Crystal structure and physical properties of UAuSi and UAu, Rainer Pottgen," Vinh Hung Tran,b Rolf-Dieter Hoffmann," Dariusz Kaczorowskib and Robert Trocb "Anorganisch-Chemisches Institut, Universitat Miinster, Wilhelm-Klemm-Strasse 8, 0-481 49 Miinster, Germany bW.Trzebiatowski Institute for Low Temperature and Structure Research, Polish Academy of Sciences, PO Box 937, 50-950 Wroctaw, Poland UAuSi and UAu, have been prepared by arc-melting of the elemental components and subsequent annealing. The crystal structure of UAuSi, which has been previously reported to crystallize in the TiNiSi-type structure, was determined from X-ray powder diffraction data: LiBaSi-type structure, P6m2, u=419.5(2) pm, c=397.2(2) pm, 1/=0.0605 nm3, Z= 1and R,(I)=0.018. It is derived from the AIB,-type structure by an ordered arrangement of the gold and silicon atoms on the boron sites.The binary compound UAu, (AlB,-type) is confirmed. Magnetic susceptibility measurements indicate spin-glass behaviour and paramagnetism for UAuSi and UAu,, respectively. Both compounds are metallic conductors. The binary uranium silicide p-USi, adopts the hexagonal structure of A1B2.172 The same structure was also reported for UAu, .3,4 However, Dommann and Hulliger' reported UAu, with the CeCd,-type structure,' which is derived from that of AlB, by simply shifting the gold position from z=OS to z= 0.45, resulting in slightly puckered layers. Thus, the silicon atoms in P-USi, should be substitutable by gold atoms. We recently reported on U,AUS~~,~ the first compound in the pseudo-binary system US,-UAu, .Similar U2TSi3 compounds exist with other transition metal^.^-^ Another compound in this system is UAuSi," previously reported to adopt the orthorhombic structure of TiNiSi; sus-ceptibility measurements on this compound indicated spin- glass behaviour. We have reinvestigated the crystal structure of UAuSi and observed some inconsistencies with the previous data. Herein we show that UAuSi adopts the hexagonal structure of LiBaSi, an ordered derivative of the AlB,-type structure. In addition we report in detail on the physical properties of UAuSi and UAu,. Experimental Starting materials were uranium platelets (Merck, 'nuklearr- ein'), gold wire (Degussa, >99.9%) and silicon lumps (Merck, >99.9%).The uranium platelets were cleaned with concen- trated nitric acid to remove oxide impurities and were then kept under argon. The samples were prepared by arc-melting the elements in an argon atmosphere. The argon was purified by repeatedly melting titanium sponge prior to the reactions. The molten buttons were turned over and remelted three times on each side to ensure homogeneity. The mass losses after several meltings were always <0.5%. The samples were sub- sequently wrapped in tantalum foil and annealed in evacuated sealed silica tubes at temperatures between 650 and 800°C for 2 weeks. Guinier powder patterns of all samples were recorded with Cu-Ka, radiation and a-quartz (a =491.30 pm, c =540.46 pm) as internal standard.Air-sensitive UAu, was ground to powder with dried paraffin oil and placed between two scotch tapes to prevent hydrolysis. The lattice parameters (Table 1) were obtained by least-squares refinements. The indexing was facili- tated by intensity calculations." Powder diffraction measurements of the UAuSi samples were performed on a STOE STADI/P focusing monochromatic beam diffractometer with a rotating very flat sample in the symmetric transmission mode. Cu-Ka, radiation was used with a linear position-sensitive detector, a step width of 0.02" (28) and a counting time of 20 s per step. The Rietveld refinements were performed with the RIETAN program.12 The magnetic susceptibilities of polycrystalline samples were measured in the temperature range 4.2-300K using an RH- Cahn electrobalance.Electrical resistivity measurements were performed in the temperature range 4.2-300 K with a conventional four-point technique. The sample voltage was measured automatically every 20 s with an accuracy of & 1 pV. The measurements were repeated with different samples, and the results were reproducible. Results and Discussion Crushed buttons of the samples are all light grey with a metallic lustre; powders are dark grey. The silicon-rich samples up to composition UAuSi are stable in air over several months; however, UAu, was less stable and showed some hydrolysis. These samples were kept under vacuum. The sensitivity against traces of humidity of such gold compounds was also observed for the corresponding thorium compound13 as well as for the alkali-metal gold compounds KAu, ,I4 NaAu,," RbAu16 and Rb,Au, .I7 Table 1 Lattice parameters of the hexagonal binary and ternary compounds in the pseudo-binary system USi, -UAu," compound structure type alpm c/Pm cla v/nm3 ref ~ ~ ~ ~~~~~~~~ p-USi, AlB, 402.8( 1) 385.2( 1) 0.956 0.0541 1 U,AuSi, AlB, 414.5(3) 398.9(2) 0.962 0.0593 6 UAuSi LiBaSi 419.5(2) 397.2( 2) 0.947 0.0605 this work UAu, AlB, 475.6(2) 311.0( 1) 0.654 0.0609 this work UAu, AlB, 475.4 310.7 0.654 0.0608 4 UAu, AlB, 475.6 311.0 0.654 0.0609 3 UAu, CeCd, 475.6 310.9 0.654 0.0609 5 " Standard deviations in the positions of the least significant digits are given in parentheses throughout the paper.J. Muter. Chem., 1996, 6(3), 429-434 429 Crystal structures The crystal structure of UAu, is confirmed The lattice para- meters [a=475 6(2) pm, c= 311 0(1) pm] are in excellent agreement with the previous data (Table 1) 'The powder patterns of our UAu, samples always showed the AlB2 struc- ture, independent of the annealing processes, in agreement with the work of Palenzona and Cirafici3 and Tran and Troc However, Dommann and Hulliger5 reported a CeCd,-type structure for UAu, In this structure type, the gold atoms are not situated on the mirror plane at z =O 5, they are shifted to zz0 45, resulting in slightly puckered layers We have calculated theoretical powder patterns with LAZY- PULVERIX" in order to check the two possibilities For the CeCd,-type structure two reflections at 28 =63 94" and 76 74" should occur with intensities of 65 and 48 (scaled at an intensity of 100 for the strongest reflection), respectively Both reflections are calculated with intensities of 0 1 for the A1B2- type structure None of our samples showed these reflections on the Guinier powder patterns However, for comparison, the 001 and 11 1 reflections with calculated intensities of 4 8 and 7 5, respectively, are clearly visible on the patterns We therefore conclude that UAu, adopts the A1B2-type structure (Fig 1) and there is no evidence for a puckering of the hexagonal gold nets in UAu, Positional parameters and interatomic distances are listed in Table 2 For UAuSi we find a different symmetry than that reported previously lo All our powder patterns (melted samples and those annealed at 650 and 800°C) show hexagonal symmetry and an intensity distribution resembling the A1B2-type struc- ture This result was also reproduced for other samples The refined lattice parameters of the sample annealed at 800 "C are a=4195(2) pm and c=3972(2) pm However, Tran and Troc" reported the TiNiSi-type structure for UAuSi (arc- melted sample annealed at 650°C for 7 days) with the ortho- rhombic lattice constants a =418 1 pm, b =798 2 pm and c = 724 1 pm These values correspond to the doubled orthohex- agonal Cell of AlB2 Uortho =ahex, bortho =2Chex and Cortho = ahex$ A reinvestigation of the previously reported'' powder data indeed showed that UAuSi has the small AlB,-like hexagonal cell (Fig 2) In order to establish whether or not the gold and silicon atoms are ordered within the hexagonal network, we have performed powder diffraction measurements on the arc-melted and the annealed samples The order can be detected directly from the sub-cell intensities, since the ordered structure has the translationenglezche18 subgroup P6m2 The crystallographic relationship for such order-disorder transitions has already Fig.1 Crystal structure of UAu, 0, uranium, 0, gold The two- dimensional gold network is outlined Table 2 Positional parameters and interatomic distances (in pm) of UAu, atom P61mmm X Y Z U la AU 2d u 12 2 6 Au 315 6(1) U 311 0(1) U 475 6(2) Au 3 Au 2 Au 6U 274 6( 1) 311 0(1) 315 6( 1) 430 J Mater Chem ,1996, 6(3), 429-434 Fig.2 Crystal structure of UAuSi projected along the z direction All atoms are situated on mirror planes at z=O(U) and z= 1/2 (Au, Si) connected by thin and thick lines, respectively been discussed in detail for isotypic ThAuSi l3 For the sample annealed at 800 "C, we clearly established the ordered LiBaSi- type structure l9 2o The intensities resulting from the Rietveld powder refinements are given in Table 3 Positional parameters and interatomic distances are listed in Table 4 In contrast, the collected powder diagrams of the arc-melted sample and the sample annealed at 650°C show two very similar hexdgonal cells Several reflections already showed some splitting on the Guinier powder patterns From this result we assumed that only a certain part of the samples is ordered The Rietveld powder refinements (assuming two phases) showed about 70% LiBaSi-type and about 30% AlB,-type structures in the sample annealed at 650°C The refined lattice parameters (powder diffraction data) were a =419 25( 3) pm, c=398 14(2) pm, V=O 0606 nm3 for the LiBaSi portion and a =420 79( 5) pm, c =395 98( 5) pm, V= 0 0607 nm3 for the AlB, portion As could be expected, the cell volume of the disordered AlB, part is slightly larger than that of LiBaSi The order within the LiBaSi-type structure expresses itself by the smaller lattice parameter u and a larger lattice parameter c We conclude from this refinement, that (1) the annealing temperature of 650 "C was slightly too low to achieve complete order, or (ii) the annealing time of 2 weeks was too short for this temperature Lattice constants The lattice parameters and the cell volumes of the compounds in the pseudobinary system USi,-UAu, are given in Table 1 As can be seen easily from these data, the a and c parameters increase slightly, when some silicon atoms in p-USi, are replaced by the much larger (metallic radii 131 9 pm for Si and 1442 pm for Au, both for coordination number, CN, 12),l gold atoms The increase of both is small up to UAuSi and then the situation is different While a rises dramatically up to UAu,, c behaves in the opposite manner Crystal chemistry A part of the pseudobinary system USi2-UAu2 has been investigated by X-ray powder diffraction All patterns show hexagonal symmetry The border phases p-US121 and UAU,~ ' adopt the AlB,-type structure For UAuSi we have determined an order between the gold and silicon atoms They form a hexagonal BN-like network The previously reported inter- metallic U2AuSi3 did not show any order between the Au and Si atoms, however, ordered structures for 2 13 silicides were recently reported for the compounds Ln,RhSi, ,22 23 Ln,Pds~,,~ and U2RuSi3 The silicon atoms in P-USi, are thus substitutable by gold Table3 X-Ray powder data of UAuSi (sample annealed at 800°C); the observed (I,) and calculated (I,) intensities for both refinements (disordered AlB,-type and LiBaSi-type) are listed together with the corresponding residuals R,(I)" ~~~~~~ ~ AlB, type LiBaSi type hkl ~~~~~~ ~~ 280,,/degrees dCdA I0 IC 10-Ic I0 I, Io-Ic 100 24.45 3.6334 22350 16314 6036 24324 24783 459 101 33.35 2.6823 100000 100567 567 100OOO 98485 1515 110 43.06 2.0977 35665 36527 8 62 33121 32943 178 002 45.55 1.9883 9600 9929 329 908 1 9086 5 111 49.03 1.8554 689 716 27 145 146 1 200 50.14 1.8167 2567 2676 109 3615 364 1 26 102 52.38 1.7442 4633 4693 60 4846 482 1 25 201 55.54 1.6524 17274 17515 24 1 17760 1769 1 69 112 64.49 1.4431 15893 14683 1210 15605 15138 467 210 68.21 1.3733 342 316 26 276 264 12 202 70.08 1.3412 2128 2016 112 2170 2086 84 003 71.03 1.3256 125 119 6 22 22 0 21 1 72.77 1.298 1 12497 11776 72 1 13461 12918 543 103 76.39 1.2453 5307 4853 454 5670 5373 297 300 78.96 1.2111 3419 3092 327 3706 349 1 215 301 83.31 1.1586 658 598 60 119 112 7 212 85.92 1.1300 2590 2372 218 1776 1663 113 113 86.82 1.1206 619 570 49 116 108 8 203 9 1.97 1.0708 2572 2404 168 2792 269 1 101 220 94.48 1.0489 1594 1503 91 1917 1847 70 302 96.24 1.0343 2929 2797 132 3545 3464 81 22 1 98.81 1.0142 474 468 6 95 93 2 310 99.67 1.0077 667 660 7 510 503 7 RB(I)=0.049 RB(I)=0.018 The final residuals for the refinement with the ordered LiBaSi structure are R,, =0.1025, RE=0.0652 and x2 =2.4714.Table 4 Positional parameters and interatomic distances (in pm, calculated from the Guinier data) for UAuSi atom ~6m2 X Y Z B/A2 U Au Si la If Id 0 213 113 0 1/3 213 0 112 112 2.2(7) 3.6( 16) 1.7(35) U: 6 Au 313.2(2) Au: 3 Si 242.2(1) Si: 3 Au 242.2(1) 6 Si 2 U 6 U 313.2(2) 397.2(2) 419.5(2) 6 U 313.2(2) 6 U 313.2(2) atoms up to the other border phase, UAu,.This substitution reveals not only a drastic change in the lattice parameters as discussed above, but also a change in the coordination spheres, resulting in distinct differences in chemical bonding. A compari-son of the interatomic distances in p-USi2, U,AuSi,, UAuSi and UAu, is given in Table 5. In p-USi, the U-U distances of 385.2 and 402.8 pm are similar.This is totally different when all the silicon atoms are substituted by gold atoms. The U-U bond lengths in UAu, amount to 311.0 and 475.6 pm, respectively. While both U-U distances in p-USi, may not be considered as direct U-U interactions, the small U-U contact of 311.0 pm in UAu, is certainly strongly bonding. Similar short U-U distances have also be observed in the intermetallics U,Mn3Ge,25 U,Ti,26-28 Table5 Comparison of the interatomic distances (in pm) in the structures of p-USi,, U2AuSi3, UAuSi and UAu, compound u-u U-Au/Si Au/Si-Au/Si B-USi, 385.2 302.0 232.6 402.8 U,AuSi, 398.9 311.5 239.3 414.5 UAuSi 397.2 3 13.2 242.2 419.5 UAu, 311.0 315.6 274.6 475.6 U3Si29 and UHg,.,' They are in the same range as in a-U (average U-U bond length of 313.7 ~m).~' The coordination of the uranium atoms in P-USi, consists of 12 silicon atoms.Owing to the short U-U contacts, this coordination number increases to 14 in UAu,; 12 Au and 2 U. The U-Au/Si distances increase from 302.0 pm (p-USi,) to 311.5 pm in U,AuSi3 and then increase only slightly to 315.6 pm in UAu,. The slightly larger U-Au distance in UAu, also reflects the larger coordination number of 14. The interatomic distances within the hexagonal network increase from 232.6 (p-USi,) to 242.2 pm (UAuSi) and then increase markedly to 274.6 pm in UAu,. The increase of only 9.6 pm (the difference of the metallic radii amounts to 12.3 pm2') from P-USi, to UAuSi reflects the strong bonding between the gold and silicon atoms.The Au-Si distance of 242.2 pm is about 12% smaller than the sum of the metallic radii. It is even smaller than the Au-Si bond lengths of 249, 249,252 and 246 pm in ScAuSi (ScAuSi-type), YAuSi (LiGaGe- type), LuAuSi (S~AuSi-type)~, and ThAuSi (LiBaSi-type, iso- typic with UAuSi),13 respectively. In the Au, rings of UAu,, the Au-Au distances of 274.6 pm are about 5% smaller than the Au-Au bond lengths of 288 pm in gold rnetaL3' The coordination number for the atoms in the hexagonal network increases from CN 9 in p-USi, (6 U +3 Si) to CN 11 in UAu, (6 U+3 Au+2 Au). Physical properties UAuSi. In order to derive the influence of atomic ordering of Au and Si atoms on the physical properties of UAuSi, the temperature dependence of the magnetic susceptibility and electrical resistivity was studied on three samples of UAuSi: (i) as-quenched, (ii) annealed at 650°C and (iii) annealed at 800 "C.We have measured x(T) in magnetic fields up to 0.6 T employing both the ZFC (zero magnetic field cooling) and FC (field cooling) conditions. It appears that these three samples show almost identical temperature dependencies of the mag- netic susceptibility, similar to that already reported in ref. 10. In Fig. 3 we present the magnetic data for UAuSi (iii) only. J. Muter. Chem., 1996, 6(3), 429-434 431 / 0 50 100 150 200 250 300 TIK Fig. 3 Temperature dependence of the reciprocal magnetic susceptibil- ity of UAuSi The left-hand inset shows the FC and ZFC susceptibility behaviour at B=O05 T The field dependence of the freezing temperature, T,, is shown in the nght-hand inset As seen from the inset, the FC susceptibility shows a weak saturation tendency at low temperatures, while the ZFC suscep- tibility exhibits a maximum at Tf (freezing temperature) Thus, the temperature dependence of the magnetic susceptibility for this compound appears to be strongly dependent on the sample cooling conditions The observed anomaly at Tf may indicate a magnetic phase transition, however, this maximum is broad and its magnitude is too large to be ascribed to a simple antiferromagnetic phase transition Such a maximum might signal a transition into an anisotropic ferromagnet, but the magnetization behaviour of UAuSi (iii) taken at various tem- peratures below 40 K in magnetic fields up to 4 T does not show characteristics of a ferromagnet As seen from Fig 4, the magnetization, a@), does not show saturation at 4 2 K At the highest obtainable magnetic field of B=4 T, only the very Fig.4 Magnetization us magnetic flux density of UAuSi at (a)42, (b) 20 and (c) 40 K 432 J Mater Chem , 1996,6(3), 429-434 small magnetic moment of p=O 19 pB(atom U)-' is observed However, at temperatures between 42 and 20 K, a small remanence is present in o(B) As suggested in ref 10, the x anomaly is thought to originate from spin-glass (SG) behaviour Therefore, we have investi- gated the irreversibility line from xzFc(T) and zFC(T)taken in several magnetic fields Defining the freezing temperature, T,, as the splitting point of the xZFCand zFCcurves, we have determined Tf as a function of the applied magnetic field (see Fig 3 inset) It is clear that Tf follows well the theoretical Almeida and Thouless line (AT line)33 in the low magnetic field limit, strongly supporting the SG-like behaviour for UAuSi The extrapolated T,value to B=O is about 17 K This value of Tf is almost the same as that found in our previous electrical resistivity measurements lo It should be mentioned that the AT straight line was predicted for the case of an Ising- type SG in the infinite-range, random-bond model33 On the other hand, as shown above, the magnetic properties of UAuSi are found not to depend on the degree of the atomic order and therefore such an interpretation for UAuSi is not quite adequate Also, at present it is difficult to claim that any non- stoichiometry of this compound is a possible reason of the SG-behaviour observed SG-behaviour is found for numerous uranium- and rare-earth-metal-based intermetallics crystallizing in hexagonal structures, such as CeCd,-type (UCUS~),~~AIB,-type (U2TSi3),7 and Ln(Al,Ga), 35 For all these hexagonal phases the XY-type mechanism seems to be responsible for the SG- formation Also, the existence of some randomness in the interactions between the U-U or Ln-Ln atoms, since a statisti- cal distribution of the non-magnetic atoms in the unit cell of a given compound can lead to the SG properties This mechanism has been postulated for CePd3B0336 and CePtGa, 37 For all samples of UAuSi, the magnetic susceptibility is reversible above T, and reaches almost the same values As seen from Fig 3, the x-'(T) function shows modified Curie- Weiss behaviour, yielding pexp=3 14 pB (U atom)-', @= 17 1 K and xo=O 3 x emu mol-' These values are in agreement with those reported in ref 10 In contrast to the x(T) behaviour, the temperature depen- dence of the electrical resistivity changes for different sample preparation methods The electrical resistivity measurements as a function of temperature, p(T), for the three samples of UAuSi are given in Fig 5 While the resistivity of the samples which are non-annealed or annealed at 650 "C increases with decreasing temperature in a Kondo-like manner, the resistivity of the sample annealed at 800 "C, I e of that showing full 1 a0921 ' ' ' ' ' 0 50 100 150 200 250 300 TIK Fig.5 Temperature dependence of the reduced electrical resistivity for the three different UAuSi samples The spin freezing temperature is indicated by arrows 1, as-cast, 2, annealed at 65OoC, 3, annealed at 800 "C crystallographic order, decreases distinctly with decreasing temperature (metal-like behaviour). Also important differences occur in the magnitude of p. The resistivity of samples (i) and (ii) is about an order of magnitude larger than that of sample (iii). These differences certainly reflect some variation in the degree of the crystallographic order of the Au and Si atoms.Nevertheless, there exists a common feature for all these samples, namely, the p anomaly occurring at about 17 K. This temperature should be associated with the Tf value deduced from the extrapolation to B=O T (see Fig. 3). Note that the occurrence of the resistivity maximum is one of the most distinctive features of the metallic-type SG.38 Thus, this behav- iour may be characteristic of samples (i) and (ii). UAu,. The low-temperature properties of UAu, have already been reported in a number of previous investigations of the U-Au system.4.39-41However, serious controversy concerning the magnetic properties observed for this compound still remains. For example, Ott et were the first group to report Pauli paramagnetism of UAu,, but they assigned the CeCd,-type structure for their sample.On the other hand, the annealed UAu2 sample had the hexagonal A1B2-type structure, tivity pointed to the existence of a weak ferromagnetic compo- 1 0 TIK Fig. 7 Temperature dependence of the specific resistivity of UAu, (annealed, 800"C, 2 weeks, B = 0 T). The inset shows the derivative dpldT us. T below 50 K. 2 r I 1 I 1I , .?01 and both the magnetic susceptibility and the electrical resis- --................... . . . ...... nent below 25 K4 Moreover, Kondo-type behaviour or spin- fluctuation effects have also been considered by Canepa et aL4' or recently by Kontani et respectively. In the present investigation we have used the annealed UAu, sample for the measurements. Its magnetic susceptibility us temperature curve is displayed in Fig.6. Above T= 50 K the susceptibility behaviour for this sample obeys the Curie-Weiss law yielding peXp= 2.98 pB(atom U)-' and 0= -190 K. These values agree well with those previously reported for a non- annealed sample4' but they are slightly different from those found for an annealed sample by Kontani et aL41As illustrated in Fig. 6, the susceptibility of our UAu, sample exhibits a pronounced upturn below T = 30 K. This behaviour, which is a consequence of oxidation, will be analysed in detail in a forthcoming paper.42 The temperature dependence of the electrical resistivity for the annealed sample of UAu, is displayed in Fig.7. This curve is similar to that recently reported by Kontani et aL4' p(T) shows a pronounced increase in resistivity at low temperatures, and then a broad shoulder followed by saturation in the high- temperature region. This result resembles the p( T) behaviour characteristic of materials dominated by spin-fluctuation effects.43 The temperature derivative of the resistivity, dp(T)/dT exhibits a distinct maximum at 18 K. Some tiny anomalies are also observed at 8 and 40 IS. They are probably due to impurities and were also observed by Kontani et 400 300 c 100 Lo -I 60 100 150 200 250 300 TIK Fig. 6 Temperature dependence of the inverse magnetic susceptibility of UAu, (annealed, 800 "C, 2 weeks, B= 0.7 T) 0 30 60 90 120 150 TIK Fig.8 Temperature dependence of the magnetoresistivity of UAu, (annealed, 800"C, 2 weeks, B= 1 T). The field dependence of the magnetoresistivity at 4.2 K is given in the inset. In order to clarify the nature of the anomalies observed for UAu, we undertook measurements of the electrical resistivity at B= 1T. It is interesting to note that the transition observed at 40 K starts to develop distinctly at B= 1 T, but no visual change has been observed in the transitions at 8 and 18 K, respectively. The magnetoresistivity, Ap/p = [p(T,lT)-p(T,O)]/p(T,O), measured as a function of temperature, is displayed in Fig. 8. At 4.2 K, the magnetoresistivity is negative and decreases linearly with increasing applied magnetic field without any tendency to saturation (see Fig.8 inset). As the temperature is increased, the Ap/p curve increases, starting from a value of -8%, goes through zero at about 15 K, and then reaches a distinct maximum at 18 K. At higher temperatures, this curve remains almost unchanged, showing a positive value. The negative magnetoresistivity observed at low temperatures is in agreement with some ferromagnetic character of the sample. We thank Professor Dr. W. Jeitschko for his interest and support of this work. We are also indebted to Dr. W. 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