 4.1 0.8 10 267 255 268 185 191 118 7.92 [2 H, d, arom.], 8.62 [2 H, d, arom.] 7.47-7.81 [l H, m, arom.], 7.98 [2 H, d, arom.],7.72 [2 H, d, arom.], 8.08 [2 H, d, arom.] 8.57 [l H, s, arom.], 11.53 [l H, s, NH] 'Values relative to urea (standard) determined by the powder method. 3350 (NH) 3350 (NH), 1540, 1318 (NO,) 3330 (NH), 1340 (NO,) 1520, 1350, (NO,), 1205 (PhO) 1510, 1350 (NO,) 3290 (NH), 1570, 1340 (NO,) J.MATER. CHEM., 1994, VOL. 4 Table 3 Physical and SHG properties of the triazine compounds, 2a-d and 3a )-,ax IR (KBr) powder SHG (ethanol) mp 'H NMR Vmax compound efficiency" /nm /"c 6, (DMSO-d6) /cm-' 2a 0.37 327 261 4.0 [3 H, s, OCH,], 7.97 [2 H, d, arom.], 8.25 3370 (NH), 1560, 1330 (NO,) [2 H, d, arom.], 11.2 [I H, s, NH] 2b 0.02 337 194 0.9-1.33 [6 H, m, 2CH,], 3.0-3.8 [4 H, m, 2 3370 (NH), 1520, 1320 (NO,) CH,], 7.9 [2 H, d, arom.], 8.15 [2 H, d, arom.], 10.57 [1 H, s, NH] 2c 9.4 326 246 1.27 [3 H, t, CH,], 2.43 [3 H, S, C=OCH,], 3360 (NH), 1720 (C=O), 4.17 [l H, s, CHI, 7.93 [2 H, d, arom.], 8.23 1580, 1340 (NO,)[2 H, d, arom.], 11.37 [I H, s, NH] 2d 3 x 10-3 314 200 2.38 [3 H, s, CH,], 3.93 [3 H, s, CH,], 7.78 3350 (NH), 1570, 1340 (NO,) [l H, d, arom.], 8.10 [2 H, d, arom.], 11.2 [1 H, s, NHI 3a 0.1 331 223 3.97 [6 H, s, 20CH,], 7.97 [2 H, d, arom.], -8.18 [2 H, d, arom.], 10.67 [l H, s, NH] 3b 0.04 328 >320 7.97 [2 H, d, arom], 8.20 [2 H, d, arom.] "Values relative to urea (standard) determined by the powder method.Table 4 Physical and SHG properties of the triazine compounds, 4a-b Amax I R (KBr) powder SHG (ethanol) mP 'H NMR "ma, compound efficiency" /nm /"c bH (DMSO-d6) /cm -4a 0.06 -149.5 3.95 [6 H, s, 20CH,], 7.52 [2 H, d, arom.], 8.0 -[2 H, d, arom.], 10.07 [1 H, s, CHO] 4b 4 x 10-4 --3.88 [6 H, s, 20CH,], 7.53 [2 H, d, arom.], -8.05 [2 H, d, arom.], 8.55 [l H, s, CHI "Values relative to urea (standard) determined by the powder method. Table 5 Physical and SHG properties of the 2-(nitroanilino)-4,6-diphenyl-1,3,5-triazinetriazinecompounds, 5a-b Amax IR (KBr) powder SHG (ethanol) mp 'H NMR "max compound efficiency" /nm /"c 6, (DMSO-d6) /cm-5a-1 104 342 268 5a-2' 0.2 342 268 7.5-7.65 [6 H, m, arom.], 7.81 [1 H, s, NH], 3330 (NH), 1530, 1321:) (NO,) 8.0 [2 H, d, arom.], 8.34 [2 H, d, arom.] "Values relative to urea (standard) determined by the powder method.bObtained by recrystallization from toluene. 'Obtained by recrysta,llization from DMF. Found: C, 64.8; H, 4.7; N, 19.2%. Calc. for C,,H,,N,O, (]:I adduct of 5a with DMF); C, 65.2; H, 5.0; N, 19.0. Mass s,pectrum m/z: 369 (Mi). imax(ethanol)/nm (&/dm3 mol-' cm-'); 265 (6.05 x lo4), 342 (3.63 x lo4). Table 6 Physical and SHG properties of the triazine compounds, 6a-f Lax IR (KBr) powder SHG (ethanol) mp 'H NMR Vmax compound efficiency" /nm 1°C 6, (DMSO-ds) /cm-' 6a 11 3 70 >300 7.40 [l H, d, CH=], 7.57-7.93 [7 H, m, =CH 1560, 1335 (NO,) and arom.], 8.20 [2 H, d, arom.], 11.23 [l H, s, NHI 6b 0.5 368 >300 7.43 [I H, d, CH=], 7.63-7.83 [7 H, m, =CH 3000-3200 (OH), 1554, 1355 and arom.], 8.23 [2 H, d, arom.], 9.27 [1 H, (NO,) s, NH], 10.77 [2 H, s, OH]6c 0.6 376 >300 3.88 [6 H, S, 20CH3], 7.37 [l H, d, CH=], 1550, 1345 (NO,) 7.47-7.97 [7 H, m, =CH and arom.], 8.17 [2 H, d, arom.], 10.02 [1 H, s, NH]6d 8.9 379 >300 3.33 [6 H, S, 2CH3], 7.40 [I H, d, CH=], 1540, 1340 (NO,) 7.53-8.00 [7 H, m, =CH and arom.], 8.27 [2 H, d, arom.], 10.20 [1 H, s, NH] 6e 6.5 393 >300 7.37 [2 H, d, CH=], 7.50-7.97 [14 H, m, =CH 1540, 1345(NO,) and arom.], 8.22 [4 H, d, arom.], 10.17 [2 H, s, NHI 6f 0 380 >300 3.34 [3 H,S, CH,], 7.37 [I H, d, CH=], 1540, 1345 (NO,) 7.50-8.00 [S H, m, =CH and arom.], 8.34 [4 H, m, arom.], 10.17 [2 H, s, NH] "Values relative to urea (standard) determined by the powder method.sesses the dicyanovinyl group,13 which is a strongly electron- withdrawing group. However, this compound is SHG-inactive. These are typical examples underlying a general understanding that the crystal structures cannot be designed only from pertinent molecular structure^.^^^ For the 4-ANS derivatives, their SHG activities reveal behaviour similar to that of the 4-NA derivatives. Both dichlorotriazine derivatives of 4-NA and 4-ANS (la and 6a) show high SHG activities while the monochlorotriazine derivatives (3a, b, d and 6b-d) show almost no net SHG activities. The dimethyltriazine of 4-ANS (6d) shows an SHG activity ca.nine times greater than that of urea. 6e appears to be of interest related with 'lambda (A)-shape' molecular which are proposed to be effective for the construction of non-centrosymmetric crystal structures. Although 6e reveals an SHG activity 6.5 times greater than that of urea, the similar A-shaped compound (6f) with the 4-ANS and MNA substituents shows no net SHG activity. A particularly interesting observation is that a crystal form (5a-1) obtained by recrystallization of 2-(4-nitroanilino)-4,6-diphenyl- 1,3,5-triazine (5a)from toluene shows the highest SHG activity of all the triazine compounds investigated, being 104 times more active than urea.Similarly, recrystallization of 5a from tetrahydrofuran gives a crystalline material of SHG-active form. In contrast, recrystallization of 5a from DMF gives another crystal form (5a-2), which shows no net SHG activity. In the latter, solvent molecules are incorporated in a 1 : 1 ratio by hydrogen bonding (vide infra). Similarly, 5a gives an SHG-inactive material upon recrystallization from aprotic polar solvents such as N-methylpyrrolidone, 1,3-dimethyl-2-imidazolidinone,ethyl acetate, acetone and 1,4-dioxane. With regard to the cut-off wavelength of optical absorption, note that the dichlorotriazine modification of 4-NA leads to a 47 nm blue shift of the optical absorption without a signifi- cant loss of net SHG activity.Similarly, all the triazine compounds containing the 4-NA chromophore show absorp- tion maxima that are shorter by 33-66nm than the parent 4-NA. In particular, the absorption maximum of If is shorter by ca. 100 nm than that of 4-NA, while maintaining a high SHG activity in the powder. For the 4-ANS chromophore, triazine derivatization also results in 10-35 nm blue shifts with moderately high SHG activities. Although absorption maxima in solution may be often different from those in the solid state, the reflectance spectrum of 5a-1 showed that the cut-off wavelength in powder is 425 nm, shorter by 52-53 nm than that of 4-NA (478 nm) or MNA (477nm). This is certainly beneficial for the generation of blue-green light. Another benefit in the triazine derivatization might be given by the relatively high melting points.For instance, the triazine compounds containing the 4-NA chromophore melt at tem- peratures higher by 47-163°C than 4-NA. Similarly, the melting points of 6a-f are higher by >55"C than that of 4-ANS. The high melting points of the triazine compounds are advantageous for the design of thermally stable SHG materials. Polymorphism and X-Ray Crystal Structure of 244-Nitroanilino)-4,6-diphenyl-l,3,5-triazine(5a) The triazine compound (5a) gave two different types of crystals depending on recrystallization solvents, in which one (5a-1 from toluene) is SHG-active but the other (5a-2 from DMF) is inactive. Table 5 lists the observed SHG intensities of 5a-1 and 5a-2 relative to that of urea.Moreover, note that the formation of the different crystals is not due to simple polymorphism but arises from the incorporation of solvent molecules (DMF) in a 1 :1 ratio in 5a-2, as confirmed by J. MATER. CHEM., 1994, VOL. 4 Fig. 1 Crystalline structure of 5a-2 elemental analysis, 'H NMR spectra for DM SO-d, solutions and thermogravimetric measurements. On the other hand, 5a-1 does not contain any solvent molecules. Annealing 5a-2 at 110°C for 3 h under vacuum gave another solid material (5a-3) containing no solvent molecules, which shows no SHG activity. We obtained a single crystal of SHG-inactive 5a-2, which was of good quality for X-ray crystallographic analysis. The molecular packing in a unit cell is shown in Fig.1. The molecular packing clearly shows that the two phenyl groups and the 4-nitroanilino moiety are almost coplanar with the triazine ring and that DMF molecules are incorporated in a 1 :1 ratio by forming hydrogen bonds between N(4)-H Ff 5a and the carbonyl oxygen of DMF with a distance of 1.95 A. The crystal data show that 5a-2 belongs to the space group P2,,, of a centrosymmetric structure (Table 7). As a conse- quence, the components of the molecular dipole moments along the c axis cancel. These results well agree with the lack of SHG activity of 5a-2 in microcrystalline powders. On the other hand, all attempts to obtain a single crystal of 5a-1 of sufficient quality for X-ray crystallographic analysis failed, although recrystallization from toluene gives 5a-1 as fine needles.Table 8 lists the selected d-spacing data and the Table 7 Structure details for 2-(4-nitroanilino)-4.6-diphenyl-1,3,5-triazine (5a-2) recrystallized from DMF chemical formula C2IH,,N,O2formula weight 369.38 colour colourless crystal system monoclinic sp!ce group p2,m44 14.464 (5)bib 10.659 (3) CIA 15.456 (3) Bldqgees 105.42 (2) V/A3 2297.3 (9) z 4 Dcalclg cm -1.068 p( Mo-Ka)/cm-0.67 ;./A 0.7 1069 reflections measured 5767 reflections used 1075 [l >3.00(1)] residuals R 0.052 Rw 0.057 b=4Fo/a2!Fo2)1crystal dimensions/mm 0.50 x 0.20 x 0.50 J. MATER. CHEM., 1994, VOL. 4 Table 8 Selected XRD data (d-spacing and relative intensities), for different crystalline samples of 2-(4-nitroanilino)-4,6-diphenyl-1,3,5-triazine (5a) 5a-1' 5a-2" 5a-3" d ,'A 1/10 d/A I/Io d/A III" 3.21 19 3.28 55 3.50 69 3.69 12 3.38 26 3.51 65 4.58 11 3.94 100 3.62 38 4.84 78 4.04 24 3.67 42 8.08 32 4.36 79 3.71 42 9.26 36 4.85 37 4.21 37 16.36 100 5.23 32 4.91 36 6.04 97 5.08 72 7.06 22 5.14 48 7.57 35 5.34 57 8.91 25 6.93 41 9.22 36 9.01 48 12.24 61 9.49 99 10.21 57 20.7 1 100 "5a-1 and 5a-2 indicate the samples recrystallized from toluene and DMF, respectively, and 5a-3 was obtained by annealing 5a-2 at 110 "C for 3 h under vacuum.relative intensities in XRD analysis of 5a-1-3. The d-spacing data of the crystals are substantially different from each other, indicating that they have different crystal structures. Presumably, the crystalline structure of SHG-active 5a-1 should be non-centrosymmetric. It is suggested that thermal liberation of solvent molecules from 5a-2 would cause a crystal change while retaining a centrosymmetric structure, but does not give 5a-1.Conclusions A series of 2-(4-nitroanilino)-l,3,5-triazinecompounds were prepared in order to investigate structure-activity relation-ships in SHG. The triazine compounds show various activities in powder SHG depending on the structures and have absorp- tion maxima at <350 nm, which are shorter by 30-50 nm than those of the parent nitroanilines. The melting points of these compounds are higher by 47-1 63 "C than 4-nitroaniline.2-(4-Nitroanilino)-4,6-diphenyl-1,3,5-triazine recrystallized from toluene (5a-1) is ca. 100 times more active in SHG than urea, an activity comparable with that of 2-methyl-4-nitro- aniline. Moreover, the absorption edge of this compound is <410 nm, much shorter than that of the parent compound, 4-nitroaniline. This is clearly advantageous for efficient gener- ation of 'blue light'. Another prominent property of this material is its high melting point (269 "C), implying ihermal stability towards laser-induced degradation of the crq'stalline structure by local heating. Recrystallization of 5a from DMF (5a-2) gives an SHG-inactive crystal, in which solvent mol- ecules are incorporated in a l : l ratio by hydrogen bonding.The crystal structure of 5a-2 was determined. We non intend to achieve the growth of a single crystal of 5a-1 large enough for preparation of an SHG device. Finally, note that the chlorotriazine compounds (la-f, 2a-e and 6a, e and f ) have reactive chlorine substituents which would be potentially accessible to further polymer derivatization of the t riazine-substituted chromophores. We gratefully acknowledge Miss Mieko Shimizu (KICR) for her assistance in measuring SHG powder intensities of the compounds. Thanks are due to Mr. Hiroshi Maki (DIC Central Research Laboratories) for carrying out thth X-ray crystallographic analysis of 5a-2. References 1 D. S. Chemla and J. Zyss, Nonlinear Optical Properties OJ Organic Molecules and Crystals, Academic Press, Orlando, 198 7, vol. 1, vol. 2, and references therein. 2 W. Tam, B. Guerin, J. C. Calabrese and S. H. Stevensox Chenz. Phys. Lett., 1989, 154, 93. 3 R. Takatsuka, T. Unishi and I. Honda, J. Polyni. Sci Poljm. Chem. Ed., 1977,15, 1785. 4 S. K. Kurtz and T. T. Perry, J. Appl. Phq's., 1968.39, 3798. 5 H. Yonehara, W-B. Kang and C. Pac, Nonlinear Opt., 199 3,4.357. 6 H. Yonehara, W-B. Kang, M. Shimizu, T. Kawara, C. Pac, Y. Tachikawa and H. Maki, Nonlinear Opt., 1992.2, 149. 7 H. Yonehara, Y. Tachikawa, C. Pac and H. Maki, Nonlinmr Opt., 1993,6, 51. 8 J. N. Ashley, H. J. Barber, A. J. Ewins, G. Newbery and A. D. H. Self, J. Chem. Soc., 1942, 103. 9 R. Hirt, H. Nidecker and R. Berchtold. Helc. Cltirn. Acia, 1950, 31, 1365. 10 J. R. Dudley, J. T. Thurston, F. C. Schaffer, D. Holm Hansen, C. J. Hull and P. Adams, J. Am. Chem. Soc., 1951,73, 2986. 11 C. C. Teng and A. F. Garito, Phys. Rec. B, 1983,28,6766 12 J. L. Oudar and D. S. Chemla. J. Chem. Phys., 1977,66,2664. 13 G. Mignani, A. Kreamer, G. Puccetti, I. Ledoux, G. SouLL. J. Zyss and R. Meyrueix, Organometallics, 1990,9,2640. 14 H. Yamamoto, T. Hosomi, T. Watanabe and S. Miyata, I. Chem. Soc. Jpn., Chemistry and Industrial Chemistry, 1990, 789. Paper 4/02312I; Received 19th Apd, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401571

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( lo), 1579-1580 Structure of LiN(CF3S0J2, a Novel Salt for Electrochemistry Jan L. Nowinski, Philip Lightfoot* and Peter G. Bruce Department of Chemistry, University of St Andrews, St Andrews, Fife, UK KY16 9ST The crystal structure of lithium bis(trifIuoromethanesu1fonimide) [LiN(CF,S02)J has been solved from X-ray powder diffraction data and- refined by the Rietveld method: The salt crystallises in the orthorhombic system, space group Pnaa, a =9.6351 (2) A, b =5.41 54(1) A, c =16.2639(3)A. The anion, previously uncharacterised crystallographically, has two-fold symmetry around the nitrogen atom, with short [1.56(1) A] S-N distances and a large S-N-S angle [1129(1)"]. This is consistent with other similar salts and suggests a high degree of double-bond character and delocalisation around N.Li + is tetrahedrally coordinated by four oxygens from four neighbouring anions. The development of rechargeable lithium batteries, smart windows and electrochromic display devices is critically dependent on finding suitable liquid or solid polymer electro- lytes. The electrolytes in turn consist of a lithium salt dissolved in a non-aqueous liquid solvent or a coordinating polymer such as a polyether. Recently a new inorganic lithium salt, LiN(CF3S02)2, lithium bis(trifluoromethanesulfonimide), LiTFSI, has been synthesized. It yields significantly higher conductivities in both liquid and solid-polymer solvents com- pared with the previous maximum values from salts such as LiC104, LiCF,S03 and LiPF,.Dissolved in poly(ethy1ene oxide) it is believed to be the best candidate for use in practical devices employing a solid polymer electrolyte.' One of our interests in this field is to determine the crystal structures of these complexes which, since single-crystal specimens are generally unavailable, has been achieved by X-ray powder It is of fundamental interest also to investigate the structural properties of the salts themselves in order to understand fully the solid-state properties of the complexes. To this end we have recently determined the structure of LiCF,SO,.' Given the very considerable interest in LiN(CF,S02)2 in both liquid and solid-polymer electro-chemistry, we have determined the crystal structure of this anhydrous salt which, again due to the lack of single crystals, has been solved by the newly emerging powder diffraction method.5.6 + Structure Determination The sample of anhydrous LiN(CF,S02)2 was obtained from 3M.X-Ray powder diffraction data were collected on a Stoe STADI/P diffractometer operating in transmission niode. A powdered sample was mounted in a 0.5 mm glass capillary and data were collected over the range 5"<28<85' using Cu-K,, radiation. The powder pattern was indexed uing the program TREOR.7 Owing to a chance relationship hetween the unit-cell parameters (cz 3b), unambiguous space group determination at this stage proved impossible due to system- atic overlap of non-equivalent reflections. In particular, absences in the reflection class hkO could not be detrtrmined with certainty and unsuccessful attempts to solve the si ructure were made in Pnam before Pnaa was tried successfully. Data suitable for structure solution were extracted from the powder pattern by a structure-independent profile fitting routine' using the program GSAS.' This procedure led to 189 indepen- dent reflections which were used as input to the direct methods program SIR88.I' The position of the S atom was dettrrmined from this.All remaining atoms were found by successive difference Fourier and Rietveld refinement cycles. Final con- vergence was achieved at R,, =6.7%, x2 = 1.50,for 46 variables and 309 reflections spread over 4000 data points Fig. 1). Isotropic thermal parameters were assigned to all atoms, with that for Li being fixed.-0.2-I I I I I I I II Fig. 1 Final Rietveld fit for LiN(CF,SO,),. (+) Raw data; (-) calculated model. The difference curve is shown below Table 1 Final refined atomic parameters tor LiN(CF,S02)?, space group Pnaa, a=9.6351(2) A, b=5.4154(1) A, c=16.2639(3) A atom Y J' Z u(iso)/A2 S N C 0(2) F( 1) F(2) Li O(1) F(3) 0.0811(2) 0.01 lO(9) 0.0949( 9) 0.2209 (4) 0.1455(4) 0.1740(4) -0.1490(4) -0.0180(4) -0.0291(4) 0.0692( 4) 0.25 0.2501(14) -0.1240(9) -0.0032 (7) 0.1079(8) 0.4428( 6) 0.33 10( 6) -0.25 0.1883( 2) 0.25 0.0884(5) 0.1690(3) 0.2052(2) 0.0346(3) 0.1040( 3) 0.0707( 3) 0.25 0.008(1) 0.006( 3) 0.025(4) 0.01 3( 2) 0.017(2) 0.041(2) 0.038(2) 0.035(2) 0.025 Table 2 Selected bond distances (/A)and angles (/degrees) s-O( 1) 1.451 (4) C-F( 1) 1.264(8) s-O(2) 1.429 (4) C-F( 2) 1.316( 9) S-N 1.55 7 (4) C-F( 3) 1.305( 9) s-c 1.901 (7) Li-O( 1)x 2 1.95(1) Li-0 (2) x 2 1.97(1) O(1)-S-0(2) 118(1) O(1)-Li-O( 1) 99(1)0(1)-S-N 108( 1) O(1)-Li-0(2) x 2 114(2) O(1)-s-c 104(1) O( 1)-Li-O(2') x 2 115(2) 0(2)-S-N 117(1) O(1)-Li-O( 2) lOl(1) O(2)-s-c 104(1) S-N-S 129( 1) N-S-C 105( 1) F( 1)-C-F( 2) 113(1) F(2)-C-F( 3) 108(1) F(1 )-C-F( 3) 114( 1) F( 2)-C-S 107(1) F( 1)-C-S 108( 1) F(3)-C-S 107(1) n Fig.2 The (CF,S02)N- ion viewed approximately down the two- fold axis at N, showing a transoid conformation Results and Discussion Final refined atomic coordinates and thermal parameters are given in Table 1 and selected bond distances and angles in Table 2.The anion lies on a two-fold axis centred on N and parallel to the a axis, with a transoid conformation of the two CF, groups with respect to the S-N-S group when viewed down the two-fold axis (Fig. 2). This is the first crystallo- graphic determination of the structure of this anion. Th,e S-N-S angle of 129( 1)" and short S-N distance of 1.56 A (compare the average value of 1.64A in the RSO,NR', moeity'') reflect the significant double-bond character of the S-N bond. The negative charge of the anion is not located on the N alone but is delocalised via the double bonds onto the neighbouring SO, moeities. The structure of the (CF3S02)2N-anion may be compared with related anions (CF,SO,),CH-,12 (CF,SO,),C-,I3 (CH,SO,),N-l4 and RS02NC1-15.Bond lengths and angles are in excellent agree- ment with those reported in these studies; the corresponding S-No distance and S-N-S angle in (CH,SO,),N-are 1.59 A and 122", respectively. As observed in the case of RS0,NCl-,15 it is found that 0 rather than N is the coordinat- J. MATER. CHEM., 1994. VOL. 4 0 Fig.3 Unit-cell packing of LiN(CF3S02)2 viewed down the b axis. Each Li is coordinated tetrahedrally by four oxygens from four different neighbouring anions, two above and two below ing atom of the anion. Hence, Li is coordinated in a tetrahedral manner by four oxygens from four different anions. A view of the unit-cell packing is shown in Fig. 3. Isolated chains of anions held together by intervening cations run parallel to the b axis.The transoid conformation of the anion is in contrast to that found for the anion in Rb(C'F,S02),CH,12 which adopts a cisoid conformation in order to act as a bidentate ligand to Rb' . The authors are grateful to the 3M company for supplying the salt. P.G.B. thanks the Royal Society for the award of a Pickering Research Fellowship. References P. G. Bruce and C. A. Vincent, J. Chem. Soc., Faruduy Trans., 1993,89,3187. P. Lightfoot, M. A. Mehta and P. G. Bruce, J. Mdrr. Chern., 1992, 2, 379. P. Lightfoot, M. A. Mehta and P. G. Bruce, Science. 1993, 262, 883. P. Lightfoot, J. L. Nowinski and P. G. Bruce, J Am. Chetn. Soc., in the press. M. Tremayne, P. Lightfoot, M.A. Mehta. P. G. Bruce, K. D. M. Harris, K. Shankland, C. J. Gilmore and G. Bricogne, J. Solid State Chem., 1992, 100, 191. P. Lightfoot, C. Glidewell and P. G. Bruce, J. hfliter. Chem., 1992, 2, 361. P.-E. Werner, L. Eriksson and M. Westdahl, J. Appl. Crystallogr., 1985, 18, 360. A. Le Bail, H. Duroy and J. L. Fourquet, Mater. Res. Bull., 1988, 23,447. A. C. Larson and R. B. Von Dreele, Los Alamos Laboratory Report No. LA-UR-86-748,1987. M. C. Burla, M. Camalli, G. Cascarano, G. Giacovazzo, G. Polidori, R. Spagna and D. Viterbo, J. Appl. C'rystullogr., 1989, 22, 389. 11 F. H. Allen, 0.Kennard, D. G. Watson, L. Brammer, A. G. Orpen and R. Taylor, J. Chem. SOC.,Perkin Trans 2, 1987, S1. 12 K. T. Davoy, T. Gramstad and S. Husebye, Actn Chem. Srand., Sect. A, 1979,33, 359. 13 L. Turowsky and K. Seppelt, Inorg. Chem., 1988,27,2135. 14 A. Blaschette, D. Schomburg and E. Z. Wieland, Anorg. Mg. Chem., 1989,571,75. 15 M. M. Olmstead and P. P. Power, Inorg. Chem., 1986,25.4057. Paper 4/03568B; Received 13th June, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401579

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(10), 1581-1584 Preparation of Gold-dispersed Vanadium Oxide Thin Films by an Alternate Spin-coating Method for Electrochromic Applications Katsumi Nagase, Seigo Izaki, Youichi Shimizu,+ Norio Miura and Noboru Yamazoe* Department of Materials Science and Technology, Graduate School of Engineering Sciences, Kyushu University, Kasuga-shi, Fukuoka 876, Japan Thin films of V205 with dispersed gold particles were prepared from ethanol solutions of vanadyl isopropoxide and HAuCI, by means of an alternate spin-coating method. The Au content was maintained at 12-36 atom% by selection of the appropriate concentration of the alkoxide solution. After calcination at 400 "C,the mean diameter of dispersed Au particles was 7.0 nm, as observed for the film containing 24 atom% Au.Violet (cathodic)-green (anodic) electrochro- mism was exhibited by the Au (24 atom%)-V,O, film. Owing to the plasma resonance of the Au particles, the film had a strong optical absorption band centred at 570 and 61 0 nm under cathodic and anodic polarization, respectively. Electrochromism has been observed in a number of inorganic and organic materials, among which transition-metal oxides such as W03,1-5M003,6,7Ti02,8,9 NiOx,'*,'' IrOx12313and COO,'^-'^^: appear to be promising from the viewpoint of long-term stability. However, these oxides cannot give a coloration other than blue, brown or grey, with the exception of V20j16-20 which shows blue-green-yellow multichro-mism. Obtaining new colorations, especially red, is an import- ant aim in electrochromic research.So far investigations have been from two directions. One is the use of oxide-oxide composite systems. New reddish colours have been reported with CuOx-W03 ,21 v205-wo322 and V205-Ti02.23 The other is to utilize the coloration due to the plasma resonance absorption of ultrafine particles of metals such as Au, Ag and Cu, as observed with the oxide- or fluoride-based dispersions prepared by e~aporation,~~sputtering25 or the sol-gel An Au-WO, film has been reported to show a reddish coloration when polarized ~athodically.~~"' We are particularly interested in V205-based composites because of the multichromic nature of V205. We reported previously that the Au-V205 thin films, prepared by an evaporation method, show a new electrochromism of reddish ~iolet-green.~~ Subsequently we found that such an Au-dispersed film could be fabricated by a wet process using an alternate spin-coating technique.This paper deals with the new fabrication process and the electrochromic properties of the Au-V20, thin films thus obtained. Experimenta1 Gold-dispersed V205 thin films were prepared by means of an alternate spin-coating method shown in Fig. 1. The starting reagents, vanadyl isopropoxide [VO(OPr'), , High Purity Chemicals Co. Ltd.] and hydrogen tetrachloroaurate(rI1) (HAuCl,.4H20, Kishida Chemicals) were dissolved separately in ethanol to concentrations of 0.1-0.4 and 0.02mol dm-3, respectively. The vanadyl alkoxide solution was first spin- coated onto an indium tin oxide (ITO) glass (20 mm square, 10 s1 per square, Kinoene Kougaku Kogyo), at 3000 rpm at room temperature in a dry box. The coating layer was dried at room temperature for 10 min, the aurate solution was then spin-coated onto it under the same conditions.Then, the deposit was calcined in air at 400 "C for 30 min, in order to decompose the vanadyl alkoxide to crystalline V205 and the f Present address: Department of Chemistry, Faculty of Engineering, Kyushu Institute of Technology, Kitakyushu-shi, Fukuoka 804, Japan.1It has been reported that electrochromic cobalt oxide switches between olive green and brown in colour. VO(OPS), + C2H50H (Room temp.) HAuC14~4H20+ C2HSOH Drying(Room temp.) (400"C) VO(OPi), + CPH50H Calcination (400"C)dhAu-V,05 thin film Fig.1 Alternate spin-coating method for the preparation of Au-V,O, films HAuC14-4H20 to metallic Au. The latter decomposition takes place at 3300 0C.26 These serial procedures of spin-coating, drying and calcination were repeated 10 times. Filially the vanadyl alkoxide was spin-coated on top of the slack and calcined at 400°C. The total thickness of the films obtained was in the range of 150-600 nm as estimated from scanning electron microscopy (SEM, JEOL, JSM-840F). The composi- tions of the films were determined by electron probe micro- analysis (EPMA, JEOL, JXA-8621SX/MX), while the structure and morphology were examined by X-ray diffraction analysis (XRD, Rigaku Denki 4011)and transmission electron microscopy (TEM, JEOL, JEM 2000 FX).For electrochemical measurements, each Au-V205 film on IT0 was paired with a Pt counter electrode in a 1 mol dmP3 LiCIO,-propylene carbonate solution, with its potential being referred to a saturated calomel electrode (SCE). Cyclic voltammetry was performed with a potentiostat (Hokuto Denko, HA-303) and a function generator (Hokuto Denko, HB-105). Optical absorption was measured in situ on a spectrophotometer (Hitachi, 200-10, 330) in the wavelength range 400-800 nm. Results and Discussion Deposition of Au-V,O, Thin Films It was reported that a sol-gel process could be applied to the preparation of noble metal-silicate composite^.^^-^^ In this case, the starting reagents silicon alkoxide and noble-metal chlorides, were mixed in the same coating solution.In the present case, however, it was not possible to prepare such a coating solution because the vanadyl alkoxide was not suffic- iently stable in the presence of the aurate. This is why the solutions of vanadyl alkoxide and HAuC1,-4H20 were spin- coated alternately. Attempts to stack a sufficient number of alternate spin-coated layers without the calcination step in between were unsuccessful. The vanadyl alkoxide solution could not be spin-coated well onto the uncalcined aurate layer. Moreover, the whole stack turned strongly opaque after the final calcination. Thus calcination was introduced after each step of the aurate spin-coating.The concentration of VO(OPri)3 in the coating solution was varied, using a fixed aurate concentration of 0.02mol dmP3. Fig. 2 shows the relationship between the Au content (or V:Au ratio) in the composite films and the VO(OPr'), concentration. The Au content (atom%) decreased and the V :Au atomic ratio increased linearly with alkoxide concen- tration. This shows that the composition of the Au-V20, thin film can be controlled well by the concentration of the starting vanadyl alkoxide solution. The total thickness of the com- posite film containing 24 atom% Au was 270 nm as estimated from the SEM photograph of its cross-section (Fig. 3). Characterization of Deposited Films Fig. 4 shows XRD patterns of three composite films having different Au contents after calcination at 400°C. The (001) diffraction peak of V205 was very strong for the Au (12 atom%)-V,O, film, (a), suggesting that the crystal orien- tation was along the c axis perpendicular to the IT0 substrate.The (001) peak intensity was drastically weakened for the 30t \ concentration of VO(0Pr')dmol dm-3 Fig. 2 Au content and V:Au ratio of the composite thin film as a function of VO(OPr'), concentration in the coating solution (0.02 mol dmP3 HAuCI,) J. MATER. CHEM., 1994, VOL. 4 Fig.3 SEM photograph of a cross-section of an Au (24 atom%)-V,O, film calcined at 400 'C I 1-110 20 30 40 50 2fYdegrees Fig.4 XRD patterns of various Au-V,O, films calcined at 400°C: (a) 12, (b)24, (c) 36 atom% Au films containing 24 and 36 atom% Au, (h) and (c).The presence of crystalline Au was obvious from its (111) and (200) diffraction peaks. The composite films were subjected to TEM observation to investigate the state of Au particle dispersion. Fig. 5 (a) shows such a TEM photograph for the Au (24 atom%)-V,O, film after calcination at 400-C. Tiny, nearly spherical Au particles were dispersed well in the matrix of V205. The diameter of the Au particles showed a distri- bution [Fig. 5(b)],from which the mean diameter was esti- mated to be 7.0nm. The optical absorption spectra of pure and Au-dispersed V205 films and Au-V,O, composite films are shown in Fig. 6. The Au-containing films exhibit a strong ab5orption band centred at 618-638 nm ascribable to the plasma resonance of ultrafine Au particles, as was observed in the Au-V20, films prepared by the evaporation method.The peak position shifted to longer wavelength with increasing Au content in accordance with the dielectric theory of Ma~well-Garnett~~ for an insulating compound in which metal particles are dispersed. The Au-related absorption band became markedly broader and weaker as the Au content increased from 30 to 36 atom%, probably due to the grain growth of Au particles. From these results, the optimum Au content was estimated to be 20-30 atom%. Electrochromism of Au-V,O, Thin Films Fig. 7 shows the cyclic voltammograms (CV) for the thin films of pure V,O, (a) and Au (24 atom%~)-V,O, (6). Unlike other V205 films that have been investigated, the pure V,O, film prepared by the present method showed a rather broad CV curve in the potential range between -0.8 V and + 1.2 V us.SCE, the appearance of which is attributed to the lower degree of crystallization of the pure V205 films obtained here. A very J. MATER. CHEM., 1994, VOL. 4 (a1 H20nm (b) r 1 -8 20-v c .-0 c mf 10-a,Ll5 0 0 4 a 12 16 particle diameterhm Fig. 5 TEM photograph of Au particles (u) and their size distri-bution (bl for the Au (24 atom%-V,O, film calcined at 400 "C 2.0 a, C m 42 1.05:LI m --___---0-400 5b0 660 760 8bO ?Jnm Fig. 6 Optical absorption spectra of various Au-V,O, films calcined at 400'C: (a) pure V,O,, (b) 12, (c) 24, (d) 36 atom% similar CV curve was exhibited by the Au-V205 composite film, indicating that the Au-dispersed film undergoes essen-tially the same reaction as V205.Nevertheless, the electrochro-mism of the composite film was totally different from that of the pure V205 film.The film, which was green in the oxidized state, was violet when it had been polarized cathodically. The electrochromism was completely reversible and could be repeated with great stability. Fig. 8 shows the optical absorption spectra of the pure V205 film (a) and the Au (24 atom%-V205 film (b) each being polarized anodically and cathodically. For the pure V205 film, with a change in polarization from +1.2V to -0.4 V, the absorption edge shifted from ca. 500 nm to ca.400nm together with a slight increase in optical absorbance over a wide wavelength range above ca. 500nm. In the case of the Au-V,05 film, on the other hand, the absorption band ascribed to the dispersed Au particles shifted from 610nm -0.8 EN vs. SCE blue green'-2.4 N green -0 EN vs. SCE Fig. 7 Cyclic voltammograms of pure V205 film (u) .tnd Au (24 atom%-V,O, film (b) calcined at 400 "C 1.o 0.5 0 1 I I J I a 400 600 800 0c (d 425: a 2.0 m 1.O I I I I 400 600 800 Unm Fig. 8 Optical absorption spectra of pure V,O, film (a) and Au (24 atom%-V,O, film (b) calcined at 400 "C under cathodic and anodic polarization (+1.2V) to 570nm (-0.8V), together with a shift of the absorption edge of V205.This indicates that the new violet coloration is associated with a blue shift of both the plasma resonance absorption band and the absorption edge of V205 on cathodic polarization. It has been suggested that the reddish colour of Au -W0330 films on cathodic polarization results from a changt:: in the effective relative permittivity of metal grains surrounded by a cloud of protons. A similar explanation may be possible for the blue shift of the plasma resonance absorption observed in the present system. References 1 S. K. Deb, Philos. Mug., 1973,27, 801. 2 B. W. Faughnan, R. S. Crandall and M. A. Lampert, Appl. Phys. Lett., 1975, 27, 275. 3 0.F. Schimer, V. Witter, G. Baur and G. Brandt, J. Ele&rochem. SOC.,1977,124, 749.1584 J. MATER. CHEM., 1994, VOL. 4 4 T. Yoshimura, J. Appl. Phys., 1985,57, 911. 20 K. Nagase, Y. Shimizu, N. Miura and N. Yamazoe, Appl. Phys. 5 6 K. Yamanaka, H. Okamoto, H. Kidou and T. Kudo, Jpn. J. Appl. Phys., 1986, 25, 1420. 0. Z. Angel, C. Menezes, F. S. Cinencio and G. F. L. Ferreira, 21 Lett., 1992,61, 243. H. Suiyang, Z. Jikai and C. Jinyi, Proc. SPIE-1987,823,159. int. Soc. Opt. Eng., J. Appl. Phys., 1980,51, 6022. 22 S. Saito and Y. Seino, J. Electron. Commun. Soc . 1982, J65-C, 629 7 N. Baba, S. Morisaki and N. Nishiyama, Jpn. J. Appl. Phys., 1984, 23. 638. 23 (in Japanese). K. Nagase, Y. Shimizu, N. Miura and N. Yamazoe, Appl. Phys. 8 T. Ozuku and T. Hirai, Electrochim. Actu, 1982, 27, 1263. Lett., 1992,61, 243.9 M. Nabavi, S. Doeuff, C. Sanchez and J. Livage, Muter. Sci. Eng. 24 T. Yamaguchi, M. Sakai and N. Saito, Phys. Reu. B. 1985, 32, B, 1989,3.203. 2126. 10 M. Fantini and A. Gorenstein, Sol. Energy Muter., 1987,16,487. 25 R. W. Cohen, G. D. Cody, M. D. Coutts and B. Abeles, Phys. Rec. 11 S. I. Coldoba-Torresi. A. H. Goff and S. Joiret, J. Electrochem. B, 1973,8,3689. Soc., 1991, 138, 1554. 26 M. Ohtaki, Y. Ohsihima, K. Eguchi and H. .4rai, Chem. Lett., 12 S. Gottesfeld, J. D. E. McIntyre, G. Beni and J. L. Shay, Appl. 1992,1992,2201. 13 Phjx Lett., 1978, 33, 208. Y. Sato, K. Ono, T. Kobayashi, H. Wakabayashi and 27 J. Matsuoka, R. Mizutani, S. Kaneko, H. Nasu, K. Kamiya, K. Kadono, T. Sakaguchi and M. Miya, J. Ceram.Soc. Jpn., H. Yamanaka, J. Electrochem. Soc., 1987,134,570. (Japan), 1993, 101, 53. 14 L. D. Burke and 0.J. Murphy, J. Electroanul. Chem., 1980, 112, 379. 28 29 H. Kozuka and S. Sakka, Chem. Muter.. 1993,s. 222. E. K. Sichel and J. I. Gitteleman and J. Zeiez. Appl. Phy~. Lett., 15 C. N. Polo da Fonseca, M-A. De Paoli and A. Gorenstein, Adv. 1977,31, 109. Muter., 1991,3, 553. 30 E. K. Sichel and J. I. Gitteleman, J. Electron. Muter., 1979, 8, 1. 16 17 S. F. Cogan, N. M. Nguyen, S. J. Perrotti and D. Rauh, J. Appl. Phys.. 1989,66, 1333. A. M. Andersson, C. G. Granqvist and J. R. Stevens, Appl. Opt., 1989,28,3295. 31 32 P. V. Ashrit, G. Bader, F. E. Girouard, Von-Van Tuong and T. Yamaguchi, Physica A (Amsterdam), 1989,157,333, K. Nagase, Y. Shimizu, N. Miura and N. Yamazoe, Appl. Phys. Lett., in the press. 18 Y. Shimizu. K. Nagase, N. Miura and N. Yamazoe, Jpn. J. Appl. Phys., 1990, 29, L1708. 33 J. C. Maxwell-Garnett, Philos. Trans. R. SOC.London, 1904, 203, 385. 19 Y. Shimizu, K. Nagase, N. Miura and N. Yamazoe, Solid State Ionics, 1992,53-56,490. Paper 4/01470G; Received 14th March, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401581

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( lo), 1585-1589 YBCO and BSCCO Thin Films Prepared by Wet MOCVD 0. Yu. Gorbenko," V. N. Fuflyigin,"* Yu. Yu. Erokhin," I. E. Graboy," A. R. Kaul," Yu. D. Tretyakov," G. Wahlb and L. Klippeb" Chemistry Department, Moscow State University, 119899 Moscow, Russian Federation IOPW, Technische Universitaet Braunsch weig, Braunschweig, Bienroder Weg 53, Germany The newly developed technique of low-pressure single aerosol source MOCVD (wet MOCVD) has been applied to prepare thin films of YBa,Cu,O, and Bi,Sr,CaCu,O,. The influence of the deposition rate, po,-T conditions, film stoichiometry on the phase composition, orientation and superconducting properties of the films was studied and compared for both systems. Thin films of YBa,Cu,O, with high-Tc(end) =92 K,jc(77 K) =1.7 x 1O6 A cm -2; Bi,Sr,CtaCu,O, with Tc(end) =84 K, jc(77K) =1 x 1O4 A cm -2 were deposited.On the basis of the crystal chemistry approach the conditions for the various orientations were established. MOCVD is recognized as a good technique for the deposition of high-T, and j, superconducting films of YBa2Cu30, and Bi,Sr,Ca, -lCu,O, (n=2,3).'-7 One drawback of the technique is the precise control of the film stoichiometry. In the case of MOCVD with individual sources of precursor vapour a small change of the source temperature or gas flow through the source leads to a considerable change in the film stoichiometry and morphology, leading to the deterioration of the supercon- ducting properties of the film^.^.^ For the last three years the problem promoted a search for various single-source MOCVD technique^.'^^'^ Wet MOCVD'2-14 involves deposition from vapour precursors produced by the evaporation of an organic solution nebulized in a carrier gas flow.This method was found to combine the reproducibility of the vapour composi- tion with the flexibility of composition control. The mechan- ism of the process in comparison with conventional MOCVD is complicated by the influence of the solvent vapour.15 Combustion of the solvent vapour can lead to an indefinite increase in the temperature in the deposition zone of the reactor and to formation of carbon dioxide and water vapour. In this connection the adjustment of the deposition conditions to the area of the thermodynamic stability of the supercon- ducting phases is necessary, taking into consideration the partial pressure of carbon dioxide and water vapour.16 The goals of this paper are to study the features of YBCO and BSCCO thin films deposited by wet MOCVD and to compare the characteristics of the deposition process for both systems.Experimental The experimental set-up is shown in Fig. 1. An ultrasonic nebulizer was used to produce an aerosol of the organic solution with a mean dimension of the droplets <5 pm. The aerosol was transported by the carrier gas to the heated zone where the evaporation of the solvent and precursors took <reservoir ?atedpipes T = 270 "C ultrasonic source water Fig. 1 CVD system with ultrasonic evaporator place.Vapours of the precursors thus obtained reached the substrate where a film was formed. A cold-wall reactor with a radiofrequency heater on the substrate was used to niinimise the combustion of the solvent.16 itPreviou~ly,'~*~~ was shown that diglyme (CH,0CH2CH20CH2CH20CH,) is an appropriate solvent for this technique due to (1) its rather high boiling point (162"C), (2) its low viscosity and (3) the high solubility (0.1 mol-l) of thd chelates (thd =2,2,6,6-tetramethyl-heptanedion-3,5-ate) in it. Moreover diglyme forms adducts with the less volatile thd complexes of the alkaline-earth metals.17 This is the probable reason for the observed higher stability of the diglyme solutions than solutions in alkanes.The temperature of the transport line where the evaporation of the solvent and precursors takes place must be in tfie range 240-270 "C: at higher temperatures decomposition of the precursors was observed; at lower temperatures it was rmposs- ible to achieve complete evaporation of the precursors at the nebulization rate used. Parameters for the process are summa- rized in Table 1. A mobile mass spectrometer system MSQlOOl was used to measure the composition of the gas phase during thci depos- ition run. The effects of the total pressure, deposition tccmpera- ture, solution and O2 feeding and variation of solvent were studied in separate deposition runs. XRD patterns of the films were measured with a Siemens D5000 four-circle diffractometer.SEM was accomplished with a CAMSCAN4 equipped with EDX and WDX systems for microprobe analysis. The stoichiometry of the films was determined with a precision of at least 5 mol% for each metallic component. Ac magnetic susceptibility and screening measurements were used to determine T, andj,." Table 1 Parameters of the deposition process parameter deposition temp./"C total pressure/mbar partial pressure of oxygenjmbar total gas flow/l h-' concentration of solution/molI-single-crystal substrate deposition rate/pm h-' solution feeding rate/ml h-' YBCO BSCCO 700-820 700-830 5-30 15-35 0.1-5 5-25 20-40 0.01-0.1 (lOO)YSZ, (100)MgO. (100)SrTi03, (100)LaA103, (100)NdGa03 0.25-5 0.25-3 3-10 YSZ, yttria-stabilized zirconia (Zro,83Yo,l,02~y).Results and Discussion Deposition Rate and po,-T Conditions A high deposition rate is known to be very important for the technology of HTSC thin films. Wet MOCVD easily permits the attainment of higher deposition rates (up to 10 pm min-') than conventional CVD since a much higher evaporation rate of the precursors can be achieved. However, the rate of the superconducting phase formation can be lower than the deposition rate. This is essential for BSCCO films since the kinetics of 2212 and 2223 phase formation are extremely slow, as was found to be the main problem for the CVD of BSCC0.7 Nearly stoichiometric Y 123 thin films deposited at 700-820°C with rates up to 3 pm h-' contained only the HTSC phase according to XRD data.A deposition rate of < 1 pm h-' and a temperature of 750-830 "C were necessary to prepare 2212 thin films with T, values of 75-84K. The increase of the deposition rate up to 2.5-3 pm h-' resulted in the formation of films containing both 2201 and 2212 phases. If the deposition rate is too high a layer containing basically the 2201 phase grows from the vapour phase. One can suggest the following sequence of phase transformations in this system: vapour--$ Bi,( Sr,Ca),CuO, -+ Bi,Sr,CaCu,O, -+Bi2Sr2Ca2Cu,0, (2201 phase) (2212 phase) (2223 phase) The po,-T conditions determine the range over which super- conducting phases exist.lg po,-T diagrams for the pure HTSC phases are shown in Fig. 2. One can see that the upper temperature of 2212 phase existence is significantly lower than that of the Y123 phase at the same po,.So the possibility of varying the deposition temperature for bismuth-containing superconductors is more limited than for the Y123 phase because a high rate of phase formation should be also provided. 0.5-0.8 pm 2212 films with T,(end)=75-84 K were deposited at 750-820°C with a rate of 0.5-0.8 pm h-l. At the lower temperature it was impossible to deposit single- phase films at such a rate. Films thus obtained always contained an admixture of the 2201 phase. Note that epitaxial Y123 thin films were deposited by conventional MOCVD under po,T conditions close to the line of the Cu,O-CuO equilibrium.20A higher partial pressure of oxygen was necessary for thin films of the Y123 phase to be obtained on (1OO)YSZ with the highest c texture by wet MOCVD.The corresponding po,-T conditions are marked by asterisks in Fig. 2. The conditions for epitaxial growth of 2212 films lay in the range of higher partial pressures of oxygen (Fig. 2). The deposition in the range close to the line of the Cu,O-CuO 2DF 2212-\\ +++ -3.0 I I I I I I 7 8 9 10 11 12 13 lo4 KIT Fig. 2 po, z's. T diagrams for Y123 and 2212 phases. *, Conditions of epitaxial growth of the Y123 phase by wet MOCVD; +, conditions of epitaxial growth of the 2212 phase by wet MOCVD. J. MATER. CHEW, 1994, VOL. 4 equilibrium resulted in films with worse superconducting properties, e.g. a film deposited at 800 'C and po2=8 mbar had T,=65 K (deposition rate=0.8 pm h-I) and the optimal conditions for the growth of the epitaxial 2212 films are po2 = 15-25 mbar, Td=780-820 "C.A possible reason for this is the inevitable deviation of the film cation stoichiometry from the 2212 composition that reduces the temperature of the decomposition according to the data of ref. 19 and 21. For example at po, =0.21 bar the stoichiometric 2212 phase decomposes at 875-880 "C; however, a small excess of copper and calcium reduces the temperature of appearance of the liquid phase to 857 "C. Stoichiometry The stoichiometry of the films is a very important factor for both systems.20-21 The stoichiometry determines the phase composition and as a consequence the superconducting properties of the films, the critical current density being the most sensitive parameter.The basic dependences found in the conventional MOCVD of Y123 are also valid for wet MOCVD. Ba enrichment resulted in drastic deterioration of the film texture and a decrease in T, and j,." A small Cu enrichment (up to Y :Ba :Cu =1: 2 :4) leads to enhancement of the c texture and a higher j, [j,(77 K)= 1 x lo6 A cm-,] with T, being nearly constant and equal to 90 92 K. The stoichiometry effects for BSCCO MOCVD have not been studied previously. We found that an excess of Bi and alkaline-earth elements (Bi,+,Sr,Ca1+,Cu,0~,, x =0.1, 0.15) led to the formation of non-textured thin films with low T, values (20-40K). Films of the 2212 phase on (100) SrTiO, with the best properties [T,=80-84 K, jJ77 K) up to 1 x lo4 A ern-,] contained an excess of calcium and copper.The appearance of the liquid phase is considered to promote the formation of superconducting phases.,, Nevertheless, the influence of the liquid phase on the properties of HTSC thin films is not simple. Crystal growth is usually accelerated in these films even if the liquid phase fraction is small. On the other hand, aggregation of secondary phases, chemical inter- action with the substrate and formation of drops which solidify separately under cooling (Fig. 3) occur if the fraction of the liquid is high. This results in a poor film morphology and a low transition temperature. In this connection it is interesting to compare the samples with an excess of Bi and with an excess of Cu and Ca.According to the phase diagram23 an excess of bismuth results in the formation of a greater amount of the liquid phase than an excess of calcium and copper. BSCCO thin films containing an excess of bismuth (Bi,~,Sr,CaCu,,,O,) had rather poor morphologies with traces 740 760 780 800 820 840 deposition ternperature/"C Fig. 3 Dependence of 2212 film orientation on the deposition temperature: intensity =Ix/(Z200+ I,,, + +Ioo8 +I,,,,), I, =I,oo (u), (1008f~oo,,) (61, (I,,, +1115) (4 J. MATER. CHEM.. 1994, VOL. 4 of melting. The T,s of these film were lower than those of films containing an excess of copper and calcium ( Bi,Srl~,,Ca,,,Cu,~,,O,). The latter also had a better mor- phology (Fig.3). Orientation of HTSC Films Anisotropy of the superconducting properties of HTSC films means that only films with definite types of orientation are of interest for applications. The orientation of the HTSC films was influenced by practically all the main parameters in the deposition process: deposition temperature, total pressure, oxygen partial pressure, rate of deposition, substrate material. The formation of films with a predominant a orientation at low deposition temperatures was observed for both superconducting phases studied (Fig. 4). The increase in the deposition temperature led to the growth of the contribution of the c orientation. At high deposition rates highly textured films could not be prepared.A decrease in the deposition rate promoted the formation of c-oriented films (Fig. 5). These results are in agreement with the fa~t~~*~~ that the c-orientation is therniodynamically preferred since the equilibrium Gibbs energy of formation of the c-oriented grains is greater than that of a-oriented grains. Formation of the a orientation is more favourable kin- etically for Y123 as well as for 2212 phases. According to the data obtained by RHEED26 chaotic distribution of metal (a1 H10pm H10pm Fig. 4 hlorphology of the BiHTSC 1 pm films: (a) Bi2.2Sr2CaCu2,20,, (h)Bi2Sr1.85C~1.3CU2.1S0~ @ 0.66 0 ,4 0 0.45 0.90 1.35 1.80 2.25 deposition rate/ym h-' Fig.5 Influence of the deposition rate on 2212 film Orientation: intensity=z~/('200+z113 +Ills f IOOS +IOOIO), zs=1200 (a),(I,13+1115) (4,(Zoos +I00,o) (c) atoms on the surface of the substrate takes place during the first moments of deposition, then the formation oj a two-dimensional net corresponding to the ac plane can be reached by smaller displacements of the atoms than the formlation of the two-dimensional net corresponding to the ab plane.This is due to the structure of facets: the ab plane is bu,ilt from atoms of only two elements, meanwhile the ac (hc) plane is built from atoms of three or four elements (Fig. 6). During the growth of the following layers the process is repeated. Additionally, one can easily see that the atomic layers which are parallel to the ab or ac planes are not electrically neutral.Thus the HTSC phase cannot grow in the c directicm layer- by-layer but by blocks containing neutral columns of atomic layers. For example, the thinnest neutral column has the height of the unit cell in the case of the Y123 phase.2s Such a column includes two layers for growth in the a directlion and six layers for growth in the c direction (Fig. 6). In thc case of the 2212 phase such piles would include two and seven atomic layers, respectively. The latter are too large to form easily. Thus at high deposition rates the growth of predomiriantly a-oriented films is preferred (Fig. 5). At the same time the necessary conditions for c-orientation are a low deposition rate and a high diffusion mobility of the components of the film.Generally higher mobility can be reached at ]he high temperatures, especially in the presence of a liquid phase. Even if no liquid phase forms, if the deposition temperature is closer to the liquid-phase appearance temperaturll: then a higher diffusion mobility is also achieved. This apprclach was applied earlier to obtain thin films of Y123,l where composi- tion deviations result in a change in the liquid-phase appear- ance temperature of ca. 50°C. The influence of the stoichiometry of the deposited films on the orientation of the films is also important. The \ ariation of stoichiometry which can promote the formation of the liquid phase resulted also in growth of the mainly c-oriented HTSC films. The presence of the liquid phase m'tkes the transport of the components in the film easier and r'avoured growth in the thermodynamically preferred orientation. It was established that an excess of copper increased the conuibution of the c orientation in comparison with stoichiomet ric films of Y123.The influence of the excess of copper may be seen from two viewpoints. First, the additional amount af copper can induce the formation of a non-equilibrium as well as an equilibrium liquid phase. Secondly, an excess of Cu, which is Considered to be the most mobile component in thi:, system, favours the formation of a two-dimensional lattice correspond- ing to the facet ab since this facet can be built only by copper and oxygen atoms. On the other hand, it was also found that an excess of J.MATER. CHEM., 1994, VOL. 4 vapour c orientation a orientation Fig.6 Formation of blocks corresponding to the different types of the films orientation. Positions of the Y123 phase unit cell relative to the substrate surface are shown. Oxygen atoms are omitted. copper or bismuth resulted in the growth of predominantly c-oriented 2212 films (Fig. 7); however, an excess of bismuth or alkaline-earth element favoured the a orientation. So the same approach can be applied to the consideration of the influence of the stoichiometry on the orientation of BSCCO thin films. Nevertheless, note that the role of the liquid phase is probably greater in this case than in the case of the Y123 film. This is because the liquid phase can be in equilibrium with the superconducting phase at the deposition temperature of BSCCO films.The orientation of the thin films is also affected by the crystal structure of the single-crystal substrate. The presence of nearly coincident site lattices is necessary for the appearance of a definite type of film orientation. In the case of the Y123 phase, c/3=b. So the better lattice match between the film and the substrate is for the c-orientation of Y123 phase and also favours the a orientation of this phase, but for 2212 and 2223 phases there is no such relation between c and a(b). Substrates with the lattice parameters close to a of the 2212 or 2223 (SrTi03) phases consequently favour only the c orientation of the film. In ref. 16 it was established that the influence of the solvent on the deposition process is versatile: (1) a thermal effect produced by solvent oxidation; (2) cracking of the solvent molecules leading to contamination by carbon; (3) an increase in CO, partial pressure; (4)a decrease of 0, partial pressure; (5) adsorption of solvent molecules on the film surface.Processes (1)-(4) are very important for wet MOCVD in the hot-wall reactor; however, for deposition in the cold-wall reactor, intensive oxidation of the solvent was not observed according to mass-spectrometric measurements (Fig. 8). 1.1 1.2 1.3 1.4 1.5 1.6 1.7 (Bi + Cu)/(Sr + Ca) Fig.7 Influence of 2212 film stoichiometry on the film orienta-tion: intensity =1XA1200 +Ill3 +1115 +I,,,+~OO,,), 1, =(I008+loolo) (4,IZoo(b); & =780 "C,substrate ( 100) MgO 0.01- E c m--.+ .+-0 + f 0.001- v) + 2 a- m.- 2c Q (a 1 x 0.0001It- I N I 1 I 1 Fig. 8 Dependence of pco2 for 'cold' (a)and 'hot' (b) wall reactor on the molar ratio of oxygen to diglyme according to mass-spectrometric measurements Conclusions A flexible and simple MOCVD technique has been developed to prepare superconducting YBCO and BSCCO films. This technique allows easy control of the composition of the multicomponent films. Films of the different orientations were prepared. The technique may be developed for preparing a variety of complex oxide films, containing three, four or more metal components. The authors thank DAAD and the Russian Ministry of the Science and Technical Policy for financial support of this work.References 1 F. Schmaderer, R. Huber, H. Oetzmann and G. Wahl, Appl. Swf. Sci.,1990, 46,53. 2 H. Yamane, H. Kurosawa and T. Hirai, Supercond. Sci. Technol., 1989,2, 115. 3 S. Matsuno, F. Uchikawa and K. Yoshizaki, Jpn. J. Appl. Phys., 1990,29, L947. 4 Y. Q. Li, J. Zhao and C. S. Chern, AppI. Phys. Lert., 1991,58, 648. 5 F. Weiss, G. Delabouglise and 0.Lebedev, J. Al10.y~Cornp., 1993, 195,475. 6 J. Zhang, J. Zhao, H. 0. Marcy, L. M. Tonge, B. W. Wessels, T. J. Marks and C. R. Kannerwurf, Appl. Phys. Lett., 1989, 54, 1166. 7 K. Endo, Y. Moriyasu, S. Misawa, H. Yamasaki and S. Yoshida, ThinSolid Films,1991,206, 125. 8 B. Schulte, M. Maul and P.Haussler, Appl. Phys. Lett., 1993, 62, 633. 9 0. Thomas, J. Hudner and M. Oestling, J. AIloys Comp., 1993, 195,287. 10 A. R. Kaul and B. V. Seleznev, J. Phys. (Paris) IV. 1993, Colloque c3, 3, 375. J. MATER. CHEM., 1994, VOL. 4 1589 11 R. Hiskes, S. A. DiCarolis and J. L. Young, Appl. Phys. Lett., 1991, 19 L. M. Rubin, T. P. Orlando and J. B. Vander Sande, Appl. Phys. 59, 606. Lett., 1992,61, 1977. 12 13 14 15 16 17 18 W. J. Lackey, W. B. Carter and J. A. Hanigofsky, Appl. Phys. Lett., 1990, 56, 1175. S. Matsuno, F. Uchikawa, S. Utsunomiya and S. Nakabayashi, Appl. Phys. Lett., 1992,60,2427. F. Weiss, K. Froehlich, R. Haase, M. Labeau, D. Selbmann, J. P. Senateur and 0.Thomas, J. Phys. (Paris) ZV, 1993,Colloque C3,3, 321.W. Decker, Yu. Erokhin, 0. Gorbenko, I. Graboy, A. Kaul, A. Nurnberg, M. Pulver, R. Stolle and G. Wahl, J. Alloys Comp., 1993,195,291. G. Wahl, W. Decker, A. Nurnberg, M. Pulver, R. Stolle, Yu. Erokhin, 0.Gorbenko, I. Graboy, A. Kaul, M. Sommer and U. Vogt, Proc. Int. Symp. CVD-XZZ, Honolulu 1993, ed. K. F. Jensen and G. W. Cullen, Electrochemical Society, 1993, p. 264. A. A. Drozdov, N. P. Kuzmina and S. I. Troyanov, Muter. Sci. Eng., 1993, B18, 139. F. Weiss, E. Senet and M. Langlet, J. Less-Common Met., 1990, 164.1393. 20 21 22 23 24 25 26 27 V. Fuflyigin, A. Kaul and S. Pozigun, J. Phys. (Paris) lV, 1993, Colloque C3,3,361. 0. Yu. Gorbenko, A. R. Kaul, Yu. D. Tretyakov, V. I. Scritny, S. A. Pozigun and V. A. Alekseev, Physica C, 1991,190, 180. J. C. Toledano, P. Strobe1 and D. Morin, Appl. Supercond., 1993, 1, 581. K. Schulze, P. Majewskij and B. Hettich, 2. Metalkuirde, 1990, B17, 836. C. P. Burmester, L. T. Wille and R. Gronsky, Proc. of ZCAM91 (Al), Strasbourg, 1991, Elsevier, Amsterdam, 1991, pp. 313-31 8. S. J. Pennycook, M. F. Chisholm and D. E. Jesson, Physica C, 1992,202, 1. T. Terashima, K. Shimura, T. Satoh and Y. Bando, &r. CrystaI Growth, 1991, 115, 745. R. Feenstra, T. B. Lindemer, J. D. Budai and M. D. (ialloway, J. Appl. Phys., 1991,69,6569. Paper 4/02305F; Received 19th April, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401585

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994,4(10), 1591-1594 Investigations into the Growth of AIN by MOCVD using Tri-ferf-butylaluminium as an Alternative Aluminium Source Anthony C. Jones,*" John Auld," Simon A. Rushworth," David J. Houlton" and Gary W. Critchlod a Epichem Limited, Power Road, Bromborough, Wirral, Merseyside, UK L62 3QF lnstitute of Surface Science and Technology, University of Loughborough, Loughborough, Leicestershire, UK LE77 3TU Thin films of AIN have been deposited at 500 and 600°C by atmospheric-pressure MOCVD using the precurscirs tri- tert-butylalurniniurn (Bu',Al) and tert-butylarnine (Bu'NH,). Growth rates of 0.5 pm h-' were obtained at 500 "C. Post-growth oxidation of the AIN films was prevented by the deposition of a protective A1 overlayer using Bu',Al.Aluminium nitride (AlN) is an important material with a variety of applications such as passive barrier layers and substrates in silicon integrated circuits, high-frequency acous- tic wave devices, high-temperature windows and dielectric optical enhancement layers in magneto-optic multilayer struc- tures.' In addition, the ternary alloy Al,Ga,-,N has a large potential application in optoelectronic devices operating in the UV- blue spectral region., The development of these various applications is critically dependent on the capability to deposit thin films of AlN at low to moderate substrate temperatures. Conventional ceramic processes, such as the direct nitriding of A1 powder at high temperature (> 1440°C) are unsuitable for the controlled deposition of thin A1N 1aye1-s.~ Therefore, the physical vapour deposition technique of vacuum sputter- ing is generally employed.However, this suffers from the disadvantages of limited scale and poor conformal step cover- age. There has thus been a concerted effort4 to develop metal- organic chemical vapour deposition (MOCVD) techniques which have the advantages of large-area growth capability, excellent conformal step coverage and precise control of layer thickness. The deposition of AlN by MOCVD has traditionally been carried out using mixtures of trimethylaluminium (Me3AI) and ammonia (NH3).5,6 However, the high thermal stability of NH3 necessitates the use of high substrate temperatures (typically >9OO"C).This leads to the problem of nitrogen loss from the AIN film which is only partially alleviated by the use of high V :I11 ratios (e.g.>2000 :1). AIN growth has been achieved at lower substrate tempera- tures (400-800 "C) using a variety of 'single-source' precursors, which already contain an intramolecular (Al-N) bond. These include [A1(NR2)3]2, [HA1(NR2),], (R =Me, Et),7 [Me,AlNH,],,* [Et2A1N,]39 and [Me,AlNR,], (R =Pr').'' However, these precursors have only very low vapour press- ures (< 1Torr at room temperature) which necessitates the heating of source and reactor inlet lines and the use of high- vacuum MOCVD reactors. It is therefore desirable to develop alternative precursors which may be more conveniently utilized in MOCVD, and the volatile nitrogen source hydrazine (N2H4) has been used in combination with Me3AI to grow A1N at temperatures as low as 220°C.'' However, N2H4 is an extremely toxic (TLV(,ki,,0.01 ppm) and unstable compound which has been reported to decompose on contact with stainless steel.These factors are likely to seriously restrict its large-scale application in MOCVD. The successful deposition of A1N from the single-source precursors [Me2A1NR2]," and [Et2A1N3]39 has encouraged us to investigate methods of forming similar species in situ in the vapour phase prior to layer growth. This approach aims to combine the advantages of convenient source temper'atures and high growth rates associated with the use of high l'apour pressure reagents, with the low growth temperatures ,tssoci- ated with single-source precursor molecules.Thus, we have recently demonstrated" the successful deposition of ,!IN in the temperature range 400-600 "C by atmospheric pressure MOCVD using the volatile primary alkylamines, tert-butylamine ( Bu'NH,) and isopropylamine (Pr'NH,) in combi- nation with Me3A1. It was proposed'2 that the directly honded species [Me,AlNHR], was formed in the gas phase prior to AlN deposition, and the recent report13 of AlN growth by high-vacuum CVD using [Me,AlNHR], (R =Bur, Pr') strongly supports this proposal. Similarly, the combination of Me3A1 and trimethylsilylazide (Me3SiN3) proved suitable for the deposition of A1N at 300-450 OC.I4 Significantly. Auger electron spectroscopy (AES) failed to detect silicon in the films, and this was attrib~ted'~ to the formation of diinethyl-aluminium azide (Me2AIN3) in the gas phase, together with tetramethylsilane (Me,%) which allows the efficient tr,tnsport of Si species away from the growth zone.However, the A1N films deposited from mixtures of Me3A1-RNH, or Me,Al-Me,SiN, were found to contain oxygen (2.0-8.0 atom%), due possibly to post-grovl th oxi- dation, together with variable levels of residual carbon (2.7-17.0 atom%). The carbon contamination was attri-buted12,14 to the use of the methyl-based A1 precursor Me3A1, which has been shown to lead to significant levels ot carbon contamination in A1 films15 and AlGaAs epitaxial layers16 grown by MOCVD. Recently, some of the present authors have shown that the new A1 precursor tri-tert-butylaluminium (Bu',Al) can be used to deposit high-purity A1 in the temperature range 300-450 "C by low-pressure CVD.I7 This has encouraged us to investigate But3Al as an alternative precursor to Me3Al for the deposition of AlN by MOCVD, and these results are presented herein.In an effort to prevent post-growth oxidation of the AlN films, But3Al was also used to deposit a protective A1 overlayer, which provides evidence of its usefulness and versatility as a new A1 source for MOCVD. Experimental General Techniques AES was carried out on a Varian scanning Auger spectrometer. The atomic compositions quoted ?re from the bulk of the film (depth from surface >2000 A) and were obtained by combining AES with sequential ion bombardment until com- parable compositions were obtained for consecutive data points.Film thicknesses were estimated by the time taken to sputter through the layer using Ar +-ion bombardment. Proton nuclear magnetic resonance ('H NMR) data were obtained on a Bruker WM 250 spectrometer operating at 250 MHz and microanalytical data (C, H, N analysis) were provided by the Microanalytical Services Department of the University of Liverpool. Scanning electron microscopy (SEM) was performed on a Cambridge Stereoscan 360 microscope. Aluminium Nitride Film Growth The reagents used were Bu',Al, synthesized as described previ~usly'~and Bu'NH,. The Bu'NH, was dried and deoxy- genated prior to use by distillation over sodium under a nitrogen purge.The AlN films were deposited at atmospheric pressure in a simple cold-wall horizontal quartz reactor (Electro Gas Systems Ltd) using radiant substrate heating. The substrates used were Si( 11 1) single-crystal wafers and these were cleaned (20% nitric acid-deionized water), degreased with acetone and dried before use. Trace oxygen and moisture were removed from the hydro- gen carrier gas by passing it through a Nanochem resin purification unit. The Bu'NH, was further purified during use by passage through a Nanochem purifier. The Bu',Al and ButNH2 sources were operated at room temperature (22°C) and were mixed in a 'T-piece' at the reactor inlet. This was heated to 60 "C to prevent condensation of any adducts formed in the gas phase.In order to prevent post-growth oxidation of the deposited AlN films, a protective A1 overlayer was subsequently deposited at low pressure ( 15 Torr) using the But,Al precursor alone. A full summary of growth conditions is given in Table 1. Results and Discussion A1N films were successfully deposited using Bu',Al and Bu'NH, at substrate temperatures between 500 and 600 "C. Below 500"C, the A1N growth rate was found to be prohibi- tively low, whilst at temperatures >600 "C film growth was limited by severe reagent depletion. The atomic composition of the films was determined by AES and these data are summarized in Table 2. These data show that all the films have an A1 :N ratio close to unity, although in film 3 nitrogen is present in slight excess.The most obvious feature of the AES data is the significant reduction of oxygen contamination resulting from the growth Table 1 Growth conditions used to deposit AlN J. MATER. CHEM., 1994, VOL. 4 Table2 Auger electron spectral analysis of AN films grown on Si( 11 1 ) using mixtures of Bu',Al and BuWH, atomic composition %) film no. A1 N C 0 1 (uncapped) 41.4 39.3 6.9 10.4 2 (A1 capping layer) 98.2 - 0.5 1.2 (A1N layer) 3 (AlN layer) 49.7 45.7 44.3 46.6 4.7 7.2 1.3 0.4 Al:N 1.05 -~ 1.21 0.98 of the protective A1 overlayer in films 2 and 3. This strongly suggests that post-growth oxidation has cxcurred in the uncapped A1N film (l),and further suggests that post-growth oxidation was largely responsible for the relatively high levels of oxygen contamination (2.0-8.0 atom%) observed previ~usly'~~~~in A1N films grown using mixtures of Me,Al-RNH, and Me,Al-Me,SiN, .The residual oxygen (between 0.4 and 1.3 atom%) remaining in the capped A1N films and in the A1 overlayer can be attributed to trace oxygen in the relatively unsophisticated MOCVD reactor used in this study. The uncapped A1N films were extremely hard and scratch- resistant and demonstrated specular surface morphology. Scanning electron microscopy (SEM) data for a typical uncapped A1N film grown at 500°C on Si(111) (Fig. 1) Fig. 1 Scanning electron micrograph of an AlN film grown at 500 -C on Si( 111)from a But3Al-Bu'NH, mixture films from mixtures of Bu',Al and Bu'NH," run no.1 2 3 (uncapped) (A1 capped) (a) AIN Growth (cell pressure 760 Torr) H, carrier gas flow through Bu',Al (sccm)b H, carrier gas flow through Bu'NH, (sccm) substrate temperature/T growth ratejpm h-' 200 50 500 0.5 200 50 500 -200 50 600 - approximate V :III ratiod 36 36 36 (h) A1 capping layer (cell pressure 15 Torr) H, carrier gas flow through But3Al (sccm) - 50 50 substrate temperature/^C duration of growth/min -- 400 3 400 1 ~~~ a But3Al and Bu'NH, sources at 22 "C; substrates Si( 111) single-crystal wafers. Standard cm3 min-'. ' Estimated from AES sputter time. Based on an estimated Bu',Al vapour pressure of ca. 2 Torr at 22 "C (vapour pressure Bu'NH, =340 Torr at 25 "C).J. MATER. CHEM., 1994, VOL. 4 indicate that the film is amorphous and structureless, with no grains ekident on a 500 nm scale. A further significant feature to emerge from the AES data (Table 2) is that, despite the use of But3A1 as an alternative to Me,Al, the A1N films still contain residual carbon at a level of between 5 and 7 atom%. These carbon levels are similar to those observed in A1N films grown using Me,Al-Bu'NH, mixtures (Cz3-9 atom%),12 which indicates that But,A1 offers no significant advantage over Me,Al for AlN growth from R3A1-RNH, mixtures. This is a surprising result in view of the marked contrast in the purity of A1 films deposited at 450 "C from But3Al (CFZ 0.2-0.5 atom%)17 compared with A1 films deposited at similar substrate tem- peratures from the methyl-based precursors Me,Al'' or Me,A1H(NMe,)l8 in which carbon levels of up to 39 atom% have been observed.This suggests that the decomposition of the [Al-R] group may not be the only factor controlling carbon incorporation in A1N films grown from R3A1-RNH2 mixtures. The decomposition of the organic radical of the primary alkylamine (RNH,) may also play a role which suggests that carbon incorporation may vary according to the nature and pyrolysis characteristics of the RNH, precur- sor. This proposal is supported by the greatly increased carbon levels (14-17 atom%) observed in A1N films grown using Me,Al-Pr'NH, compared with films grown from Me,Al-Bu'NH, mixtures.', In addition, the carbon contami- nation was shown to increase with increasing V: 111 ratios, in marked contrast to the trend generally observed in the growth of GaAs and AlGaAs by MOVPE;16 this provides further evidence that the RNH, precursor may play a critical role in carbon contamination.Information concerning the possible growth mechanism has been obtained by the ex-situ addition of ButNH, (12.9 g, 0.17 mol) to Bu,'Al (16.0 g, 0.08 mol) in dry pentane solution (25 cm3). Removal of volatiles in ~acuu left a colourless crystalline product which was highly soluble in benzene. This was shown to be the 1: 1 adduct, [But,Al(NH,But)] by 'H NMR data and elemental microanalysis (Table 3). The [Bu',Al(NH,Bu')] adduct was observed to melt at 70-80 C, and at 115 "C a gas was evolved.Further heating of the compound at 115-120°C for 30min led to a white powder which was only sparingly soluble in benzene. This precluded meaningful 'H NMR data and elemental microanal- ysis (Table 3) was also inclusive, although these data suggest that the decomposition product may have the molecular formula [Bu',AI(NHBu')], (n=2, 3). The low solubility of this compound in benzene is consistent with the proposed oligomeric structure. Table 3 Analytical data for 1: 1 adduct formed from the reaction between But,Al and ButNH, 'H NMR data (['H6] benzene) 6 0.85 (s, 9 H, N-But) 1.25 (s, 27 H, But-Al) 2.5 (s, 2 H, N-H) elemental microanalysis C (Yo) H (Yo) N (Yo) found 70.74 14.49 4.83 calcd. for [Bu',Al( NH,Bu')] 70.77 14.14 5.16 elemental microanalysis of decomposition product" C (Yo) H (Yo) N (Oh) found 66.67 13.65 5.74 calcd.for [Bu',Al( NHBu')], 67.54 13.25 6.56 ~~~~ ~ Formed by heating the 1: 1 adduct at 115-120 "C for 30 min. Decomposition product essentially insoluble in c2H6] benzene. During the growth of AlN from But,Al-ButNH, mixtures, a crystalline deposit was observed to form at the reactor inlet if this was left unheated, and it is likely that this is the which[Bu',Al(NH,Bu')] adduct. Previous studie~,'~~~~ are supported by the present work, have shown that such adducts readily form elimination products of the type [R,AlNHR'], on heating, and therefore such species may be expected to form in the hot boundary layer adjacent to the substrate.Subsequent pyrolysis of the directly bonded species [Bu',AINHBut], on or near the substrate surface leads to the deposition of A1N. This proposal is strongly supported by the recently reported growth of AlN by vacuum CVI) using [Me,AlNHR], (R=Pr', The level of carbon contamination in A1N films drrposited from [R,AlNHR], precursors, which have been either pre- synthesized or formed in situ in the gas phase, will depend strongly on the mechanism by which the alkyl radicals bound to A1 or N are desorbed from the growth surface. For Pr' and But radicals there is a ready desorption route viu the p-hydride elimination of alkene;,l however, the continued pres- ence of carbon in AlN grown from Bu',Al-Bu'NH, mixtures indicates that some surface decomposition of the Bu' radical has occurred.The decomposition of the But radical is likely to be promoted by the presence of A1 on or near thcr growth surface.,l This may lead to methyl abstraction from the But radical, leading to surface-adsorbed methyl radicals which subsequently decompose to deposit carbon. The mechanism of AlN deposition from [R,AINHR'], species may be similar to that occurring in the growth of GaAs from the single-source molecule [M~,G~AsBu',],.~~In these studies it was proposed that the facile P-hydride elimin- ation of alkene from the bulky and sterically hindered tert-butyl group leads to the formation of a strong intraniolecular 111-V bond during pyrolysis which facilitates the growth of stoic hiometric GaAs.The precise mechanism of A1N deposition from R3A1-RNH, mixtures has not been established. However, the low growth temperatures (400-600 "C) and low V: IT1 ratios used in this, and previous12 studies, compared with those med for Me,Al-NH, combinations, suggests that 'directly bonded' species of the type [R,AlNHR], are the active precursors to A1N deposition. Conclusions A1N films have been deposited by atmospheric -pressure MOCVD using But3Al in combination with ButNHz. Growth rates of 0.5 pm h-' were obtained at substrate temperatures of 500 "C. The use of a protective A1 overlayer, depoS(ited from But3Al, was shown to lead to a significant reduction in oxygen contamination of the AlN films.However, residuiil carbon was present at levels of between 5 and 7 atom%, and it is suggested that the decomposition of the organic radical in ButNH, may play a role in carbon incorporation. This work was supported by the Department of I'rade and Industry under the LINK/ATP initiative and the Teaching Company scheme. D.H. is a Teaching Company Associate (University of Keele, UK). References 1 L. M. Sheppard, Cerum. Bull., 1990,69, 1801. 2 M. A. Khan, R. A. Skogman, R. G. Schulze and M. Gershenzon, Appl. Phys. Lett., 1983, 43, 492. 3 L. V. Interrante, L. E. Carpenter, C. Whitmarsh and W. Lee, Mater. Res. Soc. Symp. Proc., 1986,13, 359. 4 L. Baixia, L. Yinkui and L. Yi, J. Muter. Chem.. 1993, 3, 117.1594 5 M. Morita, S. Isogai, N. Shimizu, K. Tsubouchi and N. Mikoshiba, Jpn. J. Appl. Phys., 1981,19, L173. 6 M. Morita, M. Useugi, S. Isogai, K. Tsubouchi and N. Mikoshiba, Jpn. J. Appl. Phys., 1981,20, 17. 7 Y. Takahashi, K. Yamashita, S. Motojima and K. Singiyama, Surf. Sci., 1979,86, 238. 8 L. V. Interrante, W. Lee, M. McConnell, N. Lewis and E. Hall, J. Electrochem. Soc., 1989,136,472. 9 K. L. Ho, K. F. Jensen, J. W. Hwang, W. L. Gladfelter and J. F. Evans, J. Crystal Growth, 1991, 107, 376. 10 D. C. Bradley, D. M. Frigo and E. A. D. White, Eur. Put. Appl., 1989 EP 0 331 448. 11 M. Mizuta, S. Fujieda, T. Jitsukawa and Y. Matsumoto, Proc. Int. Symp. on GaAs und Related Compounds, Las Vegas, Nevada, 1986, IOP Publishing Ltd, Bristol, 1987.12 A. C. Jones, J. Auld, S. A. Rushworth, E. W. Williams, P. W. Haycock, C. C. Tang and G. W. Critchlow, Adzi. Muter., 1994,6,229. 13 M. M. Sung, H. D. Jung, J. K. Lee, S. Kim, J. T. Park and Y. Kim, Bull. Koreun Chem Soc., 1994, 15, 79. 14 J. Auld, D. J. Houlton, A. C. Jones, S. A. Rushworth and G. W. Critchlow, J. Muter. Chem., 1994,4, 1245. J. MATER. CHEM., 1994, VOL. 4 15 D. R. Biswas, C. Ghosh and R. I,. Layman, J. Electrochem. Soc., 1983,130,234. 16 T. F. Kuech, E. Veuhoff, T. S. Kuan, V. Deline and R. Potemski, J. Crystal Growth, 1986,77,257. 17 A. C. Jones, J. Auld, S. A. Rushworth and (3. W. Critchlow, J. Crystal Growth, 1994, 135,285. 18 A. C. Jones, S. A. Rushworth and J. Auld, paper to be presented at ICVGE-8, Freiburg, Germany, July 1994, proceedings to be published in J. Crystal Growth. 19 S. Amirkhalili, P. B. Hitchcock, A. D. Jenkins. J. Z. Nyathi and D. J. Smith, J. Chem. Soc., Dulton Trans., 1981, 377. 20 AA Al-Wassil, P. B. Hitchcock, S. Sarisaban, J. D. Smith and C. L. Wilson, J. Chem. Soc., Dalton Trans., 1985. 1929. 21 J. S. Foord, N. K. Singh, E. T. Fitzgerald, G. J. Davies and A. C. Jones, J. Crystal Growth, 1992,120, 103. 22 J. E. Miller, K. B. Kidd, A. H. Cowley, R. A. Jones, J. G. Ekerdt, H. J. Gysling, A. A. Wernberg and T. N. Blanton, Chem. Muter., 1990,2, 589. Paper 4/03214D; Received 31st Muy, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401591

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( lo), 1595-1602 Role of Additives in the Sintering of Silicon Nitride: A 29Si,27AI,25Mg and *'Y MAS NMR and X-Ray Diffraction Study K. J. D. MacKenzie* and I?.H. Meinhold New Zealand Institute for Industrial Research and Development, PO. Box 37-370, Lower Hutt, New Zealand Multinuclear MAS NMR in conjunction with X-ray diffraction (XRD) has been used to study the role of A1203, Y,p03and MgO, both singly and in combination, in the sintering of silicon nitride at 1500-1 800 "C. Under the present experimental conditions, Al,03 enters the silicon nitride to form a low-z fi'-SiAION, whereas MgO reacts both with the cixidised surface SiO, layer to form forsterite (Mg,SiO,), and with the Si3N4 to form an X-ray amorphous Mg-Si-0-N phase characterised by a broad 25Mg signal at about -50 ppm and a fast ,'Si relaxation time.Y,O, forms an yttrium-rich Y-Si-0-N phase at 1500 "C which progressively becomes silicon-rich at higher temperatures. The "Y spectra of these phases are broad and could be detected only in samples containing added Yb203 to shorten the relaxation timtl?. When used in combination, the A1,03/Mg0 and Y,O,/MgO pairs behave similarly to the separate components, in terms of intergranular phase formation, but A1,03/Y,03 forms Y,oAI,Si,0,BN4, for which the ,'Si and MAS NMR spectra are reported. Silicon nitride is a versatile high-technology ceramic which has found use as refractories, cutting tools, engine wear parts and biomedical implants. In practice, it is prepared as a powder which is then formed to the required shape by pressing or casting and sintered at high temperatures, with or without the application of pressure, to a hard, dense material.Because sintering is difficult in pure silicon nitride, a variety of additives have been used, which react to produce a phase which is liquid at the sintering temperature. Three of the most common additives used to promote sintering in Si3N4 are MgO, A120, and Y203, introduced either singly or in combination. The addition of up to 5 wt.% of MgO has been used to produce fully dense Si3N4 bodies by hot pressing.' The Mg-Si-A1-0-N phase diagram2 predicts the formation of forsterite (MgSiO,) in the presence of free Si02 which is normally present as a surface impurity.The silicate is molten above 1550 "C, and promotes liquid-phase sintering. The phase diagram also predicts the further reaction of the molten silicate with Si3N, to produce Mg-Si-0-N liquid,2 which when recrystallised by re-heating at 1350"C is reported to give a mixture of enstatite (MgSiO,) and silicon oxynitride (Si2N20).3 Y203 has been used, both on its own and in combination with Al,03 to produce grain-boundary phases which are more refractory than magnesium silicates, and therefore more appropriate to higher temperature applications. The Y-Si-A1-0-N phase diagrams4 indicate the formation of several stable phases within this system, including an yttrium- nitrogen melilite ( Y2Si303N4), a nitrogen apatite Y5Si,012N (designated 'H-phase'), and a substituted yttrium aluminate Y4Si207N2 (designated 'J-pha~e').~ Other phases which com- monly occur within this system are Y2Si207 and yttrium aluminium garnet (Y3A15012).The presence of Al,O, in the system lowers the temperature of the Y,03-Si02 eutectic and decreases the viscosity of the liquid Y-Si phases. Combinations of A120, and Y203 have been reported to produce a liquid phase at temperatures as low as 1400"C;6at higher tempera- tures, some of the alumina reacts with the Si3N, to form SiAlON and increases the yttrium concentration of the liquid.6 The crystalline intergranular phases which eventually appear are reported to be similar to those formed with Y203 alone, and include N-melilite, H-pha~e,~,' J-phase7 and K-phase (YSiO,N).' The use of MgO in combination with Y203 has a similar effect to that of A1203 in lowering the viscosity of the liquid phase.The crystalline compounds reported in this system are Y2Si3O3N, (N-melilite), Y,Si20,N2 (J-phase), YSi02N (K-phase), Y2Si207 and Mg5Y,Si502,.9 One of the problems in studying the reactions of the intergranular phases formed by the sintering additives is that they are often non-crystalline; to study these phases by X-ray diffraction methods, they may be re-crystallized hy heat treatment at lower temperatures, which may also ai€ect the physical properties of the sintered body. Solid-statc NMR with magic-angle spinning (MAS NMR) is a technique which is useful for studying the atomic environments of atoms in non-crystalline and poorly crystalline phases.The 1eported MAS NMR data for a number of the phases which may be encountered in Si3N4, sintered with various additives, are summarised in Table 1. Despite the existence of this body of information about the pure phases implicated in sintering processes, very little in situ work has been published on the NMR spectra of these phases in actual sintered bodies, probably because of the small quantities of interganular material involved. To solve this sensitivity problem, C'arduner et all2 spun cylinders of Si,N4 sintered with Y203 and A1203 in their NMR rotors, and recorded their 29Si and 2741 MAS NMR spectra. The aim of the present work is to extend the studies of the in situ phases formed in Si3N4 sintered with additions of MgO, A1203 and Y203 and mixtures thereof, using ;L combi-nation of X-ray powder diffraction to monitor the ciystalline phases and solid-state MAS NMR to study both the CI ystalline and non-crystalline components. Recent improvertients in 25Mg2' and "Y MAS NMR23 have re-kindled an interest in the glassy grain boundary phases in samples containing MgO and Y20,, and these are of particular interest in the present study.Experimental The silicon nitride used in this work (H.C. Starck, grade LC 10) was blended with the additive (A1203, MgO or Y203) by ball-milling under ethyl alcohol for 4 h in polypropj lene jars using Si,N, milling media. Several experiments using longer milling times (up to 16 h) did not give significantly different results, suggesting that the additives are adequately dispersed by milling for 4 h.Initial experiments used 10 wt.% of each additive, corresponding to 12.5, 28 and 6.4 mol% of Al,03, MgO and Y203 respectively, but other experiments were also J. MATER. CHE_M.,1994, VOL. 4 Table 1 MAS NMR parameters for systems containing Si,N,, Al,O,, MgO or Y,O, phase 6 ref. 29Si NMR (wrt TMS) b-Si,N, -48.5 10-12 r-Si,N, -47.1, -49.5 10-12 Y,Si,O,,N (H-phase) -73.7 to -74.9, -67.4 to -67.5 12,13Y,Si,O,N, (J-phase) -73.7 to -74.4 12-14 YSi02N (K-phase) -64.7 to -65.3 12,13Y,Si,03N4 (N-melilite) -56.7 to -57.1 12,13 Y10A12Si3018N4 -76.1 this work Y Si,N, -42.3 to -42.8, -45.5 12,13Y,Si05 -79.8 to -80.0 12.13 y-Y2Si,0, -92.8 13 Si,N,O -63.0 13 F-SiAlON -47.6 to -48.8 15 SiAlON glass (ca.Si,4AI,0,0N) -113 (broad) 16 Mg,SiO, -62 17 MgSiO, -82 18 MgSiN, -44.4 this work MgAlSiN, -42.5 (broad) this work NMR [wrt A1(H20),3+] P-SiAlON 66 to 69.1, 106 to 112.5, -6.6 to 10 15,19 SiAlON glass (ca. Si24A160s0N) 35, 50, 0 16 Y10A12Si30 1SN4 30, 54.7, 105 this work Y,A1,09 9.4, 114, 0.8 19 Y3A15012 74, 0.8 19 YAlO, 9.4, 0.8 19 Al,,O,?N5 (approx.) 114, 65, 12 19 MgAlSiN, 101.8, 112.3, 10.1 this work "Mg NMR (wrt MgSO,) MgO 26 20,21 MgSiO, -18.0 21 MgSiN, 79, 45, -57 (broad) this work MgAlSiN, 45, -59 (broad) this work "Y NMR (wrt YCl,) y203 314, 272.5 22 Y3A15012 222 22 YAlO, 214.5 22 Y,SiO, 237, 148 22 a-Y,Si20, 114 22 j?-YzSi207 208 22 ;t-Y,Si20, 198 22 6-Y2Si,0, 122 22 Y,Si,O,N, (N-melilite) 185 (broad) 23 Y4Si20,N, (J-phase) 202 (broad) 23 carried out using different additive concentrations, e.g. up to positive pressure during the firing.The heating rate was 15 mol% Y203. Additions of 3 wt.% Yb203 (ca. 0.03 mol per selected to take 120 min to reach the sintering temperature mole of mixture) were made to the Y-containing samples to (1500-1800°C) at which the sample was held for a further decrease the 89Y relaxation time;23 89Y spectra could not be 120 min before being cooled rapidly by turning the furnace obtained for samples without this Yb203 addition. Other off. Temperature measurement was by boron graphite-graph- samples were also prepared containing 10mol% each of Y203 ite coaxial thermocouple, with periodic checks also made by and A120,, Y203 and MgO, and A1203 and MgO.After the optical pyrometry. sample had been milled, the solvent was removed in a vacuum After measurement of their radial firing shrinkage by vernier rotary evaporator and the powder brushed through a 600 pm callipers, the pellets were broken, their densities and porosities sieve before being pressed at 100 kPa into 2.0 g pellets of measured by a water penetration method, and a portion 200 mm diameter. The green densities of all the pellets, both ground to pass a 100 mesh sieve for examination by X-ray doped and undoped, were very similar (1.62-1.68 g ~m-~). powder diffraction using a Philips PW 1700 computer-Since the primary focus of this study was the chemistry and controlled goniometer with a graphite monochromator and composition of the intergranular phases, the addition of other Co-Kol radiation.The room-temperature MAS NMR compounds was avoided, to minimise possible complications. measurements were carried out at 11.7 T on a Varian Unity Thus, the low pressing pressure and consequent low green 500 spectrometer using a 5 mm Doty probe spun at typically density resulted from the decision not to use an organic 10 kHz as follows: "Si, spectrometer frequency 99.3 MHz, 90" binder. Pressureless sintering was carried out in a boron pulse of 6 ps, recycle times of up to 3000 s, referenced to nitride powder bed in an alumina pot with lid, using a graphite tetramethylsilane (TMS); 27Al, spectrometer frequency resistance furnace (Thermal Technology Inc).The furnace was 130.3MHz, 15" pulse of 1 ps, recycle time 0.1 or 1 s, referenced evacuated to 100 kPa or better for at least 0.5 h before filling to Al( H20)63+ ;25Mg, spectrometer frequency 30.6 MHz, 60" with oxygen-free nitrogen which was kept flowing at a slight (solids) pulse of 3 ps, recycle time 0.1 s, up to 500 000 transients J. MATER. CHEM., 1994. VOL. 4 acquired, referenced to saturated MgS04 solution; "Y, spec-trometer frequency 24.5 MHz, 90" pulse of 18 ps, recycle time 100 s, referenced to aqueous YC1, solution. Results and Discussion The change in radial shrinkage with temperature for the series of samples containing 10 wt.% of the single additives is shown in Fig.1. By comparison with the undoped control samples, all three additives exert a positive influence on the sintering behaviour, increasing with temperature, and in approximate proportion to the molar concentration of the additive. A similar trend is seen in the bulk density, with an inverse trend in the apparent porosities. The absolute values of density and porositj indicate that the most highly sintered of these samples has achieved ca. 80% of theoretical density, a consequence of the low unsintered densities resulting from the decision not to use a binding agent. However, the changes in density with sintering conditions are of more interest than the absolute densities which could be achieved, since the primary purpose of this work was to study the intergranular phases rather than to produce highly densified materials.Typical X-ray powder diffraction traces of the various sintered bodies are shown in Fig. 2. XRD indicates that the starting material contains predominantly a-Si3N4; with increasing temperature, in the absence of additives, the pro- portion of p-Si3N4 increases only slightly. With the addition of 10 wt.% Y,03, conversion to p-Si,N, is much enhanced at 1600-C and is complete by 1700°C. The X-ray patterns also indicate the presence of small amounts of other crystalline phases: Y,Si,012N (H-phase) at 1500"C, giving place to Y2Si3N40, (N-melilite) at 1600-1750 "C [Fig. 2(f),(g)]. Samples prepared specifically for "Y NMR (see later) contain- ing higher concentrations (15 mol%) of Y203 behave similarly to the 10wt.% samples, except that they form Y4Si20,N2 (J-phase) in preference to H-phase at 1500°C [Fig.2(e)].At 20.0 h 15.0 -(II.-10.0 0.0 I I I I1 I I 1500 1600 1700 1800 71°C Fig. 1 Radial shrinkage of Si,N, pellets sintered in nitrogen with 10wt.Yo additive at 150O-175O0C, and 120 min dwell time at sintering temperature. Additives: (M) MgO, (+) A1,0,, (A)Y,03, (0)blank. 18OOcC, the Y-containing samples were found by XRD to contain elemental Si, and a degree of melting had occurred. Similar results were found at 1800 "C for samples containing A1203 [Fig. 2(c)] and MgO. The formation of elemt~~tal Si has previously been reported by Deeley et a/.' in Si,N4 sintered in graphite at 1850cC,and by D~tta~~ in commercial pressureless sintered (Kyocera) Si3N4.The reasons for its formation in the present samples are not clear, but may be due to some peculiarity of the reducing atmosphert in the graphite furnace, acting in combination with the porous nature of the pellets, which are of lower green density than would normally be encountered in sintering experiments. The addition of 10 wt.% MgO similarly promoted the formation of P-Si3N4, this conversion being complete by 1700 "C . At the lower temperatures, there was also X-ray evidence of a small amount of poorly crystalline forsterite [Fig. 2(d)] which progressively decreased with increasing temperature. At 1800"C, the sample still contained predominantly Si.N4, but decomposition to Si had progressed appreciably. In samples containing 10 wt.% A1203, conversion to p-Si3N4 was also well advanced by 1600°C and complete by 1700°C. At 1500"C, a small amount of a-A1203 was also detected [Fig. 2(a)], but by 1600"C this decreased signifi- cantly. No other discrete Al-bearing phases were dtbtectable by X-ray diffraction, but slight shifts in the positions of the P-Si3N4 peaks suggest the incorporation of some A1 into this phase to form b'-SiAION (IFig. 2(b)].Careful measurements of the X-ray peak positions, using silicon as the angular calibrant, were used to deduce the z values of these p-SiAlONs from the relationship given by Ekstrom et These values indicate very little incorporation of A1 at 1500"C, consistent with the presence of unreacted A1203in thi:, sample.At increasing temperatures, the SiAlON z values progressively increase, up to a maximum of 0.64 at 1700T. The XRD results, which are generally consisttmt with predictions from the phase diagrams, are summarised in Table 2. 29SiNMR Fig. 3 shows a selection of typical 29Si MAS NMR spectra. The major features of all these spectra are the 4 and P-Si,N4 resonances at -47 and -49 ppm (a) and -48 ppni (p), the relative areas of which were used to provide an indication of the progress of the r to p transformation with temperature (Fig. 4). Fig. 4 indicates that all three additives facilitate the a-p transformation above 1500"C, the greatest efft:ct being obtained with Y203 and MgO.The effectiveness of Y203 as a transformation additive is in contrast to its performance as a sintering aid (Fig. l), suggesting that the mechanisms of these two processes are influenced as much by the chemistry of the system as by the formation of phases which ;ire liquid at the reaction temperature. The 29Si spectra of the samples containing A120, [Fig. 3(a),(b)]show no additional features attributable to aluminosilicate or oxynitride phases, but the spectra are not inconsistent with the presence of D' SiAION, in which the Si resonance occurs at -47.6 to -48.4 ppm,15 i.e. for practical purposes indistinguishable from P-Si,N,. The 29Si spectra of samples containing MgO [Fig. 3(c)] contain, in addition to the two polymorphs of Si,N4, an additional resonance at about -62 ppm, corresponding to that of f0r~terite.l~ The relative intensity of this I esonance decreases as the heating temperature increases, possibly due to the vapour-phase removal of MgO in the flowing nitrogen atmosphere at higher temperatures.These MAS NMR obser- vations are consistent with the XRD results. Samples containing 6.4 mol% Y203 showed no 29Si reson- ances other than those of Si,N4, despite clear XRD evidence J. MATER. CHEM., 1994, VOL. 4 IN N tf) I lB lBlB 10 20 30 40 50 60 70 80 28(Co-Ka)/degrees Fig. 2 Typical XRD traces for Si2N4 sintered for 2.0 h with various additives: (a) A1203, 1500-1600 "C; (b)A1,03, 1700-1750 T; (c) A1203, 1800°C; (d) MgO, 1600-1650°C; (e) Y203, 1500°C; (f)Y,03, 1600°C; (g)Y203, 1700-1750°C; (h)A1203+Y203, 1700'C.Additive contents: (u)-(d) 10 wt.%; (e)-(g) 15 mol%; (h) 10 mol% each component. Key to phases: A, r-Si3N4 (PDF no. 9-250), B. P-Si3N4 (PDF no. 33-1160); B', P'-SiAlON (PDF no. 25-1492); C, corundum (PDF no. 10-173); Si, silicon (PDF no. 27-1402); F, forsterite (PDF no. 34-189); J, J-phase (PDF no. 32-1450); N, N-melilite (PDF no. 28-1457); Y, Y,,A12Si3018N4 (PDF no. 32-1426). Table 2 Phase formation in Si3N4 sintered with additives, by XRD and MAS NMR additive" T/"C XRD phases NMR phases 1500 A, B, C 1600 B', AlN(tr) 1700 B', AIN 1750 B', AIN 1800 B', AlN, Si 1500 A, B, F, Mg-Si-0-N 1600 B, A, F, Mg-Si-0-N 1700 B, F(tr), Mg-Si-0-N 1750 B, Mg-Si-0-N 1800 B, Si, Mg-SI-0-N 1500 A, B, J 1600 B, J, N 1700 B, N 1750 B, N 1800 B, Si 1700 B', Mg-Si-(1-N, MgO 1700 B, N, Mg-Si-0-N, MgO 1700 B, y Key: A =r-Si3N4, B =P-Si3N4, B' =P'-SiAlON, C =corundum, (r-A103), F =forsterite (Mg2Si04), J =J-phase (Y4Si,0,N2), N =N-melilite (Y,Si303N4),Y =Y,,,A12Si30,,N4, A1N =aluminium nitride, Si =silicon, Mg-Si-0-N =glassy phase containing these elements.Tr =trace. a All additive concentrations 10 wt.%, except Y203 (15 mol%) and binary combinations (10 molyo each component). J. MATER. CHEM., 1994, VOL. 4 -48.9-4i 4 -4 -47 A (f) I, I1 I ,0#l.l.ltl~ -40 -60 -80 ' -20 -60 -100 -140 29~i6 wrt TMS Fig.3 Typical room-temperature 29Si MAS NMR spectra of Si,N, sintered with various additives at various temperatures: (a) A120,, 1500°C; (b) Al,O,, 1750°C; (c) MgO, 1500°C; (d) Y203, 1500°C; (e) Y,03, 1700 "C, (f)MgO +Al,O,, 1700"C.Additive content: (a)-@) 10 wt.%; (d)-(e) 15 mol%; (f) 10 mol% each additive. Recycle time for spectra (u)-(e),3000 s, recycle time for spectrum (f) 10 s. 100 80 h. $! 2 60 m.-? 4c 2c I I I I I IC 1 30 1600 1700 1800 TI% Fig. 4 Transformation of r-to P-Si,N, estimated by ,'Si MAS NMR, in the presence of 10wt.% of various additives, as a function of temperature, and 120 min dwell time at sintering temperature. Additives: (M)MgO, (+) Al,03, (A)Y203,(@) blank. of H-phase at 1500 "C and N-melilite at > 1600 "C By con- trast, the spectrum of the sample containing higher concen- trations of Y203 (15mol%) heated at 1500°C show, in addition to the double peaks of a-Si3N4, a weak resonance at -74.4 ppm [Fig.3(d)], corresponding to the reported position for J-pha~e.'~,'~ When the sample is heated at higher tempera- tures, this resonance is replaced by a broad feature at -57 ppm [Fig. 3(e)], corresponding to the reported position of N-melilite.'23'3 Thus, the 29Si NMR spectra of samples containing appreciable Y203 are consistent with 1 he XRD results, but at lower Y concentrations, the low sensitivity and broadness of these resonances makes MAS NMR 1t:ss useful than XRD as a diagnostic technique for the intergranular yttrium silicate phases. 27A1NMR The 27Al NMR spectra of A120, samples sintered for 2 h at various temperatures are shown in Fig.5. At 1500"C, the A1 is predominantly six-coordinate ( 14 ppm), corresponding to unreacted corundum, a-A1203 [Fig. 5(u)], but the smidl, broad resonance at 60 ppm indicative of tetrahedral A1 indicates that some degree of reaction has taken place. By 1600°C,this reaction is well advanced [Fig. 5(b)],the broad tetrahedral and octahedral resonances at 65 and 7 ppm, respectively, corresponding well with those of P'-SiAlON (66 to 69.1 and -6.6 to 10 ppm, re~pectively'~,~~). This spectrum also contains a hint of residual corundum at 12 ppm and a shoulder which may mark the beginning of the P'-SiAlON peak at 104 ppm; this resonance, which corresponds to A1-N bonding is much better developed at 1700°C [Fig.5(c)]. By 1750°C [Fig. 5(d)] the degree of A1-N bonding has become more significant, as reflected in the more highly positive shifts in all the resonances, and the dominance of the resonance at 113 ppm sug,gests the appearance of A1N as a discrete phase.lg By 18OO"C, A1N has become the only A1 phase detectable by NMR [Fig. 5(e)], the silica having been removed by the vapour-phase formation of SiO and the progressive reduction to elemental Si, confirmed by XRD and 29Si NMR. None of these samples >,how the 102.5 I I I I 1 r A" l l I I 1 I 1 L- 200 0 -200 200 0 -200 27AI 6 wrt AI(H20)6& Fig. 5 Typical room-temperature 27Al MAS NMR spectra of Si,N, sintered with 10 wt.% A1,0, for 120 min at (a) 1500'C, (0)1600"C, (c) 1700"C, (d) 1750"C, (e)1800"C.(f) Sample sintered with 10 mol% each of A1,0, and Y203for 120 min at 1700"C, (Y,oA12Si.,018N,). spectrum reported for a highly siliceous SiAlON glass,16 in which a resonance ascribed to five-coordinate A1 was found at 35 ppm, in addition to four- and six-coordinate resonances at 50 and 0 ppm, respectively. Since the principal reaction product found here (p-SiAlON) is itself refractory and difficult to sinter, the addition of A1203would not by itself be expected to assist sintering by the formation of any phase which is liquid at the sintering temperature. 2sMgNMR A selection of 25Mg MAS NMR spectra of samples containing MgO and sintered at various temperatures is shown in Fig.6. The low concentrations of Mg-bearing phases in these samples and the low sensitivity of "Mg make these natural-abundance spectra noisy, but at all temperatures, a sharp resonance is seen at ca. 33-36ppm, most probably resulting from the presence of unreacted Mg0.20,21 The other major feature of these spectra is the broad resonance at ca. -69 to -85 ppm, the origin of which was the subject of several further experi- ments. The XRD and 29Si NMR results suggest that the crystalline Mg-bearing phase in these samples is forsterite, Mg,SiO,. A similar broad feature at about this position has been observed in chrysolite heated to 850 "Cwhich contained a significant amount of crystalline forsterite;21 it is unlikely, however, that the 25Mg resonance in that sample was due to forsterite, and probably arises from some Mg-bearing impurity. To examine this point further, mixtures of MgO and Si02 in molar proportions corresponding to both forster- ite and enstatite (MgSiO,) were melted in air at 1600 "C and quenched in water.XRD and 29Si NMR of these materials showed them to be very crystalline forsterite, with unreacted MgO also present, but the 25Mg spectra showed only a single sharp resonance due to MgO. A similar 25Mg result was found for a sample in which part of the SiO, was replaced by Si,N4, which also contained principally forsterite, according to XRD. These results suggest that forsterite is not responsible for the "Mg spectra of these samples.A similar broad feature at ca. -50 ppm was found in equimolar mixtures of Si3N4 I I I , I , I , 1-A I , I , I , I , I 400 0 -400 '400 0 -400 25Mg6 wrt MgS04 solution Fig. 6 Typical room-temperature 25MgMAS NMR spectra of Si,N, sintered with 10wt.% MgO for 120 min at (a) 1500"C, (b) 1600"C, (c) 1700'C. (d) Equimolar mixture of Si,N, and MgO reacted in air for 120 min at 1650°C. (e) Synthetic crystalline MgSiN, and ( f)MgAISiN,. J. MATER. CHEM., 1994, VOL. 4 and MgO fired at 1650 for 2 h both in nitrogen and air [Fig. 6(d)] which according to XRD contained only Si3N, and a small trace of forsterite. The broad ,'Mg spectrum therefore appears to be associated with the X-ray amorphous or glassy phase predicted from the phase diagra~i~.~ This phase may contain four-coordinate Mg; the broad resonance at 62 ppm [Fig.6(d)] is in the region reported for tetrahedral Mg in spinel, MgA12022 and in akermanite, C'a,MgSi,O, .27 The "Mg resonance arising from six-coordinate Mg in the sintered silicon nitride samples falls in a similar range as that of MgSiN, [Fig. 6(e)] and MgA1SiN3 [Fig. 6(j')], suggesting that the Mg-containing glassy phase is stabilised by nitrogen. The results of heating equimolar Si,N,-MgO mixtures suggest that the amount of glassy phase formed is not restricted by the amount of Si available, as would be expected if the sole Si source was the oxidised surface layer on the nitride particles, and that further reaction with Si,N4 was viir the molten magnesium silicates thus formed.2' Rather, the glassy phase appears to be the product of a direct reaction of MgO with the Si3N4 itself, with the small amount of forsterite also produced resulting from a different reaction with the surface SO2, since, by contrast with the glassy phase, its concentration is essentially independent of the concentration of available MgO.Thus, to summarise, the mechanism by which MgO assists sintering appears from 29Si NMR and XRD (but not "Mg NMR) to involve the formation of some forsterite, probably by reaction between the MgO and the oxidised surface layer of the nitride. However, the 25Mg NMR spectrum provides evidence of further reaction between the MgO and Si,N, to form a glassy Mg-Si-0-N phase which is not readily recrys- tallised, and at higher MgO concentrations contains appreci- able proportions of four-coordinate Mg."Y NMR Fig. 7 shows typical *'Y MAS NMR spectra of silicon nitride samples containing 15 mol% Y203, with 3 wt.O/,l Yb,O, added before firing to facilitate the acquisition of the spectra by A , I I I I I L 800 400 0 -400 89~ solution6 wrt YCI~ Fig. 7 Typical room-temperature *'Y MAS NMR spectra of Si,N, sintered with 15 mol% Y,O, and 3 wt.% Yb,O, for 120 min at (u) 1500"C, (b)1600"C, (c) 1750°C. J. MATER. CHEM., 1994, VOL. 4 shortening the "Y relaxation time. The spectra are all very broad. and similar to those observed in equimolar mixtures of Y203 and Si3N4 heated together.23 The position of the 89Y resonance in the sample heated at 1500 "C [Fig.7(a)] is close to that reported for the J-phase (202 ~pm~~), consistent with the 29Si NMR and XRD results (Table 2). As the heating progresses to higher temperatures, the position of the centre- of-gravity of the broad "Y resonance tends downwards, able. This is confirmed by the similarity of the 29Si spectrum to that of the Y203-containing sample [Fig. 3(e)],although if forsterite is present its resonance at -62 ppm could be obscured by the broad N-melilite feature centred at -57 ppm. The 25Mg spectrum is similar to Fig. 6(c), showing the reson- ances of unreacted MgO and the broad resonance centred at ca. -70 ppm; possible confirmation that this represents an N-stabilised Mg glass was found in the 29Si spectrum acquired towards the position reported for pure N-melilite (185 ~pm~~).with a delay of 10 s [similar to Fig. 2(f)], which shows a Concomitantly, minor spectral features such as that at 526ppm and a possible shoulder upfield of the major peak become more easily distinguishable [Fig. 7(b),(c)]; these fea- tures are associated with the spectrum of N-melilite.23 Thus, although the usefulness of the 89Y MAS NMR spectra is limited by their broadness, their general features support the conclusions of 29Si NMR and XRD, that the intergranular phases formed during sintering with Y203 at lower tempera- tures are Y-rich, but become progressively Si-rich as the temperature is raised. This sequence is at variance with that proposed by Hirosaki et ~l.,~who reasoned that if the reaction is between the surface Si02 layer and the Y,03, the liquid initially formed should be Si-rich, and become progressively Y-rich with increasing temperature.It is, however, clear the sole source of Si is not the surface oxide, and significant reactions occur between Y203 and Si3N4, especially during the comparatively long soaking times of the present experi- ments. The reaction products also appear to be dependent on the reaction atmosphere, being different and less reproducible when the heatings are carried out in a carbon pot. The phases detected by MAS NMR of the various nuclei are summarised in Table 2. Effect of Additives in Combination A brief survey was made in which the techniques developed above were applied to more complex Si3N4 systems containing 10 mol% each of the additive pairs A1203/Y203, MgO/Y203 and A1203/Mg0, heated at 1700°C for 2 h.The samples containing Y203were prepared both with and without 3 wt.% added Yb203, the former samples being used for ''Y MAS NMR. The sample containing A1203/Mg0 shrunk radially by 19.5% on firing, corresponding to a final bulk density of 2.70. The only crystalline phase detectable by XRD was P-Si3N4, with no crystalline forsterite, spinel, A1203 or MgO found. The 29Si NMR spectrum acquired using a long delay (3000 s) shows only the expected silicon nitride spectrum, but using a shorter delay (10 s) and acquiring more transients, an additional broad component appears, centred at ca.-100 ppm [Fig. 3(f)]. The 27Al NMR spectrum is very similar to that of a corresponding sample containing A1203 alone [Fig. 5(c)], except for an upfield displacement of all the resonances by about 10 ppm. There was no indication of the reported resonance of Mg5A1Si30,,N at 67 pprn." The 25Mg NMR spectrum is similar to those of samples containing MgO alone [Fig. 6(c)], containing a sharp resonance at ca. 30 ppm corresponding to unreacted MgO, and a broad feature centred at ca. -50 tentatively ascribed to an N-containing Mg-Si glass. Thus, the sintering action of this pair of additives appears to consist of a combination of their individual reac- tions (i.e. SiAlON formation by the A1203 and formation of an N-stabilised Mg glass by the MgO), with no evidence of the interaction of the additives to form new phases.The combination of MgO with Y203 resulted in a radial shrinkage and final bulk density of 15.8% and 2.5 gcmV3, respectively. XRD detected only the crystalline phases present in samples containing Y203alone (P-Si3N4 and N-melilite), with no forsterite or any other Mg-containing phase detect- broad feature centred at ca. -100 ppm and possibly associated with this glassy phase. If, as has been suggested," the composi- tion of the amorphous phase formed in this system includes Y, in addition to Mg, Si, 0 and N, its presence does not significantly change the 25Mg and 29Si spectra. Thus. as is the case with MgO/A1203, the additives appear to be acting independently of each other, at least with respect to the formation of the crystalline intergranular phases.By contrast, the addition of A1203 and Y203, wliich gives a radial shrinkage and bulk density of 20.3% and 2.74 g cmP3, respectively, results in the formation of the phase Y,OA12Si30,8N4 (PDF no. 32-1426). The 29Si spectrum shows, in addition to an intense P-Si3N4 resonance at -48.8 ppm, another peak at -76.1 ppm, which apparently coi responds to the previously unreported spectrum of Y 10A12Si3018N4. The 27Al spectrum [Fig. 5(f)] is significantly different from that of the corresponding sample containing A1203 alone [Fig. 5(c)]; both spectra contain a broad resonance at ca. 60 ppm and a sharp peak at ca. 104 ppm, but the A1203/Y203 spectrum also contains a significant resonance at .?0.7 ppm, and no resolved octahedral resonance at ca.15 ppm. Resonances at 20-30 ppm are often ascribed to five-cc lordinate Al, but the only previous report of a resonance in this position in N-containing systems is in a highly siliceous SiAlON glass of approximate composition Si24A16050N.'6 Since the only crystalline phase other than P-Si3N4 identified in this sample is Y,oA12Si3018N4, Fig. 5(f) probably represents the pre- viously unreported 27Al spectrum of this compound. The combined evidence suggests that this additive pair reacts to form a new phase, which may behave similarly to the related yttrium phases formed with Y,03 alone, while possessing some of the structural features of the SiAlON glass in which the similar A1 resonance has been reported.This result is different from that reported by Carduner et who detected Y,Si3OI2N (H-phase) and YSi02N (K-phase) by hoth 29Si NMR and XRD in solid Si3N4 samples sintered with A1203 and Y203. In other experiments with A1203/Y203 ;idditions sintered in a carbon pot, we detected the formation of Y2Si303N4 (but not H-phase or K-phase); the expt.rimenta1 sintering conditions thus clearly play a significanI role in determining the nature of the intergranular phases fix-med. Conclusions 1. Multinuclear MAS NMR in conjunction with XRD is capable of providing useful information about both the degree of a-P transformation and the nature of the intergranular phases formed when Si3N4 is sintered in the presence of additives.At ca. 1800 "C the system becomes unstable, decom- posing to elemental Si, especially when the reaction 1s carried out in a carbon pot. 2. When used alone, A120, enters the Si,N, structure at >1600"C, forming a low-z P'-SiAION which decomposes to A1N with the loss of SiO at ca. 1800°C. 3. Below 1750"C, MgO reacts with the residual SO2 in the system to form forsterite which is detectable by XRD and 29Si NMR but not by "Mg NMR; the latter suggests, however, that MgO also reacts with the Si3N4 to form m X-ray amorphous phase, suggested to be an Mg-Si-0-N gldss, from 1602 J. MATER. CHEM., 1994, VOL. 4 the similarity of its ,'Mg spectrum with that of MgSiN,.The 29SiNMR spectra suggest that this glassy phase has a short 29Si relaxation time. 4. The use of Y,O, alone produces intergranular Y-Si-O-N phases which change in composition with tempera- 5 6 7 Ceramics, ed. R. E. Tressler, G. L. Messing, C. G. Pantano and R. E. Newnham, Plenum Press, New York, 1986, p. 79. K. H. Jack, J. Muter. Sci., 1976, 11, 1135. N. Hirosaki, A. Okada and M. Mitomo, J. Muter. Sci., 1990, 25, 1872. 0.Abe, J. Muter. Sci., 1990,25,4018. ture. At 1500"C the yttrium-rich J-phase was found in samples containing 15mol% Y203, progressively converting to the more siliceous N-melilite at 1600"C. The "Y spectra, which could be detected only in samples containing added Yb203 to decrease the relaxation time, are all very broad, but in 8 9 10 G.M. Crosbie, J. M. Nicholson and E. D. Stiles, Bull. Am. Ceram. SOC.,1989,68, 1202. A. Gianchello, P. C. Martinengo, G. Tommasini and P. Popper, Bull. Am. Ceram. Soc., 1980,59, 1212. K. R. Carduner, R. 0. Carter 111, M. E. Milberg and G. M. Crosbie, Anal. Chem., 1987,59,2794. general confirm the XRD and ,'Si NMR results. 5: In terms of intergranular phase formation, the additive pair Al,O,/MgO behaves as separate entities, the A1,0, forming /j'-SiAlON and the MgO forming a glassy phase, with no evidence of spinel or other A1-Mg phase formation. The same is true of MgO/Y,03, in which the same Y-Si-O-N 11 12 13 14 G. H. Hatfield and K. R. Carduner, J. Muter. Sci.. 1989,24,4209. K. R. Carduner, R. 0. Carter 111, M. J. Rokosz, C. Peters, G. M. Crosbie and E. D. Stiles, Chem.Muter., 1989, 1, 302. R. Dupree, M. H. Lewis and M. E. Smith, J. Am. Chem. Soc., 1988, 110,1083. D. S. B. Hauck, R. K. Harris, D. C. Apperley and D. P. Thompson, J. Muter. Chem., 1993,3, 1005. phases are formed as with Y203 alone, together with the Mg-containing glass. By contrast, in samples containing Al,03/Y,03, an Al-Y-Si-O-N phase is found, with a 27Al NMR spectrum similar to that of a SiAlON glass, containing a resonance ascribed to five-coordinate Al. 15 16 17 18 R. Dupree, M. H. Harris, G. Leng-Ward and D. S. Williams, J. Muter. Sci. Lett., 1985,4, 393. R. K. Sato, J. Bolvin and P. F. McMillan, J. Am. Cerum. Soc., 1990,73,2494. J. S. Hartman and R. L. Millard, Phys. Chem. Mineral., 1990, 17, 1. M. Magi, E. Lippmaa, A. Samosan, G. Engelhardt and A-R. 6. The present NMR and XRD results are consistent with expectations from previously published phase diagrams for these systems. 19 20 Grimmer, J. Phys. Chem., 1984,88,1518. R. Dupree, M. H. Lewis and M. E. Smith, J. Appl. Crystallogr., 1988,21, 109. R. Dupree and M. E. Smith, J. Chem. Soc., Chem. Commun., 1988, 1483. We are indebted to Dr. W. A. Groen, Philips Research Laboratories, Eindhoven, for the samples of MgSiN, and MgAlSiN,. 21 22 23 K. J. D. MacKenzie and R. H. Meinhold, Am. Mineral., 1994, 79, 250. R. Dupree and M. E. Smith, Chem. Phys. Lett., 1988,148.41. R. H. Meinhold and K. J. D. MacKenzie, to be published. 24 S. Dutta, J. Am. Ceram. SOC.,1982,65, C2. References 25 T. Ekstrom, P. 0.Ka11, M. Nygren and P. 0. Olsson, J. Muter. Sci., 1989,24, 1853. 1 G. G. Deeley, J. M. Herbert and N. C. Moore, Powder Met., 1961, 26 J. Sjoberg, R. K. Harris and D. C. Apperley, J. Muter. Chem., 1992,2,433. 2 8, 145. K. H. Jack, in Nitrogen Ceramics, ed. F. L. Riley, Martynes Noordhoff, Leyden, 1977,p. 109. 27 28 P. S. Fiske and J. F. Stebbins, Am. Mineral., to be published. D. R. Messier, F. L. Riley and R. J. Brook, J. Muter. Sci., 1978, 13, 1199. 3 4 D. P. Thompson, in Tailoring Multiphase and Composite F. L. Harding and R. J. Ryder, Glass Tech., 1970,11,54. Paper 4/02639J; Received 4th May, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401595

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(10), 1603-1610 New Routes to Alkali-metal-Rare-earth-metal Sulfides John P.Cotter, Jonathan C. Fitzmaurice and Ivan P. Parkin* Department of Chemistry, Christopher lngold Laboratory, University College London, 20 Gordon Street, London, UK WCIH OAJ Ternary alkali-metal-rare-earth-metal sulfides, MRES, (M =Li, Na, K; RE =Y, La-Yb) were prepared by heating RECI, and M,S under an H,S-N, atmosphere at 750-800 "C for 10 min. The ternary sulfides form thin, mostly colourcrd plates. The structures were characterised by X-ray powder diffraction, which revealed high-temperature cubic modifications for LiHoS, and LiErS,. The disordered cubic form (NaCI type) was observed for LiLnS, (Ln =Pr-Er) and NaLnS, (Ln = La-Nd), while the ordered rhombohedra1 form (a-NaFeO, type) was observed for LiYbS,, NaLnS, (Sm-Yb) arid KLnS, (La-Yb).Alkali-metal-rare-earth-metal sulfides may also be synthesized from the rare-earth-metal chloride and alkali- metal halide under comparable conditions. Similarly, thermolysis of the rare-earth-metal sesquisulfide (y-RE,S3). oxysulf-ide (RE202S)or oxychloride (REOCI) with M2S or MCI in an atmosphere of H2S-N2 gives MRES,. The materials were characterised by powder X-ray diffraction, scanning electron microscopy (SEM), energy dispersive analysis by X-rays (EDXA), Raman, infrared, magnetic moments and microanalysis. Rare-earth-metal chalcogenides (e.g. RE2E3; RE =Y, La-Yb; E =S, Se) and ternary metal rare-earth-metal sulfides MRES, (M: Groups 1-3) are refractory materials that have a number of potential and actual technological uses, ranging from infrared lenses' (transmission range ca.1-14 pm), catalysts,2 colour phosphors3 (e.g. television sets utilize Eu-doped Y oxysulfides for the red pixels), lasers (neodymium ~ulfide),~ semiconductor dopants to ionic conductor^.^ Alkali-metal-rare-earth-metal sulfides MRES, are tra-ditionally prepared by high-temperature (900-1 100 "C) pyrol- ysis of an alkali-metal source and rare-earth-metal oxide under an atmosphere of H,S. Sodium lanthanide sulfides were prepared by direct combination of the elements at 800°C for 7 days in the presence of an excess of NaCl (Na,LnCl, also observed, Ln =lanthanide); lower ratios of NaCl resulted in the sesquisulfide.6 Lithium lanthanide sulfides (Ln =Pr-Dy) have been prepared in the cubic modification, from the reaction of a mixture of lithium carbonate and lanthanide oxide with H2S, at 900 "C for 24 h.7 This method was also utilised for the preparation of MLnS2 (Ln =lanthanide, M = Rb, Cs').Two solution-based coprecipitation methods have also been utilised: reaction of lanthanide nitrates in nitric acid with alkali-metal hydroxides' and lanthanide oxides in chloric acid with sodium chloride." Thermolysis of the precipitates at 900--1200"C in an H2S atmosphere for 2-24 h produced MLnS,. In the reactions that utilised chloric acid, a contami- nant of LnOCl was found in the product that could be removed by pretreatment with H2 and ammonium chloride at 450 'C.Lanthanum chalcogenides may also be synthesized via molecular precursor routes, where the reaction of lanthanum trisdialkylamides with H2Sin benzene followed by thermolysis in H2S produced La2S3.11 Kaner has shown that solid-state metathesis can offer an alternative method of synthesizing metal sulfide^.'^,^^ MoC1, and Na2S undergo a spontaneous exothermic reaction to produce NaCl and MoS,, with temperatures in excess of 1000"C being generated. The coproduced sodium chloride is readily removed by trituration to leave phase-pure metal sulfide. Extensions to this reaction have yielded a range of transition-metal chalcogenides and main-group s~lfides.'~ We have utilised metathesis and extended its usage to the synthesis of lanthanide and transition-metal nitrides,15 phosphides,16 arsenides and stibides." In most cases the reaction is initiated by minimal energy input such as a hot wire, grinding the reagents or by brief heating in a microwave or conventional oven.The reaction is self-sustaining once initiated and is often complete in 2-3 s, as shown by the accompanying thermal flash. We have also investigated the reaction of alkali-metal sulfides with rare-earth-metal halides in an attempt to broaden the range of materials available from the self-propagating route. Compared to the formation of lanthanide pnictides," reaction of rare-earth-metal halides with M2E (M ==Li, Na, K) are slow and not accompanied by a thermal flash. Indeed, thermolysis of the reagents at 550-700 "C in quartz ampoules was required and the products from the reactions were often contaminated with oxygen either in the form of REOCl or RE,02S (even after carbon coating of the ampoule).In this paper we report on an extension to these expcriments, where the metathesis reactions were carried out under a stream of H,S-N,, to produce rapidly a range of single phase- pure MRES, materials (depending on the stoichiometric ratio of REC1, to M2S). Experimental All reactions and reagent preparations were carried out under anaerobic or vacuum conditions. Glassware and quai tz/Pyrex ampoules were flame-dried before use. All solveiits were degassed with nitrogen. Tetrahydrofuran (thf) yas distilled from sodium-benzophenone and stored over 4 A niolecular sieves; methanol was disdilled from magnesium (activated with I,) and stored over 3 A molecular sieves.Rare-ear th-metal trichlorides, sulfur and alkali-metal halides were purchased from Aldrich and Strem Chemicals; lithium, sodium and potassium metal from BDH. All chemicals were used as received. Hydrogen sulfide (99.9%) was purchascd from Matheson Ltd. Alkali-metal chalcogenides M,E (M := Li, Na, K; E =S, Se), (Mo,5M'o.5)2E (M, M' =Li, Na and Li K) and Li2(Eo,5E'o.5)(E, E'= S, Se) were prepared from stoichiometric amounts of the elements in liquid ammonia under an inert atmosphere. Rare-earth-metal sesquisulfides (RE2&) were made by reaction of rare-earth-metal halides with H2S at 800 "C. Rare-earth-metal oxysulfides (RE202S) and oxychlor- ides (REOCl) were obtained from thermolysis of REC1, and Na2S in evacuated quartz ampoules at 550-700 "C;the phases were identified by powder XRD.X-Ray powder diffraction measurements were recorded on a Siemens D5000 difTactometer using nickel-filteretl Cu-Ka radiation (%=1.5406 A) as finely dispersed powders on a silicon mirror. An external NBS 6100 silicon standard was used for calibration. Diffraction patterns for single phases were indexed using the exhaustive (TREOR) approach. Crystallite sizes were determined using the Scherrer equation" and referenced to a standard KC1 pattern. SEM profiles and EDXA were performed on a JEOL JSM 820 instrument using the Kevex detection system (Quantum detector Kevex delta 4 and Quantex 6.2 software).Infrared spectra were recorded on a Nicolet 205 (CsI) using CsT pellets (100-600 cm-I). Raman spectra were recorded on a Dilor XY spectrometer. The samples were measured as powders dispersed at the focal point of the microscope attachment. The 514.53 nm line of an argon laser (50 mW) was the excitation source; the slit width was 300 pm. Magnetic moment measurements were measured on a Johnson Matthey balance (by the Evans method). Sulfur microanalyses were determined by the departmental service at University College. Thermolysis studies were accomplished in a brick tube furnace and the temperature was monitored by an external thermocouple; the apparatus is represented schematically in Fig.1. The thermolysis system, comprising a quartz tube and alumina boat were heated to 750 "C under vacuum and cooled to room temperature under a flow of nitrogen prior to the addition of reagents. Reactions were initiated in this apparatus at 750-800 "C under a flow of H2S-N, (30 :70) for a period of 10min. The products were allowed to cool under the H,S-N, flow. Preparation of MRES2from RECl, and M2Sin H2S(M =Li, Na, K; RE =Y, La-Yb) Alkali-metal sulfide M,S (1.5, 3.0 and 5.0mmol for M=Li, Na and K, respectively) and anhydrous rare-earth-metal tri- drying agent p205 I1I uu MI bubbler NaOH PbfOAc), Fig. 1 Schematic diagram of the thermolysis apparatus J. MATER. CHEM., 1994, VOL. 4 chloride (1.0 mmol) were ground together in an agate pestle and mortar.The powder was placed in an alumina boat inside a quartz tube that was sealed by Youngs taps. The tube was connected to the H,S-N, system as shown in Fig. 1. The apparatus was heated to 800 "C over 30 min under a flow of H,S-N, and maintained at that temperature for 10 min before it was cooled to room temperature. The resulting fused mass was triturated with thf (50ml) (for LiRES,), methanol (NaRES,) or water (KRES,) to remove the coproduced alkali- metal halide and any remaining M2S. The (mostly coloured) powders were then collected by filtration, washed with acetone (20 ml) and diethyl ether (20 ml) and dried in L'CICUO. The alkali-metal lanthanide sulfides were characterised by X-ray powder diffraction and magnetic moments measurements (Tables 1-3), Raman, infrared, SEM/EDXA (Fig. 2) and sulfur microanalysis.Sulfur microanalyses [%(calculated value in parentheses)] are as follows: LiSmS,, 27.7 (28.9); NaCeS,, 26.5 (28.2); KSmS2, 23.6 (25.1); LiHoS,, 26.7 (27.2); NaSmS,, 24.6 (26.8); KYbS,, 22.1 (23.1). [The samples analysed by microanalysis were also examined by EDXA which showed no oxygen present (1-2% detection limit).] Preparation and isolation of mixed alkali-metal lantha- nide sulfides of the form (Li, -xNax IRES, and (Li, -,K,)RES, (Table 4) were as described above using RECl, and Lio.5Nao.5S2 as starting materials. and Lio,5Nao,5S2 W1Oprn Fig. 2 SEM profile of KPrS, after washing Table 1 X-Ray powder diffraction data and magnetic moment measurements for LiRES, obtained from thermolysis of a mixture of REC1, and Li,S (2 :3) in an H,S-N, atmosphere at 750-800 "C phase colour of detected material LiYS, white La33 yellow Ce,S, red Pr2S3' lime green LiNdS, light green LiSmS, pale yellow LiEuS,' metallic grey LiGdS, white LiTbS, pink LiDyS, white LiHoS, pink LiErS, white LiYbS, yellow 'An eTcess of Li,S @a.5 x) give! Pr,S, 5.930 A). c =18.68 A. c =18.63 A. e = lattice/space group measured (-t0.005) ref. 7 PclbS(* 0.08)/ PB PcadM3+I/PB cubic, Fm3m 5.461 5.473 0 0 cubic, 1*3 8.722 8.727 0 0 cubic, 143d 8.626 8.636 2.12 2.11 8.567 8.576 3.08 3.58 5.613 5.628 3.61 3.62 5.555 5.588 1.35 0.85 cubic Fm3m 5.553 5.514 5.497 5.606 5.530 5.505 4.80 7.28 9.13 0 (Eu2' 7.94) 7.94 9.72 5.462 5.474 9.93 10.63 5.429 3.898' 10.11 10.61 5.415 3.875d 8.93 9.59 hexagonal, R3m 3.808' 3.842f 3.89 4.54 and LiPrS, (cubic, a =5.677 A;ref.a =5.687 A). Moisture-sensitive; decomposes to EuS (cubic, a = 18.560 A. c=18.54 A. J. MATER. CHEM., 1994, VOL. 4 1605 Table 2 >;-Ray powder diffraction data for NaRES, obtained from thermolysis of a mixture of REC1, and Na,S (1 : 3) in an H,S-N, atmosphere at 750-800 ^C lattice parameter/A measured' ref. 7 phase (s) colour of lat tice/space detected material group a C (I C NaY S, white hexagonal, R3m 3.936 19.832 3.968 19.89 NaLaS, yellow 5.868 5.881 NaCeS, red cubic, Fm3m 5.819 5.832 NaNdS, pale green 5.767 5.768 NaSmS, white 4.018 19.846 4.056 19.87 NaEuS, orange 4.035 19.916 4.042 19.92 NaTbS, white hexagonal, Rjm 3.988 19.880 3.989 19.87 NaHoS, white 3.934 19.870 9.949 19.86 NaYbSZb yellow 3.904 19.869 3.902 19.91 NaLaS, + yellow cubic, Fm3m 5.868 5.881 La,% cubic, 1_*3 8.720 8.727 y-Pr,S, lime green cubic, Z43d 8.567 8.576 NaNdS, + green cubic, Fm3m 5.767 5.768 -;'-Nd,S, cubic, 143d 8.503 -8.525 NaSmS, + yellow hexagonal,_ R3m 4.01 8 19.846 4.056 19.87 y-Sm,S, cubic, Z43d 8.43 1 8.437 The last four entries list multiphase products obtained from LnCl,/Na,S (2 : 3)." k0.005 A. NaYbS, (u= 3.897 A,c= 19.762 A)obtaiaed from Yb,02S/'3Na,S/H,S. Table 3 X-Ray powder diffraction data for KRES, obtained from thermolysis of a mixture of RECl, and K,S (1 : 5) in an H,S-N, attnosphere at 750-800 'C lattice parameter/A measured" ref.7 -phase(s) colour of lattice/space -detected material group a C U C KYS, white 4.020 21.862 4.023 21.85 KLaS, yellow 4.257 21.874 4.264 21.89 KPrS, pale green 4.190 21.858 4.185 21.75 KSmS, white hexagonal, R3m 4.190 21.857 4.107 21.76 KEuSz red-brown 4.094 21.841 4.094 21.85 KDYS, brown 4.029 21.858 4.030 21.83 KHoS, white 4.005 21.817 4.010 21.80 KYbS, yellow J 3.968 21.841 3.964 21.82 -y-La,S3 yellow cubic, 143d 8.720 -8.727 + x-La,& orthorhombi_, Pnum 7.577h 4.150 7.584' 4.144 -;'-Sm,S, orange cubic, 143d 8.43 1 -8.437 + x-Sm,S, orthorhombic, Pnam 7.379d 3.981 7.382' 3.974 The last tyo entries list multiphase products obtained from LnCl,/K,S (2: 2)." f0.005 A. ' b= 15.870 A. ' b= 15.860 A. h= 15.381A. h = 15.378 A. Table 4 X-Ray powder diffraction data for MI-,M,LnS, obtained from thermolysis of a mixture of RECl, and M,-,M',S (1 : 3) in ari H,S-N, atmosphere at 750-800 'C measured latLice parameter/A" phase colour of lattice/space unit-celb detected material group a C v~,)lume/A~ (Li,Na)SmS, white 4.019 19.557 273.6 ( Li,Na)ErS, hexagonal, Rjm 3.888 19.155 253.9 (Li,Na)YbS, green 3.874 19.223 249.8 (Li,K)SmS, green 3.883 19.632 256.4 (+ ;I-Sm2S3) (Li,K)TbS, grey hexagonal, R3m 3.894 19.048 250.1 ( Li,K)YbS, green 3.850 18.585 238.61 " f0.005 A. Alternative Methods of Preparing MRES, (i) Reaction of ground powders of lanthanide halides and alkali-metal hlaides ( 1:4) under comparable conditions (H,S, 8OO"C, 20min) was also found to produce MLnS,.The MLnS, materials were isolated and analysed, as previously described for reactions involving M,S and LnCl,. (ii) Reaction of rare-earth-metal sesquisulfides (yRE,S,), oxysulfides (RE20,S) or oxychlorides (REOC1) with an excess of M2S or MCl (ca. five-fold) in an atmosphere of H2S-N, (H,S, 800 "C, 20 min) gives MRES,. (iii) Reaction of rare-earth-metal halides with lithium selen- ide Li,Se and H2S under comparable conditions produces LiRES, (and no LiRESe,). For lithium sulfide selenide Li2(So.5, Se,,,), the products ranged from the sesquisulfide (y-Nd,S,) to a mixture of phases (y-Sm,S, and LiSmS,) to LiGdS,.Results and Discussion Reaction of rare-earth-metal halides with M,S (M =Li, Na, K) at 750-800 "C for 10 min under a flow of H,S produces crystalline alkali-me tal-rare-eart h-me tal sulfides, rare-earth- metal sesquisulfides RE,& or a mixture of the two phases which is dependent on the stoichiometry. Optimisation of the stoichiometry produces the single phase MRES, with yields being virtually quantitative. The idealised reactions may be summarised as follows (note: these equations are balanced for the initial ratios of metal halide to alkali-metal sulfide actually used; Ln stands for the cases where the reaction does not involve Y and RE stands for the cases which include Y and the lanthanides. The reactions may not be due totally to the direct exchange of the ions, uide infra). H,S/750 "C2LnC1, +3Li2S -y-Ln,S, +6LiC1 (1)(Ln =La, Ce, Pr) (In the case of PrC1,+5Li2S, a mixture of the sesquisulfide (y-Pr2S3) and LiPrS, was observed.) H,S:750 'C2REC1, +3Li,S +H,S -2LiRES, +4LiC1+ 2HC1 (RE=Y, Nd-Yb) (2) H,S/750 "C4LnC1, +6Na,S +H,S -2NaLnS, +yLn,S3 + lONaCl +2HC1 (3)(Ln =La, Nd, Sm) H Si750 CREC1, +3Na,S -NaRES, +3NaC1+ (3Na) (4)(RE=Y, La-Yb) H S'750'C2LnC1, +3K2S -Ln,S, +6KC1 (5)(Ln =La, Sm y and a phases) H,S/750 'CRECl, +3K,S -KRES, +3KClf (3K) (6)(RE=Y, La-Yb) SEM analysis of the alkali-metal-rare-earth-metal sulfides indicates that prior to trituration some MCl is evident.Washing removes the alkali-metal halide to reveal either uniform cubes or flat hexagonal crystallites of typical dimen- sion 10 pm (Fig.2). Analysis of the backscattered electrons indicates a homogeneous sample composition. Compositional EDXA studies of the triturated material, both as single spots and maps across the surface gave only the rare-earth metal, alkali metal (for sodium and potassium as lithium below the threshold detection limit) and sulfur and matched within experimental error (& 3% of stoichiometric ratio) the single J. MATER. CHE.M., 1994, VOL. 4 phases detected by powder X-ray diffraction. Chlorine was evident by EDXA in the unwashed material. In none of the samples was oxygen observed by EDXA (1 -2% detection limit). The X-ray powder diffraction patterns (Fig. 3) for MRES, showed two different phases: the disordered cubic (NaC1 type) and hexagonal (a-NaFeO,) lattices consistent with the litera- t~re.~The rare-earth-metal and alkali-metal cations occupy random (Na') sites in the NaCl lattice; the hexagonal phase is derived from a rhombohedral deformation of the cubic lattice with a cubic close-packed S2-arrangement with octa- hedral voids occupied by Na' and RE3+ and alternating cation layers perpendicular to the threefold a xis.The cubic lattice parameter decreases in a nearly linear fashion as the ionic radii of the rare-earth-metal decrease; a similar trend is observed for the hexagonal lattice u parameter. An exception is noted for europium, where it is possible that Eu3+ and Eu2+ coexist in both LiRES, and NaRES,.The hexagonal lattice parameter c remains essentially constant as a function of the lanthanide ionic radii. In contrast to the previously observed hexagonal phases for LiHoS, and L~EI-S,,~ high-temperature cubic modifications were observed. A comparison of the lattice constants for the cubic (a,) and hexagonal (ah) cells7 reveals that ah=a, J2/2 and Ch=uC2J3. This suggests that the cubic structure has a marked structural resemblance to the rhombohedral structure. Observed and calculated intensities for the hexagonal phase were qualitatively compatible, except for (001) lines, where prefered orientation of the crystallites as thin platelets parallel to the (001) direction leads to enhancement of these lines. The crystallite sizes for LiRES,, determined by the Scherrer equation" from the powder diffraction line uidths and com- pared too a standard KC1 pattern, were of the order of 520-600 A.CrystalliteQ sizes for NaRES, and KRES, were in the range 300-400A. Where the sesquisulfide was an additional phase [Fqn. (3) and (5)], the crystallite size was reduced (250-350 A). These crystallite sizes are comparable to those obtained by metathetical reaction^.'^.^^ A general reduction in crystallite size is observed on moving form LiRES, to NaRES, for comparable reaction conditions and may be due to the better diffusion of lithium in the melt in forming the material. The formation of mixed alkali-metal lant hanide sulfides offers the possibility of altering electronic properties.It was possible to prepare compounds of the type Li,-,Na,LnS, and Li, -,K,LnS, for Ln =Sm-Yb (Table 4). There is a trend towards the formation of LnS, (x =1.5-1.75) for the lighter lanthanides (eg. y-La,S,, y-Ce,S, and Nd,S,). The lattice types all belong to the hexagonal R3m phase and show a reduction in unit-cell volume from NaLnS, (5%) in the case of Li,-,Na,LnS, (Ln=Sm, Er, Yb) and from KLnS, (20%) for Li, -,K,LnS, (Ln =Sm, Tb, Yb). The individual phases, e.g. LiYbS, and KYbS,, are both yellow; the mixed phase is green and suggests the modification of the optical and elec- tronic properties from the individual compouFds. In both cases the crystallite size was reduced (100-250 A). The MRES2 materials and y-RE,& phase materials were also characterised by FTIR and Raman spectroscopy.The y-sesquisulfides, which belong to the cubic Th,P, type structure, all have similar Raman spectra with wavenumbers which decrease in a nearly linear fashion with lanthanide ionic radii. Disorder (i.e. randomly distributed vacancies) in the structure leads to broadening (20-40 cm-') of certain lines. The band in the region 230-250cm-' may be assigned to the A,, mode;,' it is somewhat narrower than other bands but is still very broad compared to other sesquisulfide phases. This band is also evident in the cubic NaCl type LiLnS, phases, with very broad band widths (ca. 80cm-'), consistent with a J. MATER. CHEM., 1994, VOL. 4 20 30 40 50 60 70 80 90 2Wdegrees Fig.3 X-Ray powder diffraction pattern for LiYbS, (a), NaYbS, (b)and KYbS, (c). Note only the first 20 lines were indexed on this pattern. disordered structure and an observed increase in the wave- number from Nd to Er (e.g. LiSmS,, 246cm-'; LiDyS,, 264 cm-l; LiHoS,, 268 cm-'). Typical Raman spectra for RE,S, and MRES, are shown in Fig. 4. For the hexagonal lattice, there are four infrared-active polar modes (2A,, +2Eu) and two Raman-active modes (A,,+E,).,' The band widths (ca. 10-20 cm-') in the Raman 1057 h u).-c C 20 1000 500 wavenumber/cm-' Fig. 4 Raman spectra of (a) Pr,S3 and (b)LiDyS, spectra are consistent with an ordered structure; the wave- number for the A,, mode (of weaker intensity compared to Eg) increases from La-Yb (e.g.NaNdS,, 236 cm-'; TcaSmS,, 263 cm-'; NaDyS,, 276 cm-'; NaYbS,, 284 cm- '). The stronger intensity E, line is located in the region 204-210 cm-'. A similar trend is observed for the KRES, derivatives, e.g. (Alg) KPrS,, 252 cm-'; NaSmS,, 262 cm-l; KYbS,, 281 cm-'. The stronger intensity E, line is also located in the region 204-210 cm-'. The mixed alkali-metal lanthanide phases Li, -,Na,LnS, also show the Raman-active (Algand E,) modes but the peaks are broadened, e.g. Yb 288 cm-' (21 cm-') and 206 cm-' (42 cm-') (where the figures in brackets represent peah widths at half height). Infrared spectra for the hexagonal lattice type were all similar. A broad band centred in the region 310 cm- and a sharper band located at ca.150cm-' may be assigned to E, modes. The A,, stretching mode may be correlated wirh peaks at 220 cm-' and 330 cm-'. The magnetic moment measurements for MRES2 were consistent with the rare-earth metal in the RE3+ ovidation state (Table 1). An exception was noted for LiEuS, (p==4.8pB) where the lattice parameter also indicates a mixture [of Eu2+ and Eu3+ oxidation states. The potassium analogue (p= 3.41 pB)agrees more closely with the higher oxidation state. Sulfur microanalyses were consistent with the formula MRES, and agreed with the phases observed by powder XRII. Reaction of rare-earth-metal(II1) chlorides with H2S at 750 "C for 10 min in the apparatus shown in Fig. 1 produced rare-earth-metal sesquisulfides (RE,S3) in quantative yield.A mixture of two phases (a-and y-Sm,S,) was observed for samarium, while the single-phase materials y-Ln,S3 ( Ln =Pr, Nd, Gd) and 6-Ho2S3 were obtained. Reaction of REC1, with M2S in evacuated quartz ampoules at 750°C produced a mixture of up to three phases, i.e. REOC1, RE2S3 and RE,O,S J. MATER. CHEM., 1994, VOL. 4 due to the oxophilic nature of the rare-earth-metal chloride (through oxygen removal from the quartz) at elevated reaction temperatures. In order to minimise the role of oxygen contamination, the reaction of REC1, with M,S was investigated in an H,S-N, flow stream at 800°C. (If the reaction was carried out with no H2S flow then some RE,02S was also observed.) The predominant phase MRES, was observed independent of the containment vessel (e.g.alumina boat); the formation of M LnS, corresponded with previous observations for Ln = Pr-Yb' [eqn. (2)] with the sesquisulfides formed for La, Ce and Pr [eqn. (l)].Increasing the stoichiometric ratio of Li2S to PrCl, to 5:1 resulted in a mixture of y-Pr,S, and LiPrS,. The europium analogue was found to be moisture-sensitive, forming EuS. It was necessary to optimise the formation of NaRES, and KRES, by increasing the stoichiometric ratio of M,S (M=Na, three-fold excess; M=K, five-fold) [eqn. (4) and (5)]to REC13. Lower ratios of the alkali-metal sulfide, in both cases, resulted in the incorporation of the sesquisulfides [eqn. (5)] for potassium and a mixture of the sodium lantha- nide sulfide and sesquisulfides [eqn.(3)], with a concomitant decrease in the crystallite size of the products (Scheme 1). The role of H,S is probably two-fold, both in controlling the oxygen contamination and being intimately involved in the reaction. It is apparent that the formation of RE,S, can be derived either directly from the reaction of REC1, with H,S, or by the metathesis of REC1, with M2S. It is likely that metathesis is in part responsible for this reaction as MC1 was always detected in the reactions and it is unlikely that it would be formed unless some exchange of ions has taken place. The incorporation of alkali-metal chloride in the fused material is indicative of a flux being formed. The differences of ionic mobility within the flux are probably the reason that an excess of reagent is required in the preparation of both the sodium and potassium lanthanide sulfides.The idealised equa- tions (4) and (6) with sodium and potassium being formed are incorrect in that the native metal was not observed after the reaction but some unreacted M,S; the additional M,S was required to insure sufficient Na or K was present so that a single phase MRES, was produced. Owing to the ambiguity of the reaction pathway, we studied the thermolysis of lanthanide chlorides with alkali-metal chlorides at 800°C in H,S. The reactions were also found to produce alkali-metal-rare-earth-metal sulfides MRES, in quantitative yield. These reactions are intrinsically quite simple in that they do not require any prior synthesis, unlike those involving M,S, and they may be applied to a range of materials, such as the synthesis of Group 2 and transition- metal mixed lanthanide sulfides.Hence the source of sulfur in the product comes directly from the H,S. It is impossible to RECl3 + MZS RECI, + Li2Se H2SI RE202S + M+t~lCl rule out H,S as the sulfur source for the reaction of REC1, and M,S (M=Li, Na, K) indicating that no metathesis reaction occurs. This would make the idealised equations (1)-(6) highly speculative. However, as stated above, the observation of significant amounts of MC1 (M=Li, Na, K) in the products from the reaction of RECl, and M,S indicate that a metathetical pathway is in part responsible for the reaction. Further reactions of M,S and RECI, in the apparatus shown in Fig.1 without an H2S flow did produce MRES, as the primary product but also a small amount of Ln,02S. A detailed EDXA analysis of the starting materials has shown that oxygen is not present (< 1-2%) and that any oxygen incorporation in these reactions comes from the containment vessels. The use of H2S in these reactions purges any oxygen from the product presumably by the elimination of water. The reaction of M2S and RECl, was not accompanied by a thermal flash, as previously seen for metathesis and the products were contained as a fused mass within the open alumina boats, rather than being spread over the reaction vessel walls. Hence there is no direct analogy between these reactions and the metathesis observed between alkali-metal pnictides with lanthanide halides17'18 and by Kaner between transition-metal chlorides and sodium sulfide. Hess's law calculations22 indicate that the reactions are less exothermic than reactions between high oxidation state t ransition-metal halides and sodium sulfide.It was also possible to make a range of alkali-metal-rare- earth-metal sulfides by the reaction of RE,S, and RE20zS with M,S or MC1 under an atmosphere of H,S-N, at 800 'C. The routes to alkali-metal lanthanide sulfides are also sum-marised in Scheme 1. It would appear that provided the stoichiometry is correct, almost any source of the rare-earth metal and alkali metal can be utilised in the formation of alkali-metal lanthanide sulfides and that this is the prefered thermodynamic product for a number of reactions.The ther- molyses proceeded rapidly at 800 "C, with the formation of a molten flux incorporating the lanthanide and the alkali-metal precursors, which allows for rapid diffusion and formation of MLnS,. Reaction of mixed alkali-metal sulfides [ie. (Li,,,Ko,5)2S and (Lio.5Nao,5)2S] with LnC1, (Ln=Sm, Er, Tb, Yb) and H,S at 800 "C produced the mixed alkali-metal lanthanide sulfides MI-,M',LnS, (M, M'=Li, K; Li, Na) in good yield (Table 4), as assessed by X-ray powder diffraction and EDXA. Conclusions The reactions of rare-earth-metal(rI1) chlorides with alkali- metal sulfides at 750-800 "Coffer a rapid route to pure single- phase crystalline alkali-metal-rare-earth-metal sulfides, or in RECS RECI, + MCI lH* RE2S3+MCI LnCI, + Li,S M = Li.Na, K, RE = Y or lanthanide Scheme 1 Routes to alkali-metal rare-earth sulfides. (Note: Reactions involving lithium selenide or lithium sulfide selenide and RECl, and H,S at 750 C formed only LiRES,. Reaction of REC1, and M,S at 750°C without H,S flow primarily produced MRES2, but invariably formed a minor phase of RE,O,S.) J. MATER. CHEM., 1994, VOL. 4 1609 some cases rare-earth-metal sesquisulfides. The stoichiometry of the reaction has to be closely monitored to insure that the required phase is produced. The reaction probably proceeds by combination of a metathetical ionic exchange and a heterogeneous reaction with H2S. The reaction occurs at a temperature that is 100 "C lower than conventional prep- 8 9 R.Ballestracci and E. F. Bertaut, Bull. Soc. Fr. Mineral. Cristallogr., 1964. 87, 512; P. N. Kumta and S. H. Risbitd, Prog. Crystal Growth Charac., 1991,22, 321. W. Bronger, W. Bruggemann. M. von der Ahe and D. Schmitz, J. Alloys Compounds, 1993,200,205. R. Ballestracci and E. F. Bertaut, Colloq. Int. Centre. N,it. Rech. Sci., 1967, 157,41. arations and also shows versatility in the preparation of M, -,M,'LnS, materials. 10 11 M. Sato, G. Adachi and J. Shiokawa, Muter. Rex Bull., 1984, 19, 1215. G. C. Allen, M. Paul and M. Dunleavy, Adc. Muter., 199.', 4,424. The Leverhulme Trust (fellowship to J.C.F.), the Royal Society (glove box purchase) and the University of London Central Research Fund are acknowledged for financial support.12 13 P. R. Bonneau, R. F. Jarvis and R. B. Kaner, Nuture (,',ondon), 1991,349,510. P. R. Bonneau, R. F. Jarvis and R. B. Kaner, horg. Chew, 1992, 32, 2127. N. Williams of the Open University is thanked for assistance with the SEMiEDXA studies and D. Ciomartan of UCL for help with the Raman spectra. 14 15 16 I. P. Parkin and A. T. Rowley, Polyhedron, 1993,12,2961. I. P. Parkin, A. Hector and J. C. Fitzmaurice, J. Ch(1m.Soc., Dalton. Trans., 1993, 2435. 1. P. Parkin and A. Rowley, J. Muter. Chem., 1993,3,689 17 J. C. Fitzmaurice, A. Hector, A. Rowley and I. P. Parkin, 1. Muter. References 18 Chem., 1994,4,289. J. C. Fitzmaurice, A. Hector, A. Rowley and I. P Parkin, 1 2 3 4 5 6 K. L. Lewis, J. A. Savage, K. J. Marsh and A. P. C. Jones, New Optical Materials, Proc. SPIE-Int. Soc. Opt. Eng., 1983,400, 21. C. Y. Yeh and C. Sarini, US Pat. 4,560,804. B. T. Kilbourne, A Lanthanide Lanthology, Molycorp Inc, White Plains. NY, 1993, Part 1. A. Bornstein and R. Reisfeld, J. Non. Crystalline Solids, 1982, 50, 93. T. Ogura, A. Mikami, K. Tamaka, T. Taniguchi, M. Yoshida and S. Nakajima, Appl. Phys. Lett., 1986,48, 1570. T. Schleid and F. Lissner, Eur. J. Solid. State. Inorg. Chem., 1993, 30, 829. 19 20 21 22 Poljlhedron, 1993, 13, 235. H. P. Klug and L. E. Alexander, X-Ray Difraction Proc edure fur Poljcrystalline and Amorphous Muteriuls, Wiley, New J ork, 2nd edn., 1974. D. S. Knight and W. B. White, Spectrochim. Actu, Purt A, 1990, 46,381. P. Bruesch and C. Schuler, J. Phjx Chem. Solids. 1971,32, 1025. D. D. Waymann, W. H. Evans, V. B. Parker, R. H. $chumm, I. Halow, S. M. Bailey, K. Chumey and R. L. Nuttal, The NBS Tables of Chemical Thermodynamic Properties, ,Zmerican Chemical Society, Washington DC, 1982. 7 R. Ballestracci, Bull. Soc. Fr. Mineral. Cristallogr., 1965, 88, 207; M. Tromme, C.R. Acad. Sci. Paris., Ser. C, 1971, 273, 849; Paper 4/02815E; Receiued 12th MJ~,1994

ISSN:0959-9428    

DOI:10.1039/JM9940401603

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(10), 1611-1617 1611 Preparation of Zinc Oxide and Zinc Sulfide Powders by Controlled Precipitation from Aqueous Solution Tito Trindade," Julio D. Pedrosa de Jesus*aand Paul O'Brienb a Department of Chemistry, University of Aveiro, 3800 Aveiro, Portugal Department of Chemistry, Queen Mary and Westfield College, Mile End Road, London, UK El 4NS ZnO and ZnS powders were prepared from aqueous solutions of zinc salts in the presence of ethylenediamine or triethanolamine. The morphology of the powders was analysed by scanning electron microscopy and infrared spectroscopy. The effect of the experimental conditions upon the size and shape of the particles is described with a special emphasis on the role of the organic ligand. There is considerable current interest in developing micro- crystalline powders suitable for use as precursors for ceramic materials.This work forms part of various efforts to produce high-performance ceramics of homogeneous composition and microstructure, as such materials should have improved reliability as compared to conventional materials.' The electri- cal properties of zinc oxide are also markedly affected by the characteristics of the precursors used to prepare an individual sample, these properties are crucial in defining the suitability of zinc oxide for use in varistor^.^-^ Zinc sulfide is another example of a material for which the control of particle formation may be of importance as this material is used in luminescence devices and pigments6 Chemical methods have been applied to the synthesis of powdered materials, and submicrometre particles of narrow size distribution, high purity and uniform morphology have been achie~ed.'.~.~ Although technological interest in such controlled synthesis is recent, many natural systems synthesize biominerals with remarkable properties.' Matijevic' showed that well defined hydrated metal oxide particles, as well as other inorganic solids, can be precipitated from homogeneous aqueous solution by several methods including the thermal treatment of solutions of coordination complexes.The method is based on forming a complex sufficiently stable to prevent spontaneous precipitation at room temperature, i.e. masking the metal ion with a suitable ligand.Raising the temperature promotes the dissociation of the complex and allows the controlled generation of free metal Table 1 Experimental conditions for the preparation of ZnO samples using zinc nitrate at pH 13.0 sample aging temp./"C aging time/min organic ligand A 150 120 en B 150 120 tea C 100 120 en D 100 120 tea E 100 120 - ion which, on hydrolysis, leads to precipitation. Well defined particles of iron oxides," nickel and cobalt both as metal or oxide" have been prepared by such methods. If an appropriate chalcogenide source is added, such methods can be used to prepare sulfides and selenides.8 Needle-like crystals iof ZnO were obtained by the hydrothermal decomposition of aqueous solutions of Na,Zn-EDTA', and spherical particles of ZnO were prepared using a spray pyrolysis method.', The con- trolled preparation of ZnS particles has been achieved l4 using thioacetamide as a source of sulfide, but with no other added organic ligand.This paper reports the synthesis and characterizlition of ZnO and ZnS powders by precipitation from homogeneous solution, and the effect of experimental conditions on the morphology of the products is discussed. The work is part of a programme aimed at collecting systematic information about the role of chelating agents in the control of powder morphologies which should help to underpin the mechanism initially proposed by LaMer and DinegarI5 for the fo-rmation and shape control of colloidal particles.Experimental Chemicals The following chemicals were used without further purifi- cation: zinc nitrate hexahydrate, Zn( NO,), 6H,O ( Merck); zinc sulfate heptahydrate, ZnS0,.7H20 (Merck); zinc chloride, ZnC1, (Merck); zinc acetate dihydra,te, Zn (CH,C02), .2H20 (Merck); sodium hydroxide, NaOH pellets (Pronalab); nitric acid 65%, HNO, (Merckj; sodium chloride, NaCl (Merck); sodium nitrate, NaNO, (Merck); sodinm sulf- ate decahydrate, Na,SO,.lOH,O (Merck); sodium acetate, NaCH,CO, (Merck); ammonia, NH, (Merck); ethanolamine, NH2CH2CH20H, (Merck); triethanolamine (tea), N(CH2CH20H), (Merck); diethylamine, NH(C€€,CH,), (Merck); ethylenediamine (en), NH,CH,CH,NH, ( Merck); diethylenetriamine (dien), NH(CH2CH,NH2), (BD H); tri-ethylenetetramine (trien), NH,CH,CH,NHCH,CH,NH-Table 2 Experimental parameters used in the prepartion of ZnO powders ligand pH (20 "C) anion" time/min T/"C I( NaNO,)/niol kg- tea en 10.0 11.0 NO3-so42- 5 30 70 80 0.3 1 0.56 - 12.0 c1- 60 90 0.81 13.0 CH3C02- 120 100 1.06 - 240 150b not controlled " The respective zinc salt was used and an excess of the sodium salt (0.025 or 0.05 mol) was added to 50 cm3 of solution.Hydrothermal conditions. Table 3 Results from precipitation experiments of aqueous solutions of zinc nitrate and organic ligands at room temperature and pH 12.0 ligand" solid ammonia zinc hydroxide diethylamine zinc hydroxide monoethanolamine zinc hydroxide triethanolamine none ethylenediamine none diethylenetriamine none triethylenetetramine none nitrilotriacetic acid none EDTA none a [ZnZ+]=0.02mol dmP3; [ligand]: [Zn'+]=5: 1.Table 4 Results from precipitation experiments of aqueous solutions of zinc nitrate and organic ligands at 100 and 150 "C, aging time 2 h and pH 12.0 ligand" solid triethanolamine ZnO ethylenediamine ZnO diethylenetriamine none triethylenetetramine none nitrilotriacetic acid none EDTA none [Zn2+]=0.02mol dmP3; [ligand]: [Zn2+]=5:l. (a) H 1.OPm -1.o pm Fig. 1 SEM images of ZnO obtained by the hydrothermal treatment (150'C) in the presence of: (a)ethylenediamine; (b)triethanolamine J. MATER. CHEM., 1994, VOL. 4 -6.00 pm Fig. 2 SEM images of ZnO obtained at 100 'C in the presence of (a) ethylenediamine; (b) triethanolamine CH,CH, (Riedel-de-Haen); EDTA, (H02CCH2)2-NCH2CH,N(CH2C0,H), (Carlo Erba); nitrilotriacetic acid trisodium salt, N(CH,COONa)3 (ICN); 2-mercaptoethanol, HSCH2CH20H (Merck); o-phthaldialdehyde.C,H,( CH0)2 (BDH); thioacetamide, CH,CSNH, (Merck). Sodium hydroxide solutions were prepared (1 and 4mol dmP3) using boiled deionized water. Nitric acid solutions of the same molarity were prepared by dilution of the concen- trated acid. Stock aqueous solutions of zinc (0.04mol dm-3) and ligand (0.2mol dmP3) were prepared. All the solutions were filtered using 0.45 pm Millipore membranes before use. Determination of en was performed by the colorimetric method described by Hihara et using a Varian DMS 300 spectrometer.The stock solutions were diluted 1000-fold for the determination. Particle Preparation In a typical screening procedure equal volumes of ligand and Zn" stock solutions were mixed and the pH adjusted to 12.0. The final mixture was left to stand with stirring, at room temperature, for at least 1 day. Thermal treatment of the solutions was performed only for those which did not give a spontaneous precipitate at room temperature. Heating pro- cedures included refluxing (e.g.for 2 h) or heating the solutions in PTFE reaction bombs, in an oven (e.g. 2 h at 150 C). Another procedure involved the addition of sodium hydrox- ide (4mol dmP3) to a solution of the zinc salt (0.02mol dm-3). Initially a precipitate formed which dissolved at pH> 13.5.This solution was filtered and then refluxed; a precipitate then formed. The precipitates formed by both procedures were removed by filtration with 0.45 pm Millipore J. MATER. CHEM., 1994, VOL. 4 Fig. 3 SEM images of ZnO after the milling of the powders: (a) sample A; (b)sample B; (c) sample C; and (d) sample D 6.00pm Fig. 4 SEM image of ZnO obtained at 100°C in the absence of organic ligand membranes, washed with deionized water and dried in a desiccator over silica gel at room temperature. The various powders obtained were analysed (see Table 1 for sample identification). The effect of other experimental parameters associated with the preparation of the ZnO powders was studied by systematically varying the reaction conditions (see Table 2).ZnS powders were prepared using procedures similar to those used in the preparation of ZnO. Thioacetamide was used to generate the sulfide ions. Particle Characterization The X-ray analysis of the powders was performed using a Philips PW 1729 generator using nickel-filtered Cu- Ka radi-ation. JEOL JSM-35C and Hitachi S-4100 scanning electron microscopes were used. The materials were prepared for microscopy by resuspension in water followed hy ultra-sonication. Aliquots of the suspensions were then kept on high-purity aluminium rods at room temperature in a desiccator over calcium chloride until the liquid had evapor- ated. The infrared spectra were recorded with a Mattson Polaris FTIR spectrometer using pellets of 1 mg of the sample and 150 mg of spectroscopic-grade KBr.Results and Discussion Preparation of ZnO and ZnS Powders Preliminary studies of the thermal and hydrothermal treat- ment of aqueous solutions of zinc in the presence 1i)f a wide variety of ligands have been reported.17 The resulth of these studies suggest that en and tea are the most promising metal- ion buffers for a detailed study. Using similar systems but with different experimcntal con- ditions Matijevic and co-workers have obtained’ ’,’’ ZnO particles with morphological characteristics similar to those described in our preliminary report.17 However, in the present paper the influence of a wider range of experimental conditions on the powder characteristics are reported.The addition of an organic ligand to a solution of a zinc salt may prevent or inhibit the precipitation of zinc hydroxide. Fig. 5 SEM images of ZnO prepared (a) in the presence of an excess of sulfate ions and ethylenediamine and (b) from a solution with an ionic strength of 1.06 mol kg-' and in the presence of ethylenediamine In order to select ligands which prevent the spontaneous, immediate formation of solid, precipitate-screening experi- ments were carried out as described in the Experimental section. The results of these studies (all carried out at room temperature) are summarized in Table 3 for a [ligand]: [Zn2+] ratio of 5 : 1. The presence of the amines, aminopoly- carboxylic acids and tea prevents the precipitation of the solid at room temperature and these ligands were chosen for more detailed investigation.The white precipitates obtained from the solutions at room temperature exhibit very poorly defined X-ray diffraction patterns with no well defined peaks, the material formed being X-ray amorphous. The composition of the samples was con- firmed as hydrated zinc oxide by comparing their infrared spectra with that of the precipitate obtained by mixing solutions of sodium hydroxide and zinc nitrate. Zinc oxide was observed to precipitate after solutions containing tea or en were heated at pH 12.0-13.0. Hydrated zinc hydroxide precipitate is unstable at temperatures >35 "C with respect to ZnO, in alkaline solution2' and at high pH the formation of the oxide is expected.Ligands other than e thylenediamine or trie thanolamine to tally inhibited precipi- tation under the conditions studied, even using higher aging temperatures, see Table 4. There appears to be only one report in the literat~re'~ of the preparation of ZnS particles of uniform morphology by the thermal decomposition of solutions of thioacetamide and zinc. The procedure reported involved a two-step process. No added organic ligand was used, other than the thioacetamide used as the source of sulfide ion. This method14 involved the preparation of ZnS seeds from a solution of zinc nitrate, nitric J. MATER. CHEM., 1994, VOL. 4 Fig. 6 SEM images of ZnS prepared at 60 'C for 2 h from solutions containing thioacetamide and (a) ethylenediamine or (b)triet hanolamine acid and thioacetamide, which were allowed to age further at an elevated temperature.The fact that the ligands en and tea had a marked effect on the morphology of the particles led us to investigate the effect of these chelating agents on the preparation of ZnS. When alkaline or acidic solutions of thioacetamide were heated in the presence of metal ions, metal sulfides precipi- tated. In a formal sense the reaction is a consequence of the hydrolysis of thioacetamide, which may be represented as: CH,CSNH, +2H,0+CH3C02NH4 +H2S ( 1) In an initial attempt to prepare ZnS particles, alkaline solutions containing the organic ligands en or tea, zinc nitrate and thioacetamide were heated at 60 or 1OOT. However, only for acidic conditions was the preparation of well defined ZnS particles successful.Influence of Preparation Methods on Powder Characteristics The powders obtained by thermal treatment were identified as being zinc oxide (wurtzite, hexagonal) or zinc sulfide (sphalerite, cubic). The materials were identified by comparing their diffraction patterns with published data.21 The peaks for the zinc oxide samples were well defined, indicating that the samples were of good microcrystallinity. The best patterns were obtained for the powders prepared in closed vessels at the higher temperatures (samples A and B, Table 1).The ZnS particles were all of ill defined crystallinity, although the characteristic reflections of the cubic form could be observed.The morphology of the particles was analysed by scanning electron microscopy (SEM). The ZnO particles obtained at J. MATER. CHEM., 1994, VOL. 4 150"Cwere present as twinned crystals in which the hexagonal crystal habit of the wurtzite type structure is evident (Fig. 1). These particles are similar to those obtained by other authors using hydrothermal methods.l23l8 When the reflux treatment was applied to the reaction mixtures a different morphology was observed with smaller polycrystalline particles. The effect of the ligand on the particle morphology is remarkable (Fig. 2): ZnO particles obtained in the presence of tea are nodular aggregates, but when en is used the particles are acicular.Preferential growth in the c direction has possibly occurred from a particle centre where twinning took place. When the powder samples obtained in the presence of en either from reactions at 150"C, or from the reflux method, were mechanically milled the particles break from several sites [Fig. 3(u)-(c)]. The same milling procedure applied to samples prepared in the presence of tea by the reflux methods deaggregates the material into smaller nodular particles, some of them being broken while others remain intact [Fig. 3(41. In the absence of organic ligands, spontaneous precipitation may occur in alkaline solutions at room temperature. Zinc hydroxide is slightly soluble in water and becomes more soluble as the pH is increased (or lowered) above certain values.The dissolution of the precipitate is possible in a strongly alkaline medium with the formation of hydroxycom- plexes such as [Zn(OH),]- and [Zn(0H),l2- in solution.22 When a clear zinc solution was refluxed at pH 13.5, a powder formed (sample E), which was identified by X-ray diffraction as ZnO. SEM of this powder (Fig. 4) reveals prismatic particles of a shape and size distinctly different from those obtained in the presence of the organic ligands. Fig. 7 SEM images of ZnS prepared at 100"C for 30 min from solutions containing thioacetamide and (a) ethylenediamine or (b)triethanolamine wavelength/pm 10 15 25 1 I 1200 1000 800 600 400 wavenu mber/cm-' Fig. 8 FTIR spectra of the ZnO samples: (a) sample A, (b)sample B, (c) sample C and (d)sample D in KBr pellets Some published work on the preparation of well defined particles in the presence of organic ligands has used equilib- rium arguments23 while in other cases the chemic;d degra- dation of the organic ligand has been involved." In an.attempt to clarify the fate of the ligand for the present systems en was quantitatively determined in the solutions from which ZnO precipitation had occurred. The results of these experiments showed that the en does not decompose. An effort was made to determine if parameters such as pH, the anion, the reaction time, the temperature and the ionic strength affected the ZnO powders. The experimental param- eters summarized in Table 2 were studied and the results showed no evidence for a pronounced effect of these variables on the characteristics of the ZnO particles, in contriist to the ligand or the nature of the hydrothermal treatmeni.In two cases slight effects were noticed. Slightly different %no par- ticles were obtained in the presence of en and sulfate ions [Fig. 5(a)]. This could be related to the complexing effect of this anion, which is more pronounced than for the other anions studied in this work. In one experiment at an ionic strength of 1.06 mol kg-', in the presence of en, the ZnO particles were deaggregated [Fig. 5(b)]. The ZnS obtained from the alkaline medium was Df poorly defined morphology and at the highest pH value used (pH 13.0) a gelatinous precipitate was obtained.However, when thermal treatment at 60 "C was performed on solutions at pH 6.0, nearly spherical submicrometre particles were obtained for both en and tea ligands (Fig. 6). The effect of the ligand on the shape of the ZnS particles was not as marked as the results for the ZnO powders. The SEM results for ZnS suggest that the best defined and most uni- form morphologies are obtained in the presence of tea. Agglomeration is marked in both cases. The fact that the experiments were performed at pH 6.0 means that both en and tea will be partially protonated; however, the ionization of the ligand is promoted by coordination and marked effects upon particulate formation are observed. When the tempera- ture of the solutions is increased the agglomeration of the ZnS particles is promoted and their size is greater for similar Table 5 Published2' stability constants (25 "C and Z =0.1 mol kg-l) for complexes of zinc and the ligands used in this work ligand equilibrium log K mea tea ML/M.L ML, /M.L2 ML/M.L ML2/M.L2 2.70" 4.78" 2.05 3.28 ammonia ML/M.L 2.21' ML3/M.L3 ML,/M.L~ 4.50' 6.86' ML4/M.L4 8.89' diethylamine ML/M.L ML,/ML.L 2.97 2.60 ML3/ML,.L 2.34 ML,/ML, .L 1.92 M L,/M.L4 9.83 en ML/M.L 5.7 dien ML,/M.L2 ML3 /M .L3 ML/M.L 10.6 12.6 8.8 ML,/M.L, 14.5 trien ML/M.L 12.0 EDTA ML/M.L 16.5 NTA ML/M.L MLJM.L, 10.7 14.2 OH- ML/M.L 5.0' ML2jM.L2 11.1' ML3/M.L3 ML4 /M. L4 M2L/M2 .L 13.6' 14.8" 5.0' " Z=O.5 mol kg-l.'1=0 mol kg-'. 100 75 h8 50 C N 25 n 7 8 9 10 11 12 13 PH Fig. 9 Speciation diagrams of the zinceethylenediamine system at room temperature (25 "C), using the values of Table 5, for the system [Zn*']:[en] = I :5. [Zn2'] =0.02 mol 1-'. Species as M,(en),(OH),. aging times. Fig. 7 shows such effects for particles obtained at 100 'C. Use of FTIR Absorption Spectra in Powder Characterization The morphological characteristics of small particles can, for ionic oxides of well defined shape, have a marked effect on the IR spectrum due to dipole oscillations induced by the vibrational modes. Applying a generalized theory of the dielectrics, Hayashi et al. successfully interpreted the exper- imental IR spectraz4 of zinc oxide.A theoretical IR spectrum for particles with a defined shape and state of agglomeration can be derived by adjusting a shape factor, g, and a filling factor, f The influence of the shape and agglomeration state of the ZnO particles on the transverse optical modes, W,, =377 cm-' and W,,,=406 cm-',can be studied. The ZnO particles described in this work present sufficiently specific morphologies for an attempt to correlate shape and ~pectra.'~-~~Even though the preparation of the KBr pellets J. MATER. CHEM., 1994, VOL. 4 must lead to some changes in the particles, sufticient morpho- logical differences remain for distinct spectra to be observed for different samples (Fig. 3). The FTTR spectra of some of the ZnO powders are shown in Fig.8. The spectra are characteristic of zinc oxide and no organic or carbonate residues were detected in the spectra. However, some differ- ences in the band profiles can be seen for the different ZnO samples. Sample D consists of small nodular particles in which aggregation is facilitated, and a single band at 443 cm-' is observed. The samples A, B and C have preferential growth along the c axis, and show splitting of the absorption band. These results agree well with those reported by other^.^^.'^ Conclusions The results of this study suggest that the organic ligand not only inhibits Zn(OH), precipitation, allowing a controlled synthesis of ZnO, but can also play a specific role in controlling the growth of the particles under different orientations.The morphological characteristics of the ZnO particles depend both on the presence of organic ligands and their chemical nature. LaMer and Dinegar" claimed that the morphology of colloidal particles is controlled by the speciation of the solution present during the nucleation and growth of the particulate materials, and this idea appears to be confirmed by our observations. The lack of thermodynamic and kinetic data for these systems under the conditions used does not allow a thorough discussion of this hypothesis. However, the stability constants available27 (Table 5 j, although determined for other conditions, show that the solution speciation should be quite different for the range of ligands used.A simple calculation (carried out with the program SPECIES, Dr. L.D. Pettit, University of Leeds) suggests that the zinc:en system (1:5) at room temperature is unstable with respect to the precipitation of the hydroxide at pH 10.85 whereas the zinc-tea (1:5) system becomes unstable at ca. pH 7.0 (Fig. 9). Undoubtedly both thermodynamic and kinetic factors are important in the control of the precipitation mechanism of ZnO and ZnS particulates. SEM results show clearly that the organic ligands strongly influence the powder properties of both materials when they are obtained from solutions where they are present. This work was financed by Junta Nacional de Investigacao Cientifica e Tecnolbgica (Portugal). References 1 G. W. Kriechbaum and P.Kleinschmith, Ado. Muter., 1989, 61, 1416. 2 M. L. Levinson and H. R. Philipp, J. Appl. Phys., 1975,46, 1332. 3 F. A. Selim, T. K. Gupta, P. L. Hower and W. G. Carlson, J. Appl. Phys., 190,51, 765. 4 R. J. Lauf and W. D. Bond, Bull. Am. Ceram. Soc., 1984,63.278. 5 M. A. Alim, M. A. Seitz and R. W. Hirthe, J. Am. Ceram. SOC., 1988,71, C-52. 6 Dictionary of Inorganic Compounds, ed. J. E. Macintyre, Chapman and Hall, London, 1992, vol. 3, p. 3797. 7 D. Segal, in Chemical Synthesis of Advanced Cerumic Materials, ed. A. R. West and E. H. Baxter, Cambridge University Press, Cambridge, 1989. 8 E. Matijevic, Annu. Rev. Muter. Sci.,1985, 15,483. 9 Biomineralization, ed. S. Mann, J. Webb and R. J. P. Williams, VCH, Weinheim, 1989.10 R. S. Sapieszko and E. Matijevic, J. Colloid Interface Sci., 1980, 74,405. 11 R. S. Sapieszko and E. Matijevic, Corrosion, 1980,36, 522. 12 H. Nishizawa, T. Tani and K. Matsuoka, J. Am. Ceram. SOC.,1984, 67, C-98. J. MATER. CHEM., 1994, VOL. 4 1617 13 T. Liu. 0.Sakurai, N. Mizutani and M. Kato, J. Muter. Sci., 1986, 22 T. P. Dirkse, C. Postmus Jr. and R. Vandenbosch, J. Am. Chem. 67, 3698. SOC.,1954,76, 6022. 14 D. M. Wilhelmy and E. Matijevic, J. Chem. Soc., Faraday Trans. I, 1984,80, 563. 23 24 M. J. Kim and E. Matijevic, Chem. Muter., 1989. 1, 363. S. Hayashi, N. Nakamori and H. Kanamori, J. Phys. Soc. Jpn., 15 16 V. K. LaMer and R. H. Dinegar, J. Am. Chem. Soc., 1950,72,4847. G. Hihara, H. Miyamae and M. Nagata, Bull. Chem. SOC. Jpn., 198 1,54,2668. 25 1979,46, 17. K. Yamamoto, C. Tran, H. Shimizu and K. Abe, J. Phjs. SOC. Jpn., 1979,42, 587. 17 T. S. Trindade, A. M. V. Cavaleiro, J. D. Pedrosa de Jesus and 26 M. AndrCs-Verges and M. Martinez-Gallego, J. Mater. ci., 1992, 18 19 20 J. L. Baptista, Actas do XI Encontro da Sociedade Portuguesa de Quimica, 1988, 2, 705. A. Chittofrati and E. Matijevic, Colloids Sur-, 1990,48, 65. M. Castellano and E. Matijevic, Chem. Muter., 1989, I, 78. T. P. Dirkse, in ZUPAC Solubility Data Series, 23, Pergamon Press, Oxford, 1986, p. 156. 27 27, 3756. R. M. Smith and A. E. Martell, Critical Stability Constants, Plenum Press, New York, vol. 2, 1975; 1976 vol. 4. 1982, vol. 5, 1989, vol. 6; D. D. Perrin, Stability Constants of Aretal Ion Complexes, Part B: Organic Ligands-IU PAC Chemrcul Data Series 22, Pergamon Press, Oxford, 1979. 21 Powder Djfraction File: Inorganic Phases, JCPDS, International Centre for Diffraction Data, USA, 1989. Paper 3/04719I; Receiued 5th Augirst, 1993

ISSN:0959-9428    

DOI:10.1039/JM9940401611

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( lo), 1619-1620 y-Radiation Sol-Gel Synthesis of Glass-Metal Nanocomposites Yingjie Zhu,*a Yitai Qian,a Manwei Zhang," Zuyao Chen" and Guien Zhoub a Department of Applied Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China Structure Research Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China Glass-metal nanocomposites of nanocrystalline silver in a silica glass matrix have been successfully synthesized by y-radiation combined with the sol-gel method. Most research on nanocrystalline metals has concentrated on the fabrication and characterization of single-phase materials. More recently the field of interest has broadened to include nanocomposite materials, because of their interesting electrical and optical properties,' their possible commercial e~ploitation~,~and their importance in providing models for understanding the physics of ultrafine particles.435 Different techniques have been used to prepare such materials, the most popular being the gas e~aporation,~,' RF sputtering3** and ion exchange and redu~tion.'"~ Sol-gel techniques for prepar- ing nanocomposites have also been rep~rted'l-'~ in which heat treatment or hydrogen reduction is needed after the sol-gel process.Recently, we developed a y-radiation method for preparing nanocrystalline metal^.'^,^^ In order to prepare glass-metal nanocomposites under ambient pressure and at room tem- perature, we have explored the possibility of using y-radiation combined with the sol-gel process, and the results are reported here.Experimental Solutions were prepared by dissolving analytically pure AgNO, in distilled water and adding sodium dodecyl sulfate as a surfactant, and isopropyl alcohol as a scavenger for hydroxyl radicals. The solutions were bubbled with pure nitrogen for 1 h to remove oxygen, then irradiated in the field of a 2.59 x 10" Bq 6oCo pray source with different doses. After the solutions had been y-irradiated, solutions of colloidal silver were obtained. Then the sol-gel method was used to prepare silica-silver nanocoposites. Tetraethoxysilane [(C,H,O),Si, 5 ml] was dissolved in isopropyl alcohol (10 ml) with water (5 ml), and a few drops of concentrated nitric acid were added to keep the pH at ca.2. After continuous stirring for 1.5 h, different volumes of irradiated solutions were added so that the final product would have different Ag :silica ratios. For gelation the pH of the solution was increased to 8 by addition of aqueous ammonia under mild stirring. The hydro- gels obtained were dried overnight in air. The gels were ground, washed with distilled water and aqueous ammonia solution to remove impurities and were dried again. Finally, powders of silica-silver nanocomposites were obtained. The nanocomposites were characterized by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD analysis was performed using a Rigaku Dmax yA X-ray diffractometer at a scanning rate of 0.05" s-' in the 26' range from 25" to 85", using graphite-monochromated Cu-Kx radiation.TEM images were recorded with a Hitachi H-800 transmission electron microscope, using an acceler-ating voltage of 200 kV. The amounts of silver present as metallic species in the nanocomposites were determined by atomic absorption spectroscopy using an IL-95 1 spectrometer at 2 =328 nm. Absorption spectra of colloidal silver .;elutions produced by y-irradiation were recorded on a UV-240 UV-VIS spectrophotometer using quartz cells. Results and Discussion The radiation reduction of Ag' ions in solution leads to a yellow solution of colloidal silver. This process can be written in a simplified way: Ag' +eaq- +Ago (reduction) nAg" -+Ag,, (aggregation) The primary reduction product is silver atoms produced by the reaction of Ag+ ions with hydrated electrons fxmed in solution during y-radiation. These silver atoms undergo further aggregation to progressively larger clusters, leading to the formation of colloidal silver with an intense optical absorption at ca.400 nm. Our experiments show that colloidal silver is formed at the beginning of the radiation and that the 400 nm band intensity increases with increasing radiation dose (Fig. 1). This implies that the concentration of colloidal silver increases with the radiation dose. Fig. 2 gives the XRD pattern of a typical sample containing metallic silver particles prepared by ?-irradiating a solution containing 0.01 rnol I-' AgNO,, 0.01 mol I-' C,,H,,NaSO, and 2.0 mol 1-1 (CH,), CHOH with a dose of 8.1 r< lo3 Gy.This shows that the sample consists of two phases, i.c,. metallic silver and non-crystalline silica. The broadening of the ( 111) r Unm Fig. 1 Absorption spectrum of a solution after various doses of y-irradiation. Solution: 0.01 mol 1-' AgNO,. 0.01 mol 1-' C~2H,,NaS0,, 0.5 mol 1-' (CH,),CHOH; dose rate: 1.0 x 10' Gy min-'; radiation time: (a) not irradiated, (b)5 min, (c) 10 min. J. MATER. CHEhl., 1994, VOL. 4 Table 1 Effect of experimental parameters on the size of silver particles dispersed in the silica glass matrix sample irradiation dose/ silver particle no. solution 10-3 GY size/nm 1 2 0.01 rnol I-' 0.01 mol I-' AgNO,-0.01 mol I-' CI2H,,NaSO,-2.0 AgN0,-2.0 mol 1-' (CH,),CHOH mol I-' (CH,),CHOH 8.1 8.1 6 40 3 0.05 mol I-' AgN0,-0.01 mol 1-' C,,H,,NaS0,--2.0 rnol 1-' (CH,),CHOH 8.1 10 4 0.01 mol 1-' AgN03-0.01 mol 1-' C1,H,,NaS0,-2.0 mol 1-' (CH,),CHOH 30 15 I 1 15 25 35 45 55 28ldegrees Fig.2 XRD pattern of the sample prepared by the y-radiation sol-gel method. Solution: 0.01 mol 1-' AgNO,, 0.01 mol I-' C,,H,,NaSO,, 2.0 rnol I-' (CH,),CHOH. Dose: 8.1 x lo3 Gy. Fig. 3 TEM micrograph of the sample in Fig. 2 diffraction peak of silver implies that the silver particles dispersed in the silica matrix are very small. The average particle size of silver is 6nm, as estimated by the Scherrer formula.16 The amount of silver present as metallic species in the composite glass is 1.24%, as measured by atomic absorption spectroscopy. A TEM micrograph of the sample prepared under the same experimental conditions as described in Fig.2 is shown in Fig. 3. The silica glass contains a dispersion of fine metallic silver grains which are quasi-spherical and well separated. The selected-area diffraction pattern confirms that these par- ticles consist of metallic silver. The diameters of the silver particles range from 4 to 20 nm; the average particle size is 7 nm obtained from the photographic image microstructure densitometry analysis method, which is in agreement with that calculated by the Scherrer formula (6 nm). The size of the silver particles dispersed in the silica matrix is dependent on several factors, such as the surfactant used, the concentration of Ag' ions and the y-radiation dose.The experimental results are listed in Table 1. Table 1 shows that the silver particle size decreases as the concentration of sodium dodecyl sulfate increases. When no sodium dodecyl sulfate is used, the silver particle size is as large as 40 nm (sample 2). If the concentration of sodium dodecyl sulfate is increased to 0.01 mol 1-', the silver particle size decreases to 6 nm (sample 1).This shows that an appropriate surfactant can limit the aggregation of silver particles in solution. Using a lower concentration of silver nitrate favours the production of smaller silver particles. This may be due to the lower rate of reduction of Ag' ions.For example, when the concentration of silver nitrate is increased from 0.01 to 0.05 mol l-', the silver particle size in the silica matrix increases from 6 nm (sample 1) to 10 nm (sample 3). The influence of the y-radiation dose on the silver particle size was also studied. When the dose was increased from 8.1 x lo3to 3.0 x lo4Gy, the silver particle size increased from 6 nm (sample 1) to 15 nm (sample 4). Therefore we preferred to use relatively small radiation doses in the preparation of colloidal silver. Conclusion ?-Radiation combined with the sol-gel method has been successfully used to prepare silica glass-silver nanocomposites with a narrow distribution of silver particle sizes. By appro-priate control of the conditions, this method may be extended to the preparation of other glass-metal nanocomposites as well as ceramic-metal nanocomposites.Financial support from the Chinese National Science Research Foundation and the Doctoral Fund of the Chinese Education Commission are gratefully acknowledged. The authors thank Professor Zhang Zhicheng for helpful discussions and Mr. Sun Tingheng and Mr. Quan Yucai for their help with the y-radiation experiments. References 1 D. Chakravorty, Bull. Mater. Sci., 1984, 6, 193. 2 J. H. Sinfelt,Science, 1977, 195, 641. 3 A. Anderson, 0. Hunderi and C. G. Granqvist, J. Appl. Phys., 1980,57, 757. 4 Aerosol Microphysics II ed. W. H. Marlow, Springer, Berlin, 1982. 5 Physics of Finely Diuided Mutter ed. N. Boccara and M. Daoud, Springer, Berlin, 1985. 6 C. G. Granqvist and 0.Hunderi, Phys. Rev. B, 1977,16,3513. 7 G. A. Niklasson, J. Appl. Phys., 1987,62,258. 8 B. Abeles, P. Sheng, M. D. Coutts and Y. Arie, Ah. Phys.. 1975, 24, 407. 9 D. Chakravorty, J. Non-Cryst. Solids, 1974, 15, 191. 10 D. Chakravorty, A. Shuttleworth and P. H. Gaskell, J. Muter. Sci., 1975, 10,799. 11 S. Datta, S. S. Mitra, D. Chakravorty, S. Ram and D. Bahadur, J. Muter. Sci. Lett., 1986,5, 89. 12 A. Chatterjee and D. Chakravorty, J. Phys. D, 1986.22, 1386. 13 S. Roy, A. Chatterjee and D. Chakravorty, J. Muter. Res., 1993, 8, 689. 14 Zhu Yingjie, Qian Yitai, Zhang Manwei, Chen Zuyao, Lu Bin and Wang Changsui, Mater. Lett., 1993, 17, 314. 15 Zhu Yingjie, Qian Yitai, Zhang Manwei, Chen Zuyao, Lu Bin and Zhou Guien, Muter. Sci. Eng., 1994, B23, 116. 16 C. N. J. Wagner and E. N. Aqua, Adc. X-Ray Anal.. 1964,7,46. Paper 4/00499J; Received 26th Junuurj., 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401619

出版商:RSC    

年代:1994

数据来源: RSC