摘要:
J. CHEM. SOC. DALTON TRANS. 1992 255 1The Quadruple Chains of SbO, Octahedra in Sb,Te,O,:an Example of Low Extent of Aggregation of PentavalentAntimony Polyhedra tJose Antonio Alonso,8 Alicia Castro.*s8 Renee Enjalbert! Jean Galyb and lsidoro Rasinesea lnstituto de Ciencia de Materiales de Madrid, C.S.I.C., Serrano 113, 28006 Madrid, Spain3 1055 Toulouse Cedex, FranceCentre d'Elaboration de Matkriaux et d'Etudes Structurales, C.N. R.S., 29 rue Jeanne Marvig,Single crystals of Sb,Te,O, have been grown by transport methods, and the crystal structuredetermined by X-ray single-crystal techniques. The results have been confirmed by a neutronpowder diffraction refinement. The compound crystallizes in the monoclinic system, space groupP2,/c, Z = 8, with unit-cell parameters a = 21.79(1), b = 4.849(1), c = 14.574(9) A and =109.21 (3)". The structure consists of infinite quadruple chains of vertex-sharing SbV06 octahedra,running along the b axis, bridged by (Te60zs)03 strings parallel to the [00 11 direction and byisolated Te,O,, groups.The Te" is five- and six-co-ordinated by oxygen, in very irregularenvironments making room for the free electron pairs. This structure is discussed in the frameworkof other pentavalent antimony complex oxides and contrasted with those of the known oxides inthe M,O,-TeO, (M = V, Nb or Ta) systems.Mixed oxides containing semimetallic p elements in lowoxidation state, such as SeIV, Sb"' or Te", exhibit crystalstructures with remarkably irregular oxygen co-ordinationpolyhedra for these elements, due to the presence of theelectronic lone pair which occupies a volume approximatelyequivalent to that of an oxygen atom.Systematic investigationshave been performed in order to study the stereochemicalinfluence of the lone pair in new oxides containing Te1V.2-6Usually Te" shows three- or four-co-ordination, in trigonal-pyramidal or trigonal-bipyramidal configuration, respectively,with the lone pair directed towards the unoccupied position ofboth polyhedra. Two or more more units are, very often, linkedtogether to give groups of composition Te205, Te308, Te401 1,etc. The degree of aggregation of these units is stronglydependent on the character of the metal M atom in differentM-Te-0 oxides.A beautiful example is the gradual increase in both thetellurium co-ordination and the degree of aggregation of thetellurium polyhedra in compounds of the system M205-Te02(M = V, Nb or Ta) from V2Te209 to Ta2Te209.In V2Te209'all the tellurium atoms are three-co-ordinated, the trigonalpyramids sharing corners to give isolated Te20s units; thethree- and four-co-ordinated Te atoms in Nb2Te301 formTe30B groups, whereas in Ta2Te20g4 the two kinds of Tepolyhedra, trigonal bipyramid and distorted octahedron, con-stitute two-dimensional nets of composition (Te,O 2)n.In order to study the influence of replacing the mentioned dmetals in this family by pentavalent p elements, it seemedinteresting to grow crystals of the corresponding antimony(v)compound and to study its structure.The synthesis of Sb2-Te,Og and Sb2Te07 as well as a study of the thermal behaviourand the infrared spectra have been previously de~cribed.~ In thepresent work single crystals of Sb2Te20g were grown bytransport methods. This paper describes the crystal structure,t Supplementary data available: see Instructions for Authors, J. Chem.Soc., Dalton Trans., 1992, Issue 1, pp. xx-xxv.Table 1collection and refinementPhysical and crystallographic data and parameters for dataFormulaMCrystal systemSpace groupalAblACIAPI"u/A3DclgF(o00)p( Mo-Ka)/cm-'T/Kh(M0-Ka)/AMonochromatorTake-off angle/"Detect or wid t h/mmScan typeScan width/"Prescan speed/" min-'Final scan o(l)/IMaximum time/sReflections for cell refinementRecorded reflectionshkl rangeIntensit y-control reflectionsOrientation control reflectionsIndependent reflectionsTransmission coefficient rangeEmpirical absorption correctionSignificant reflectionsRefined parametersExtinction I1 (g)RR'SzU'0,Sb2Te642.7Monoclinicf w c21.79( 1)4.849( 1)14.574(9)109.21(3)1454(1)4 x 25.8722241572930.710 69Graphite3.84 x 40.70 + 0.35 tan0100.0 1860w 2 e25; e 6-16"3972,e G 30"&30, M, - 20 to 20410,008,235 every 3600 s913, 1410,020 every 250 reflections3798Ref.83713 with I > 3o(I)1466.5 x 10-40.04 10.05 57.65/(02F + 0.07F2)0.990.71-1.02552 J. CHEM. SOC. DALTON TRANS. 1992Table 2 Positional parameters for the X-ray and neutron refinementsX-RayX0.054 58(3)0.158 M(3)0.347 18(3)0.447 50(3)0.062 44(3)0.233 23(3)0.440 45(3)0.279 98(3)0.116 5(3)-0.092 4(3)-0.010 l(3)0.001 3(3)0.102 5(3)0.233 7(3)0.211 l(3)0.123 l(3)0.208 8(3)0.294 6(3)0.426 4(3)0.382 6(3)0.262 O(3)0.316 O(3)0.365 5(3)0.596 O(3)0.528 l(3)0.49 1 2( 3)Y0.718 8(1)0.219 0(1)0.713 8(1)0.216 9(1)0.666 5( 1)0.321 5(1)0.159 7(1)0.669 3(1)0.01 l(1)0.509( 2)0.41 7(2)0.393( 1)0.577( 1)0.960(2)0.451(1)0.069( 1)0.413(1)0.862( 1).0.093( 2)0.602(2)0.042( 1)0.01 l(2)0.952( 1)0.407(1)0.559( 1)0.396( 1)Z0.719 77(4)0.666 24(4)0.510 82(4)0.669 20(4)0.947 54(4)0.919 47(4)0.890 69(4)0.690 32(4)0.740 5(5)0.639 O(5)0.701 O(5)0.908 3(5)0.854 4(5)0.71 1 O(5)0.604 9(5)0.535 8(5)0.784 8(5)0.586 O(4)0.577 5(5)0.410 5(5)0.926 6(5)0.428 7(5)0.598 l(5)0.741 4(4)0.734 3(5)0.582 3(5)NeutronX0.054 7(5)0.160 O( 5 )0.346 4(6)0.448 8(5)0.063 O(4)0.232 9(5)0.441 l(4)0.281 7(5)0.117 3(4)0.092 2(4)-0.007 l(4)- 0.002 O(4)0.102 5(5)0.231 9(5)0.211 O(4)0.125 l(4)0.210 7(5)0.295 2(5)0.428 3(5)0.383 O(4)0.261 4(5)0.316 l(4)0.366 2(4)0.597 3(5)0.526 7(4)0.492 6(4)Y0.7 15(2)0.223(2)0.7 16(2)0.217(2)0.673(2)0.3 19(2)0.16 l(2)0.669(2)0.01 3(2)0.5 12( 2)0.41 3(2)0.386(2)0.572(2)0.960(2)0.45 1 (2)0.067(2)0.416(2)0.5 7 5 (2)- 0.090(2)0.600(2)0.955(2)0.039(2)0.397( 2)0.560( 2)0.008(2)0.39 1 (2)L0.719 6(7)0.665 7(7)0.509 5(8)0.671 O(8)0.947 7(7)0.920 9(7)0.889 6(6)0.690 5(8)0.744 l(7)0.634 O(7)0.701 O(7)0.909 3(6)0.851 7(7)0.709 3(8)0.606 2(6)0.535 6(6)0.789 2(7)0.590 2( 7)0.577 7(6)0.410 5(7)0.925 l(7)0.430 6(6)0.600 O(7)0.744 2(6)0.733 4(6)0.581 5(7)J I I I I I I I I 1 1 I1 I I I I I I I1 1 I I I I I I I I I I I I I I I I I1 I I I I I1 I I I I I I I I I1 I I I I I I I U l l l ~ l ~ U ~ ~ I I I I I1 I I I I I I I I Ud I I I I I I I I I1 I I I I I I I I 1 1 I I I I20 40 60 80 100 120 14020t"Fig.1 Neutron powder diffraction profile of Sb,Te,O, at 295 K. Crosses are the raw data points; the solid line is the best calculated profile. Thedifference plot (observed - calculated) appears at the bottom.The marks below the profile indicate the positions of the allowed reflections (2667)included in the calculationssolved by X-ray single-crystal methods. The results are com- general framework of the pentavalent antimony complexpared with those obtained from neutron powder diffraction oxides, and compared with those of the known compounds indata. The structural peculiarities are discussed within the the M,0,-Te02 systemsJ. CHEM. SOC. DALTON TRANS. 1992 2553Table 3 Selected bond distances (A) and angles (") in Sb,Te,O,Sb( 1)-O( 1')Sb( 1 )-O( 3")Sb( 1)-0(2)S b( 1 )-O( 3)Sb( 1 tO(5)Sb( 1)-0(4")Sb( 2)-O( 1 )Sb(2)-0(8)Sb(2)-0(2)Sb(2F0(9)Sb(2)-O(7)Sb(2)-0(6"')Te( 1)-0(4)Te( 1 )-O( 5 )Te( 1)-0(8)Te(1)- O(3")Te( 1 jO(4"")Te( 1 to( 2")Te(2)-0( 14)Te(2 j O ( 13)Te( 2)-0(9)Te(2 j O ( 1OIv)Te( l)-0(2Iv)O( 1')-Sb( 1)-0(2)O( 1 I)-Sb( 1 )-O( 3")O(1')-Sb( 1)-O(5)O( 1 '&Sb( 1)-O(4")O(2)-Sb( 1)-0(3)O(2)-Sb( 1)-O(5)O(2)-Sb( 1)-0(4")0(3")-Sb( 1)-0(3)0(3")-Sb( 1)-O(5)O( 3")-Sb( 1 )-O(4")O(3)-Sb( 1)-O(4")O( 3)-Sb( 1 )-O( 5 )O( 1 )-Sb(2)-0(8)O( 1 bSb(2)-0(2)O( 1 jSb(2)- O(9)O( 1 )-Sb(2)-0(6"')O( 8)-S b(2)-0(6"')O( 8tSb(2)-0( 2)O( 8)-S b( 2)-O( 7)0(2)-Sb(2)--0(9)0(2)-Sb(2)-0(7)0(9)-Sb(2) O(6"')O( 6"')-Sb( 2)-O( 7)O(4)-Te( 1 )-O(5)O(4)-Te( 1)-0(8)O( 5)-Te( 1 ) 0(8)O( 14)-Te(2)-0( 13"')O( 14)-Te(2)-0(9)O( 13"')-Te( 2)-0(9)Sb( 11")-0(1)-Sb(2)Sb( 1 )-O( 3) -Sb( 1 IX)O( 9)-S b( 2)-O( 7)Sb( 1)-0(2)-Sb(2)1.9 12(7)1.934(8)1.982(8)1.989(7)2.016(6)2.030(6)1.9 lO(8)1.94 1 (7)1.952(7)1.956(6)2.004( 7)2.020(8)1.869(7)1.890(8)1.895(6)2.415(7)2.848(8)3.079(7)1.884(7)1.892( 7)1.909(7)2.8 1 l(6)3.079(7)93.8(3)90.2(3)89.3(3)89.8(3)87.4(3)103.0(3)83.6(3)88.7( 1)78.1(3)93.4( 3)89.9(3)90.9(3)104.6(3)92.5(3)89.3(3)88.7(3)92.6(3)95.7( 3)82.7(3)89.8(3)89.7(3)81.3(3)83.3( 3)88.0(3)95.1(3)93.6(3)88.0(3)92.1(3)85.9(3)95.1(3)138.0(4)133.3(4)133.4(4)Sb(3)-0(15) 1.9 14(7)Sb(3)-O( 11') 1.924(6)Sb(3)-0(12) 1.941 (8)Sb(3)-0( 14') 1.973(7)Sb( 3)-0( 1 3v"') 2.024(6)Sb(3)-O( 10) 1.964(7)Sb(4)-0(17'> 1.949( 7)Sb(4)-0( 1 1) 1.96 1 (7)Sb(4)-0( 17) 1.97 1 (7)Sb(4 j O ( 15) 1.974( 6)Sb(4tO( 16") 1.998(7)Sb(4)-0( 18) 2.01 5(8)Te( 3)-O( 12) 1.877(8)Te( 3)-O( 1 gV') 1.890(6)Te(3)-O( 18") 1.903(7)Te( 3)-O( 1 8) 2.657(7)Te(3)-0( 17") 2.740(8)Te( 3)-O( 1 1 ") 3.03 1 (7)Te(4)-0( 6) 1.8 14( 7)Te(4)-0( 10) 1.898(7)Te(4)-O(7) 1.922(6)Te(4F0(9) 2.694(8)Te(4)-O( 15) 2.923(8)O( 1 5)-S b( 3)-O( 1 1 I)O( 15 j S b ( 3 w ( 12)O( 15)-Sb(3)-0( 13'"')0 ( 1 11tSb(3)-O( 12)0(11')-Sb(3 j O ( l 0 )O( 1 l')-Sb(3)-0( 14')O( 12)-Sb(3)-0( 14')O( 12tSb(3)-0( 13'"')O( 1 OtSb( 3)-0( 14')O( 1 O)-Sb( 3 )-O( 1 3'"')0(14')-Sb(3)-0(13V11')0(15)-Sb(3w(10)O( 1 7v)-Sb(4)-0( 17)O( 1 7V)-Sb(4)-0( 15)O( 17')-Sb(4w( 16"')O( 1 7V)-Sb(4t0 (1 8)O( 1 1 )-Sb(4)-0( 17)O( 1 l)-Sb(4)-0( 16")O( 1 1 )-Sb(4)-0( 18)O(17 jSb(4)-O(16V')O( 15)-Sb(4w( 16")O( 1 l)-Sb(4w( 15)O( 17)-Sb(4)-0( 18)O( 15)-Sb(4)-0( 18)95.1(3)103.7(3)86.9(3)92.5( 3)90.2(3)97.5(3)89.2(3)84.9(3)86.9(3)84.0(3)84.0(3)8 3.5( 3)92.0(2)91.3(3)8 3.4( 3)93.8(3)84.9(3)91.7(3)95.5(3)87.4(3)91.7(3)89.5(3)88.8(3)90.2(3)0(12)-Te(3)-0(16V') 84.5(3)0(12)-Te(3)-0(18V') 91.9(3)O( 1 sV')-Te(3)-0( 18") 94.3(3)0(6)-Te(4)-0( 10) 90.0(3)0(6tTe(4)-0(7) 100.1(3)0(10)-Te(4)-0(7) 93.0(3)Sb(3)-0(15)-Sb(4) 131.8(4)Sb(4V')-O( 17)-Sb(4) 135.0(4)Sb(3"')-0(ll)-Sb(4) 133.4(4)Symmetry code: I x, 1 + y, z; I1 -.x, i + y, 3 - z; I11 x , y - 1, z; IV .Y, 2 - 1', 4 + z; v 1 - x 1 + y 3 - z; v1 1 - x, y - $, + - z; VII - x,1 - ;1', 2 - 2; VIII .x, + - I', 2 - f; IX -x, y - +, 2 - 29 2 9 2ExperimentalPreparation.-Polycrystalline Sb2Te,09 was prepared asdescribed el~ewhere.~ Single crystals were obtained by means ofa transport reaction, using TeCl, as the transport agent.Amixture of powdered Sb2Te,0g (1 g) and TeCl, (50 mg) washeated in a sealed evacuated Vycor ampoule (diameter 12 mm,Table 4 Bond valences for Sb-0 and Te-0 in Sb,Te,O, *Atom 1 2 3 4 5 6 ZSSb(1) 1.91 1.93 1.98 1.99 2.02 2.03Sb(2) 1.91 1.94 1.95 1.96 2.00 2.02Sb(3) 1.91 1.92 1.94 1.96 1.97 2.02Sb(4) 1.95 1.96 1.97 1.97 2.00 2.021.00 0.93 0.80 0.79 0.73 0.70 4.951.00 0.91 0.88 0.87 0.75 0.72 5.130.99 0.96 0.91 0.85 0.83 0.71 5.250.89 0.86 0.83 0.82 0.77 0.73 4.90Te(1) 1.87 1.89 1.90 2.42 2.85 3.081.28 1.21 1.19 0.34 0.14 0.10 4.26Te(2) 1.88 1.89 1.91 2.81 2.961.23 1.20 1.15 0.15 0.12 3.85Te(3) 1.88 1.89 1.90 2.66 2.74 3.031.25 1.21 1.16 0.21 0.18 0.10 4.11Te(4) 1.81 1.90 1.92 2.69 2.921.49 1.18 1.11 0.19 0.13 4.10* Values are d(M-0) followed by SM where S,, = (d/1.911)-6.0 fromref.14 and S,, = 1.333 (d/1.854)-5.2 from ref. 15.length 30 cm) in a temperature gradient of 750-650 "C for 3 d.Colourless needles (length 2 mm, average width 0.2 mm) werefound in the cold extremity of the ampoule. They weresuccessively washed in HCl and distilled water, and then driedat 80 "C.X- Ray DifJi.action.-The data collection was performed usingan Enraf-Nonius CAD4 diffractometer. Table 1 shows physicaland crystallographic data together with the experimentalconditions of data collection.The structure was determined by the Patterson method.Thedistinction between Sb and Te atoms was facilitated by theoxygen environment, with six homogeneous bonds around Sb.The refinements were carried out by full-matrix least-squarescalculations. Atomic scattering factors were corrected foranomalous dispersion.' Calculations were performed withSHELX l o using an ALLIANT VF-80 computer.Additional material available from the Cambridge Crystal-lographic Data Centre comprises thermal parameters andremaining bond lengths and angles.Neutron Diffraction.-Neutron powder diffraction data werecollected in the high-resolution D2B diffractometer at theInstitut Laue-Langevin (ILL)-Grenoble, using a wavelength of1.594(1) A.About 10 g of sample were enclosed in a vanadiumcan of 8 mm diameter. The pattern was collected at 295 K in theangular range 0 < 28 d 150", in steps of 0.05".The neutron diffraction profile was analysed by theRietveld' ' method, using the DBW l 2 refinement program. Theline shape of the diffraction peaks was generated with a pseudo-Voigt function. The coherent scattering lengths for Sb, Te and 0were 5.641, 5.430 and 5.805 fm, respectively.The regions 0 < 28 < 10 and 145 < 28 < 150" were ex-cluded in the refinement. A total of 121 parameters were refined,including six background coefficients, zeropoint, half-width,pseudo-Voigt and asymmetry parameters for the peak shape,scale factor, positional, thermal isotropic and unit-cell para-meters. The maximum shift-to-error value for an atomic co-ordinate in the final refinement cycle was S/o = 0.05.ResultsX - Ray Diffraction.-The final positional parameters aregiven in Table 2, and Table 3 lists selected bond distances andangles.Neutron Powder Diffraction.-The refinement of the neutro25540 991992Fig.2 (a) [O 1 01 Projection of the Sb2Te209 structure. Thick lines represent Sb-0 bonds, very thin lines Te-0 bonds at distances betewen 2.0 and3.1 A. (b) Antimony environmentspattern according to the structural model determined from X-ray data led to the final atomic parameters also included inTable 2. Fig. 1 shows the agreement between the observed andcalculated profiles. Final discrepancy factors R,, Rwp, Rexpt,, andx2 (defined in ref.11) were 4.88,6.39,4.73, and 1.82, respectively.The RI factor for the integrated intensities was 3.77 for 2667reflections. The neutron results do not show any significantstructural deviations from the X-ray model; the differences inatomic parameters are, in general, lower than two times thestandard deviation for the metallic atoms, and four times for theoxygens. A X-N Fourier difference map13 did not permitlocalization of well defined residual electronic density in theproximities of the Te atoms (lone pairs), probably due to thelarge scattering factor of both Sb and Te atoms present in thestructure.The structural description that follows refers to the dataobtained from the X-ray single-crystal study.Description ofthe Structure.-In the unit cell there are fourcrystallographically inequivalent antimony and telluriumatoms. Fig.2 shows a view of the structure projected along the[O lo] axis. All the Sb atoms are co-ordinated to six oxygensforming quasi-regular octahedra. The Sb-0 distances rangefrom 1.91 to 2.03 8, with an average of 1.98 A for Sb(1) andSb(4) and 1.96 8, for Sb(2) and Sb(3). The SbOs octahedrashare corners to give infinite quadruple chains parallel to the baxis (Fig. 3). As it can be seen in Fig. 3(a), there exist twocrystallographically non-equivalent chains. The first is built upby Sb(1) and Sb(2) octahedra (chain A), connected via 0(1),O(2) and O(3); the second contains the Sb(3) and Sb(4) octaJ. CHEM.SOC. DALTON TRANS. 1992 2555- x. .- . . ----I t t Chain Bz(a ) (b )Fig. 3 Two views of the quadruple chains of vertex-sharing SbO, octahedra: (a) [O 1 01 projection, (b) view perpendicular to a single chain (chain B)O(3")O( 4" I I ) O( 1 OTV)O(2I")Fig. 4 Oxygen co-ordination polyhedra of five- and six-co-ordinatedTe(1) and Te(2). Thin lines correspond to Te-0 bonds at distancesbetween 2.0 and 3.1 Ahedra (chain B), bonded through 0(11), O(15) and O(17). Theangle between the chains is 107". Brown's bond valences l 4 forS b O bonds, listed in Table 4, sum to a value close, on average,to the expected valence for antimony in this compound.The co-ordination number of the Te atoms can be discussedin terms of bond valence theory.I4 Table 4 includes the valencesassociated with the oxygens bonded to Te at distances lowerthan 3.1 A.Following Brown's criterion, largely applied byPhilipot in many tellurium compounds,' ' for distances largerthan 3.1 A the Te-0 interactions can be neglected, the bondvalences taking values lower than 0.09. In Sb,Te,O, each Teatom is strongly bonded to three oxygens, at distances between1.8 1 and 1.92 A, in a trigonal-pyramidal configuration. Accord-ing to Table 4, Te(1) and Te(3) are also co-ordinated to threeother oxygen atoms, in a very irregular environment allowingroom for the electronic lone pair. In the same way Te(2) andTe(4) are bonded to two additional oxygens in a W-octahedralconfiguration, where the inert pair is thought to occupy thesixth vertex.Fig. 4 shows the tellurium environment for bothkinds of oxygen co-ordination.In this way, Te atoms are grouped together by means of theweakest bonds corresponding to distances between 2.42 and 3.08A. Atoms Te(2) and Te(4) are linked via O(9) and O( 10) to giveinfinite strings parallel to the COO 13 direction. Bridges O(5)-Te( 1)-0(4)-Te( 1)-0(5) link neighbouring chains, giving rise toinfinite double strings, of composition (Te602,),. On the otherhand, pairs of Te(3) atoms are bonded via 0(18), formingisolated two-fold groups Te2010 (Fig. 5). Both kinds of telluriumgroups hold together the four-fold strings of Sb06 octahedra.DiscussionTellurium Network.-The association of tellurium co-ordin-ation units in Sb2Te209, giving rise to double strings ofcomposition (Te,O,,),, is intermediate between those foundin the isostoichiometric compounds of V and Ta,,v4 as expected.Considering the tellurium(1v) lone pair as a sphere with avolume similar to that of an oxygen atom,' the average volumeper anion in Sb,Te,OgE, (E = lone pair) is 16.5 A3.As a com-parison, the values for V,Te,Og, Nb2TeOg and Ta,Te,O, are15.6, 17.2 and 17.2 A3, respectively. This implies the existence ofvacant sites in the structure, which are physically occupied bythe electron pair. These sites are in the neighbourhood of eachTe atom, as it can be seen in Fig. 2.Antimony Network.-Pentavalent antimony forms a largevariety of ASbO, complex oxides l 6 with very different ele-ments A. All of these compounds are based on octahedral SbO,co-ordination groups, which are linked together, via commonvertices or edges, to give (Sb,O,), two- or three-dimensionalnets. The aggregation degree of the SbO, octahedra, propor-tional to the number of common oxygens, depends on thechemical nature of the elements A: with electropositive metalsthe aggregation degree of such polyhedra tends to be higher.It is useful to define a coefficient giving a quantitative idea ofthe degree of association of MOP polyhedra in a net characterizedby a repeating unit of formula M,O,.The aggregation factor(a.f.) can be defined as 2[p - (m/n)] /p. For instance, a$ = 0 forisolated polyhedra, without common oxygens, and 1 for a three-dimensional network in which each MOP polyhedron shares pcorners with p similar polyhedra (repeating unit M0,/2).In the crystallochemistry of pentavalent antimony there area large number of examples of complex oxides defined by arepeating unit (Sb,O,,), (q = 1, 2, etc.), a.f. = 1, such asKSbO, (ilmenite structure), A1Sb04 (random rutile), Ag,Sb2-0, (pyrochlore) or Cd2Sb207 (weberite).', Only for mor2556 J.CHEM. SOC. DALTON TRANS. 1992notdrawnelectronegative elements, such as Sb itself, in Sb204 (i.e.Sb"'SbvO,) can the layered structure of both a and ppolymorphs be described by a repeating unit (Sb208),,implying a lower aggregation factor, a.f. = 0.66.In Sb2Te20,, the tellurium electronegativity being still lower(Allred-Rochow l 8 electronegativities for Sb and Te are 1.82and 2.01, respectively), the Sb06 antimony octahedra are stillless associated.The repeating unit for the four-fold strings is(Sb,O,,),, with a.f, = 0.58. There are no examples, to ourknowledge, of pentavalent antimony complex oxides containingp elements more electronegative than tellurium. The lowestlimit of association of SbO, octahedra seems to be that foundin the Sb2Te2Og structure.It is also interesting to compare the M-0 nets of the knownM20,-Te0, oxides. In V2Te20g2 five-co-ordinated vanadiumatoms occupy the centre of trigonal bipyramids, which sharecorners to give infinite one-dimensional (VO,), strings, witha.f. = 0.40. The Nb06 octahedra share vertices in Nb2Te301 1,3to give double infinite one-dimensional chains (Nb209),, a.f.=0.50. As for Ta2Te2Og? Ta06 octahedra are linked via com-mon vertices forming a two-dimensional network (Ta,O 6)?,a.f. = 0.66. The aggregation degree found in Sb2Te20g isintermediate between those of the compounds of V and Ta. Thecomparison with Nb2Te3Ol1 is not strictly correct since theM/Te ratio is lower in this compound: the hypothetical oxideNb2Te2Og probably would show a higher degree of associationof the Nb06 octahedra. The different stacking of polyhedraalong the sequence chains, quadruple chains, layers can beexplained as a consequence of the gradual increase in the ioniccharacter of the M-0 bonds for M = V, Sb, Ta.There are many examples of isostoichiometric complex oxidesof NbV and TaV which crystallize in structures quite differentfrom that of the corresponding antimony(v) compound [e.g.LiSbO, (space group Pncn) DS.Li(Nb,Ta)O, (R~c)]. This factcannot be attributed to differences in electrostatic Madelungenergy, since the three cations have similar formal valences andsizes. Goodenough and Kafalas l 9 propose that the covalentcontribution to the M-0 bonds (M = Sb, Nb or Ta) isresponsible for the observed differences. Covalent o bonding ofan oxygen to two near-neighbour cations is stronger if thesecations are on the same side of the oxygen, which allows theparticipation of two oxygen p orbitals, and favours M-O-Mangles close to 90'. This is the predominant covalent contributionto the M-0 bond for SbV, which has a filled d'O core that can-not participate in 7c bonding to p orbitals of the neighbouringoxygens. On the contrary, NbV and TaV have empty d shells thatparticipate in covalent bonding to oxygens.The anion p orbitalsare shared between both covalent contributions, which weakensthe o bonding and favours 180" O-M-0 angles. In consequence,Nb and Ta stabilize different crystal structures from those of Sb.In the Sb2Te209 structure the Sb06 octahedra that consti-tute the quadruple chains are strongly tilted, showing Sb-O-Sbangles in the range 131.8-138.0" [see Table 2 and Fig. 3(b)], faraway from 180'. This is thought to be due to the strong (Tcovalent contribution to the Sb-0 bonding which tends toadjust the angles towards 90". Lower angles would involvestrong repulsions between pentavalent antimony cations.Onthe contrary, the Nb06 octahedra in Nb,TeJOll are scarcelyrotated, with Nb-O-Nb angles of 171.0 or 168.0'. This is alsotrue for Ta2Te209, which exhibits Ta-O-Ta angles of 145.5 or180.0". Both cases show clearly the partial inhibition of the (Tbonding. Thus, the covalent binding energy seems to play amajor role in stabilizing the Sb2Te209 structure.AcknowledgementsThe authors thank Dr. J. L. Soubeyroux for the collection of theneutron diffraction diagram, and the ILL for making all facili-ties available. A. C. thanks the Ministerio de Educacion yCiencia of Spain and the Centre National de la RechercheScientifique of France for support.References1 J. Galy, G. Meunier, S. Anderson and A. Astrom, J. Solid State2 J. Darriet and J. Galy, Cryst. Struct. Commun., 1973,2, 237.3 J. Galy and 0. Lindqvist, J. Solid State Chem., 1979,27,279.4 J. A. Alonso, A. Castro, E. Gutierrez Puebla, M. A. Monge, I. Rasines5 C. Pico, A. Castro, M. L. Veiga, E. Gutierrez Puebla, M. A. Monge6 A. Castro, R. Enjalbert, D. Lloyd, I. Rasines and J. Galy, J. SolidChem., 1975,13, 142.and C. Ruiz Valero, J. Solid State Chem., 1987,69, 36.and C. Ruiz Valero, J. Solid State Chem., 1986,63, 172.Stare Chem., 1990,85, 100J. CHEM. SOC. DALTON TRANS. 1992 25577 J. A. Alonso, A. Castro, A. Jerez, C. Pic0 and M. L. Veiga, J. Chem.8 N. Walker and D. Stuart, Acta Crystallogr., Sect. A, 1983,39, 158.9 D. T. Cromer and J. Waber, International Tables for X-RayCrystallography, Kynoch Press, Birmingham, 1974, vol. 4.10 G. M. Sheldrick, SHELX 76, Program for Crystal StructureDetermination, University of Cambridge, 1976.11 H. M. Rietveld, J. Appl. Crystallogr., 1969,2,65.12 R . A. Young and D. B. Wiles, J. Appl. Crystallogr., 1982, 15,430.13 P. Coppens, Top. Curr. Phys., 1978.14 1. D. Brown, Structure and Bonding in Crystals, eds. M. OKeefe andSoc., Dalton Trans., 1985, 2225.A. Navrotsky, Wiley, New York, 1981, vol. 2.15 E. Philippot, J. Solid State Chem., 1981,38,26.16 A. F. Wells, Structural Inorganic Chemistry, Clarendon Press,17 J. Amador, E. Gutierrez Puebla, M. A. Monge, I. Rashes and C. Ruiz18 A. L. Allred and E. G. Rochow, J. Inorg. Nucl. Chem., 1958,5,264.19 J. B. Goodenough and J. A. Kafalas, J. Solid State Chern., 1973, 6,Oxford, 1984.Valero, Inorg. Chem., 1988,27, 1367.493.Received 18th March 1992; Paper 2/01436
ISSN:1477-9226
DOI:10.1039/DT9920002551
出版商:RSC
年代:1992
数据来源: RSC