Transition-metal stannides with high tin content: Os,Sn,,, RhSn,, RhSn, and IrSn, Arne Lang and Wolfgang Jeitschko Anorganisch-Chemisches Institut, Universitat Miinster, Wilhelm-Klemm-Str. 8, 0-48149 Miinster, Germany The title compounds have been prepared in well crystallized form by lengthy annealing of the elemental components with the atomic ratios varying between 1:3 and 1:10. The tin-rich matrix was dissolved in dilute hydrochloric acid. The crystal structures of these stannides were determined from single-crystal X-ray data. Os,Sn,, crystallizes with a new structure type: Pbcrn, a=694.9( 1) pm, b= 1428.1(3)pm, c= 1921.1(4) pm, Z=4, R=0.023 for 1573structure factors and 102 variables. RhSn, is isotypic with P-CoSn,: 141/acd, a=632.4( 1)pm, c= 3412( 1) pm, Z= 16, R=0.016 for 231 F values and 21 variables.RhSn, has a IrGe,-type structure: P3,21, a= 677.4(2)pm, c= 861.4(2)pm, Z= 3, R=0.018 (1285 F values and 26 variables). IrSn, is isotypic with RhSn, and IrGe,: P3,21, a =679.1 (2) pm, c =857.5(3)pm, Z =3, R =0.018 (1165 F values and 26 variables). In all of these structures the transition-metal (T) atoms have well defined coordination polyhedra. The coordination polyhedron of the Rh atom in RhSn, is formed by one Rh and eight Sn atoms; in the other compounds (with higher tin content) the T atoms have (only) eight or nine Sn neighbours with T-Sn distances covering the relatively narrow range 267-303 pm. In contrast, the coordination polyhedra of the Sn atoms are difficult to define. In addition to the four (one Sn site in RhSn,) or two T neighbours (all others), each Sn atom has between ten and twelve Sn atoms in its coordination shell at distances almost continuously covering the range 286-408 pm.As a consequence the structures contain relatively large voids. Magnetic susceptibility measurements carried out with a SQUID magnetometer indicate Pauli paramagnetism for all four compounds, in agreement with the metallic conductivity found by four-probe electrical conductivity measurements for Os,Sn,, superconducting down to 2 K. In the course of our investigations of transition-metal stannides with high tin contents we have reported on MoSn,,' VSn,,, NbSn,2 and CrSn,., Recently we reinvestigated the tin-rich side of the cobalt-tin binary system, where we characterized two modifications of the new compound COS~,.~ The low- temperature a-phase was found to be isotypic with PdSn,;, for the high-temperature P-phase a new structure type was established.We have subsequently searched for isotypic stan- nides in related binary systems and found the compounds Os,Sn,, and RhSn,, which, to our knowledge, have not been reported previously. The other two stannides discussed in the present paper, RhSn, and IrSn,, have already been mentioned in the literature. A compound with the tentative composition RhSn, was observed to form a eutectic with tin, but the compound was not characterized f~rther.~ IrSn, has been investigated by "'Sn Mossbauer spectro~copy~.~ and appar- ently its trigonal crystal structure had been determined.' However, it seems that this structure has never been published.Another modification of IrSn, has been prepared at high pressure, which crystallizes with the orthorhombic PtSn,-type structure.' In the present paper we report the crystal structures and some physical properties of Os,Snl,, RhSn,, RhSn, and the ambient-pressure modification of IrSn,. Experimental Sample preparation Starting materials were powders of osmium (Ventron: 99.9%), rhodium (Matthey: 'reinst'), iridium (Ventron: 'reinst') and small granules of tin (Merck: 99%). Well crystallized samples of 0s4Snl7, RhSn, and IrSn, were obtained by annealing the transition metals with tin in the atomic ratio 1:10 in evacuated silica tubes at 550°C for 2 days, followed by 7 days at 240°C (0s4Snl7) or 5 days at 300°C (RhSn, and IrSn,) and sub- sequent quenching in water.The sample of RhSn, was prepared in a similar way. The atomic ratio was 1:3 and the annealing was at 550°C for 2 days, followed by 7 days at 330°C. The tin-rich matrix of the samples with the starting ratio 1:10 was ,RhSn, and IrSn,. The compounds do not become dissolved in moderately dilute (1:1) hydrochloric acid, which attacks the binary stannides at a slower rate. The sample of RhSn, was also treated with hydrochloric acid to separate the crystals; grinding or crushing is not well suited, because this compound has some ductility. Energy-dispersive X-ray analy- ses of the four stannides in a scanning electron microscope did not show any impurities with an atomic mass greater than that of sodium.X-Ray diffractometry Guinier powder diagrams of the samples were recorded with Cu-Ka, radiation using a-quartz (a=49 1.30 pm, c =540.46 pm) as an internal standard. The lattice constants were refined by least-squares fits. To ensure the proper assignment of the indices the observed intensities were compared with the calcu- lated" ones, ultimately using the positional parameters of the refined structures. Single crystals suitable for the collection of the intensity data were selected on the basis of Laue patterns. The data were determined with an Enraf-Nonius four-circle diffrac- tometer using graphite-monochromated Mo-Ka radiation and a scintillation counter with pulse-height discrimination. The scans were along 8 with background counts on both ends of each scan.Empirical absorption corrections were made on the basis of psi-scans. Further details of the data collection are summarized in Table 1. RhSn, was recognized to be isotypic with P-CoSn,, from the Guinier powder diagrams. The structures of the other stannides were determined from Patterson and difference Fourier syntheses using the program package SHELXL86.l' They were refined by a full-matrix least-squares program using the atomic scattering factors provided by the Enraf-Nonius SDP programs.12 The weighting schemes accounted for the counting statistics and parameters correcting for isotropic secondary extinction were optimized as least-squares parameters.The space group of RhSn, (14,lacd) is unique. The structure of Os,Sn,, was refined in the space group Pbcrn, the group J. Muter. Chern., 1996,6( 12), 1897-1903 1897 Table 1 Crystal data for Os,Sn17 , RhSn, ,RhSn, and IrSn,' 0s4Sn 17 RhSn, RhSn, IrSn, structure type 0s4Sn17 CoSn, IrGe, IrGe, Pearson symbol oP84 t164 hP15 hP15 formula mass 2778 5 458 98 577 7 667 0 space group Pbcm (no 57) 14,lacd (no 142) P3121 (no 152) P3,21 (no 152) alpm 694 9( 1) 632 4( 1) 677 4(2) 679 l(2) blpm 1428 l(3) clpm 1921 l(4) 3412( 1) 861 4(2) 857 5(3) v/nm3 1906 5 1364 6 342 3 342 4 z 4 16 3 3 calculated densitylg cm-, 9 68 8 94 8 41 9 70 crystal dimensions/pm3 20 x 20 x 40 lox 10x20 40 x 40 x 40 30 x 30 x 40 rdnge in h, k, I +11, +22, -30, +18 & 10, f10, +50 k12, f12, +15 +12, +12, f15 0120 scans up to 29 =70" 28 =70" 20 =80" 28=80" total number of reflections 17167 6097 8426 8414 highest/lowest transmission 135 145 122 1 80 unique reflections 4727 964 1421 1421 inner residual, R, 0 050 0 042 0 030 0 035 reflections with I, >3o(I,) 1573 23 1 1285 1165 number of vanables 102 21 26 26 highest residual peakle A 22 0 56 24 45 conventional residual, R 0 023 0 016 0 018 0 018 weighted residual, R, 0 023 0 015 0 022 0 020 ~~~~~~ ~~~ "Standard deviations in the place values of the last listed digits are given in parentheses throughout the paper with the highest symmetry compatible with the space group R, =O 089 for the incorrect one, even though the positional extinctions The situation was more complicated in the case of parameters were the same within three standard deviations the trigonal structures of RhSn, and IrSn, In these cases the As is usually the case for solid-state compounds, it can be structure refinements eventually resulted in the non-centrosym- expected that the samples of RhSn, and IrSn, contained metric enantiomorphous space groups f'3,21 and P3221, crystals of both space groups, P3121 and P3221, in equal respectively To facilitate comparisons, the dca of IrSn, were amounts The positional parameters were standardized using transformed to the other handedness (hkl-thkl) and therefore the program STRUCTURE TIDY l3 The final residuals, both structures are now described in P3121 The differences in atomic parameters and interatomic distances are listed in the residuals between the refinements with the wrong and the Tables 1-5 Anisotropic thermal parameters have been correct handedness were AR=0 001 and AR, =0 002 for deposited at the Cambridge Crystallographic Data Centre RhSn, For IrSn, the differences were much larger R =O 018 (CCDC) See Information for Authors, J Muter Chem, 1996, and R, =O 028 for the correct handedness and R =O 076 and Issue 1 Any request to the CCDC for this material should quote the full literature citation and the reference number Table 2 Atomic parameters of 0s4Snl7, RhSn,, RhSn, and IrSn,' 1145/17 atom X Y Z Be, Electrical conductivity and magnetic measurements Pbcm Single crystals of 0s4Snl7, RhSn, and IrSn, with the largest 8e 0 02744( 7) 0 2441 l(4) 0 11942(2) 0 323(6) dimension of approximately 0 5 mm were contacted with four 8e 0 48741(8) 0 00303(4) 0 12445(2) 0 360(6) copper filaments using a silver epoxy cement The electrical 8e 0 1353(2) 006061(7) 0 07301 (5) 0 64( 1) resistivities of these crystals were measured in the temperature 8e 0 1675(2) 041750(6) 0 09010(5) 0 59( 1) 8e 0 2087( 1) 0 61322(7) 0 17243(5) 0 68( 1) range 4-300 K using the van der Pauw technique 8e 0 3737(1) 0 18674( 7) 0 17172( 5) 0 63( 1) The magnetic susceptibilities of the binary stannides 8e 0 5589(1) 0 09903 (6) 0 00346( 6) 0 62( 1) Os,Sn,,, RhSn,, RhSn, and IrSn, were determined for poly- 8e 0 6832( 2) 0 33370( 7) 0 15041(6) 0 76( 1) crystalline samples, which did not show any impurity phases 4d 0 0480( 2) 0 3079( 1) 114 0 74( 2) on the Guinier powder diagrams A SQUID magnetometer 4d 0 2236( 2) 0 0063( 1) 114 0 90( 2) was used for these investigations at temperatures between 2 4d 0 3705(2) 0 4362( 1) 114 0 69(2) 4c 0 2598(2) 114 0 0 56(2) and 300 K with magnetic flux densities of up to 5 T 4c 0 8302( 2) 114 0 0 58(2) I4,lacd Results and Discussion 16d 0 114 0 33313(3) 0469(7) 16f 0 1599( 1) 0 4099 118 0 492( 7) The four stannides are stable in air for long penods of time 3% 0 1760( 1) 0 4270( 1) 0 54088(1) 0 667( 8) The crystals of Os,Sn,, have the shape of elongated prisms, P312I those of RhSn, and IrSn, have the form of approximately 3b 031119(6) 0 516 0 410(6) equidimensional polyhedra and the crystals of RhSn, are 6c 0 23646(4) 0 50036(4) 0 42990( 3) 0 672( 4) platelike As is usually observed, the preferred growth direc- 3a 009119(6) 0 113 0 664( 6) tions of these crystals are those with the short translation 3a 0 63043 (6) 0 1/3 0 685(6) penods All of these compounds are somewhat ductile, never- P3121 theless, they can be ground to a fine powder at room 3b 0 31220(4) 0 516 0 265(3) temperature6c 0 23334( 6) 049821(6) 0 43296(4) 0 523( 5) The well crystallized samples of the four stannides have tin- 3a 0 08982(8) 0 113 0 534(8) like metallic lustre, and as might be expected from their 3a 0 631 18(8) 0 113 0 523(8) composition, they show metallic conductivity (Fig 1) The "The equivalent isotropic thermal parameters Be, are listed in units room-temperature resistivities were found to vary between 30 of lo4pm2 and 70 p.sZ cm for Os,Sn,, and RhSn,, respectively However, 1898 J Muter Chem, 1996, 6(12), 1897-1903 Table 3 Interatomic distances in the structure of Os4SnI7" 267 3 267 4 268 8 272 0 273 4 277 7 280 7 286 8 273 9 274 4 276 4 277 1 277 6 279 0 287 5 288 7 303 0 276 4 286 8 295 2 309 6 272 0 277 1 295 2 302 1 315 7 322 4 326 8 339 1 354 6 362 0 375 5 377 0 392 1 395 2 268 8 279 0 298 0 308 6 314 3 315 1 322 4 273 4 288 7 300 8 303 3 308 6 309 6 316 1 322 2 351 1 369 8 386 7 392 1 274 4 287 5 286 5 294 7 299 6 322 6 326 8 327 9 344 6 273 9 277 7 303 3 318 7 319 7 322 6 324 5 326 5 329 0 375 5 378 2 382 6 395 2 267 3 289 5 319 7 322 2 340 4 354 6 362 4 303 0 Sn(9) 20s(2) Sn(7) Sn(8)2Sn( 3) 2Sn (6) 2Sn(2) 2Sn (4) Sn( 10) 20s( 1) Sn(l1) 2Sn(5) 2Sn(2) 2Sn( 1) 2Sn(4) Sn(l1) Sn( 11) 20s( 1) 2Sn( 5) Sn( 10) 2Sn(6) 2Sn( 1) 2Sn(2) 2Sn(3) 277 6 289 5 299 3 314 3 324 5 339 1 386 7 280 7 298 6 299 6 302 1 316 7 351 1 396 4 267 4 286 5 298 6 329 0 371 2 377 0 385 5 315 1 326 5 362 0 299 3 Sn( 10) 396 4 315 7 362 4 363 1 316 1 316 7 363 1 369 8 318 7 327 9 368 5 340 4 344 6 385 5 354 1 354 1 368 5 371 2 378 2 379 5 "All distances shorter than 415 pm are listed Standard deviations computed from those of the lattice parameters and the positional parameters are all 0 2 pm or less Table 4 Interatomic distances in RhSn," 13 pi2 cm depending on the direction," since higher resistivities can be expected for intermetallic phases than for a pure metallic Rh 1Rh 285 7 Sn(2) 1Rh 273 7 element The relative resistivities are more reliable They2Sn (2) 273 7 1Rh 274 4 2Sn( 2) 274 4 lSn(2) 294 3 decrease systematically with the temperature, as is typical for 4Sn( 1) 277 3 lSn(2) 315 7 metallic conductors The decrease for IrSn, is rather small and Sn(1) 4Rh 277 3 1Sn( 1) 322 2 this may be due to minor amounts of impurities, as is also 1Sn( 1) 286 I 2Sn(2) 329 4 suggested by the magnetic measurements 2Sn(2) 322 2 2Sn(2) 329 8 The magnetic susceptibility data of all four compounds 4Sn( 1) 336 1 1Sn( 1) 357 2 indicate Pauli paramagnetism (Fig 2) The susceptibilities at 2Sn(2) 357 3 1Sn( 1) 357 6 2Sn(2) 357 6 lSn(2) 368 6 room temperature (not corrected for the core diamagnetism) are very small They vary between 0 2 x and 3 5 x lop9m3 "All distances shorter than 420 pm are listed Standard deviations are formula unit (f u)-l The data were fitted to the modified all 0 1 pm or less Curie-Weiss law x=xo+C/(T-@) This resulted in the tem- perature-independent xo values of 3 0 x 0 06 x lop9, Table 5 Interatomic distances in RhSn, and IrSn4" 03 x and 0 8 x m3 (f u)-' for Os,Sn,,, RhSn,, RhSn, and IrSn,, respectively The moments calculated from Rh/Ir 2Sn(l) 270 11270 5 Sn(2) 2Rh/Ir 286 21286 4 the temperature-dependent terms varied between 0 11 and 2Sn( 1) 273 31274 6 2Sn( 2) 306 41304 7 0 48 pB (f u)-' for RhSn, and IrSn,, respectively Thus, they 2Sn(3) 273 81273 9 lSn(3) 312 11311 4 are much smaller than the value of 1 73 pB expected for one 2Sn( 2) 286 21286 4 2Sn( 1) 313 31313 4 unpaired electron per formula unit, and for this reason the Snl 1Rh/Ir 270 11270 5 2Sn( 1) 315 71313 5 lRh/Ir 273 31274 6 1 Sn( 3) 365 31367 6 corresponding upturns of the magnetic susceptibilities at low lSn(3) 298 61296 2 2Sn(3) 365 31364 5 temperatures should be ascribed to paramagnetic impurities lSn(2) 313 31313 4 2Sn( 1) 405 51407 6 or to paramagnetic surface states The temperature dependence lSn(2) 315 71313 5 Sn(3) 2Rh/Ir 273 81273 9 is largest for IrSn, This compound also showed a rather small lSn(3) 319 91323 0 2Sn( 1) 298 61296 2 temperature dependence of the electrical conductivity This lSn(1) 332 31332 0 lSn(2) 312 11311 4 sample may therefore have been contaminated by the homo- 2Sn( 1) 333 61333 9 2Sn( 1) 3 19 91323 0 2Sn( 1) 377 31380 1 lSn(2) 365 31367 6 geneous inclusion of an unknown impurity element lSn(3) 388 81388 1 2Sn (2) 365 31364 5 The magnetic susceptibilities were also determined with lSn(3) 401 41402 6 2Sn( 1) 388 81388 1 small magnetic flux densities, in particular at temperatures lSn(2) 405 51407 6 2Sn( 1) 401 41402 6 down to 2 K, the lowest temperature attainable with our instrument None of the samples showed highly negative "All distances shorter than 450 pm are listed Standard deviations are susceptibilities, as would be observed for a superconductor in all 0 1 pm or less a SQUID magnetometer, due to the Meissner-Ochsenfeld effect Slight discontinuities in the susceptibilities at ca 4K these values are affected by large errors of up to a factor of were ascribed to impurities of elemental tin, which becomes two, because of the difficulty in estimating the geometry of the superconducting below 3 7 K l6 small crystals and the size of the contacting areas Nevertheless, The most interesting results of the present investigation are these values compare well with those found for the tetragonal, the crystal structures of these tin-nch compounds Os,Sn,, metallic P-modification of tin, which vary between 9 and crystallizes with a new structure type The structure is rather J Muter Chem , 1996, 6(12), 1897-1903 1899 r I I II I I I I I I I 0 100 200 300 0 loo 200 300 TIK Fig.1 Relative electncal resistivities of (a) 0s4Snl7, (b) RhSn, and (c) IrSn4 between 4 and 300 K complicated with 84 atoms in the orthorhombic cell It may be visualized as consisting of several atomic layers, although the bonding within and between the layers (as can be concluded from the interatomic distances) is of comparable strength In Fig 3 we have emphasized (somewhat arbitrarily) three differ- ent kinds of layers, which we designate with the letters A, B and C Layer B is rather densely populated by a puckered network of four osmium and ten tin atoms Layer A contains onIy eight tin atoms, and only six tin atoms are sltuated on the mirror plane, which constitutes layer C Intermetallic phases are frequently characterized by close packing of all atoms If the atoms are of equal size, the structures usually contain both octahedral and tetrahedral voids, as is well known for the structures of the metallic elements, eg fcc and hcp packing The ‘tetrahedrally close- packed structures’, sometimes also called ‘o-phase related’ or ‘Frank-Kasper 22 contain only tetrahedral voids This is possible when the compounds consist of atoms with differing space requirements, resulting in different coordination numbers, as is well known for the Laves phases with the three prototype structures MgNi2, MgCu, and MgZn, The struc- ture of Os,Sn17 has a rather high tin content and therefore we cannot expect it to contain only small tetrahedral voids, however, it contains rather large voids We have not made an extensive search for such voids in the structure of Os4Sn17, however, as examples we show the positions of the five voids 1900 J Mater Chern, 1996, 6(12), 1897-1903 i I I I I I I 50 100 150 200 250 300 I4--a--1.5 1.o 0.5 1 I I I 1 I I 50 100 150 200 250 300 I I 1 I I I I 50 100 150 200 250 500 TIK Fig.2 Temperature dependence of the magnetic susceptibilities of (a) 0s4Snl7, (b) RhSn,, (c) RhSn, and (d) IrSn, measured with a magnetic flux density of 5 T Fig.3 The crystal structure of Os4Sn,,. A projection of the whole structure along the x direction is shown on the lower left-hand side. The structure may be visualized as consisting of the atomic layers A, B, C, B’, A’, B”, C’ and B”‘.Several of these layers, viewed along the translation period z, are shown on the right-hand side. Only the 0s-Sn bonds are indicated in the lower left-hand side projection. All Sn-Sn distances within the layers, shorter than 400 pm are indicated in the projection on the right-hand side. There are equally strong bonds between the layers; one such interface between the layers A, B”’ and B is shown in the upper left-hand corner. The positions of the voids V( 1)-V( 5) are indicated in the layers A, B and C. V( 1)-V( 5) (Fig. 3) and we list their positions and ‘coordi- nations’ (Table 6). The trigonal-prismatic void V( l), formed by six tin atoms at 245 pm is particularly large. It is even larger than the largest void in p-tin, which is ‘coordinated’ by four tin atoms at 225.7 pm and two more at 318.7 pm in a rather irregular arrangement, and it is almost as large as the tetrahedal void in the a(diamond) modification of tin, formed by four tin atoms at a distance of 281.0 pm.Sometimes such voids are filled by interstitial atoms; however, our final differ- ence Fourier analysis did not give any indication of this. We also found no related ‘filled’ structures in searching Pearson’s handb~ok.’~ The osmium atoms of Os,Sn,, occupy two different sites. The Os(1) atoms have eight tin neighbours in a distorted square-antiprismatic arrangement at distances varying between 267.3 and 286.8 pm with an average distance of 274.3 pm.The Os(2) atoms are situated in a monocapped square antiprism of tin atoms with a larger range of Os(2)--Sn distances extending from 273.9 to 303.0 pm. Both the larger coordination number and the larger spread should resuIt in larger Os(2)-Sn distances, and this is indeed the case with an average of 282.0 pm. There are eleven different tin sites in Os,Snl,. All tin atoms have two osmium and between ten and twelve tin neighbours (Table 3, Fig. 4). However, in contrast to the well defined coordinations of the osmium atoms, the coordination J. Muter. Chem., 1996,6( 12), 1897-1903 1901 Table 6 Location and coordination/pm of unoccupied sites [voids V( 1)-V(5)] in the structure of Os,Sn,, Pbcm X Y Z V(1)V(2) 4d 8e 0 6396 0 0929 0 2281 0 5916 114 0 9863 V(3) V(4)V(5) 4d 4d 8e 0 4840 0 9575 0 0760 0 7895 0 6442 0 9315 114 114 0 1645 V(1) 2Sn(3) 2454 V(3 2Sn(4) 2322 V(5) lSn(6) 2196 2Sn(4) 2454 2Sn(6) 2326 lSn(8) 2212 2Sn(6) V(2) lSn(2) 2455 2334 V(4 lSn(9) lSn(8) 2326 2337 lSn(2) lSn(7) 2224 2561 lSn(l1) 2339 lSn(1) 2342 2Sn(3) lSn(7) 2338 2338 lSn(1) lOs(1) 2581 2899 lSn(5) lSn(1) 2445 2472 2Sn(4) 20s(l) 2816 2888 Fig.4 Near-neighbour environments in the structure of Os,Sn,, The site symmetries are given in parentheses polyhedra of the tin sites are not easy to define The Sn-Sn distances almost continuously cover the range between 286 5 [Sn(5)-Sn(ll)] and 3964 pm [Sn(lO)-Sn(ll)] Thus, the shortest Sn-Sn distance of 286 5 pm is only slightly greater than the two-electron bond distance of 281 0 pm in the 1902 J Muter Chew, 1996, 6(12), 1897-1903 diamond (a)modification of elemental and the great variety of Sn-Sn bond lengths in Os,Sn,, is reminiscent of the Sn-Sn interactions in the P-modification of tin, where each tin atom has 4 +2 +4 Sn neighbours at 302 2, 318 1 and 376 8 pm, respectively 24 RhSn, crystallizes with a structure type which was deter- mined first for the high-temperature (p) modification of CoSn, , While there are no 0s-0s bonds in Os,Sn,,, the higher transition-metal content of RhSn, is reflected in the coordi- nation polyhedron of the rhodium atoms, which consists of one rhodium atom at the bonding distance of 285 7 pm and eight tin atoms at the almost equal distances of 273 7 (2 x), 274 4 (2 x) and 277 3 pm (4 x) The two different tin atoms of RhSn, have coordination numbers 15 and 12 The Sn(1)atoms of RhSn, are the only tin atoms in the structures of the present investigation which do not have two transition-metal neigh- bours They have four rhodium and eleven tin neighbours, while the Sn(2) atoms of this stannide are coordinated by two rhodium and ten tin atoms Again, the Sn-Sn distances cover a wide range extending from 286 1 to 368 6 pm For a further discussion of the P-CoSn,-type structure, refer to ref 3 The stannides RhSn, and IrSn, are isotypic with a structure (Fig 5) first described for IrGe, 25 The c/a ratios of these trigonal cells are slightly different, but the cell volumes are practically the same (Table l), and the interatomic distances in the two compounds are also very similar (Table 5) The rhodium atoms (the corresponding values for the iridium compound are listed in parentheses) are coordinated by eight tin atoms at an average distance of 275 8 (276 4) pm forming a distorted square antiprism (Fig 6), similar to the environ- ment of the Os(1) atoms in Os,Sn,, The three different tin atoms all have two Rh (Ir) and twelve Sn neighbours and, as was discussed above for Os,Sn,, and RhSn,, the Sn-Sn distances cover a wide range, almost continuously extending from 298 6 (2962) pm to 405 5 (407 6) pm There are no further Sn-Sn distances up to 450 pm The average Sn-Sn distances of 349 8 (350 4), 340 8 (340 5) and 352 1 (352 3) pm for the Sn(l), Sn(2) and Sn(3) atoms reflected the average Sn-Rh(1r) bond lengths of 271 7 (272 5), 286 2 (286 4) and 273 8 (273 9) pm, respectively, I e the longest Sn-Rh(1r) bonds are found for the Sn(2) atom, which has the shortest average Sn-Sn distances We conclude with a remark about the thermal parameters Usually these are strongly affected by absorption errors, which cannot fully be accounted for by the correction from psi-scan data The relative values within one compound, however, are much more reliable It can be seen (Table 2), that the thermal parameters of the transition elements are all smaller than those of the tin atoms This is also the case for the stannides MoSn,,' VSn2,2 NbSn2,2 CrSn22 and the two modifications of CoSn,,, even though the transition metals are sometimes lighter and sometimes heavier than the tin atoms We believe this reflects Fig.5 Projection of the hexagonal IrGe,-type structure of RhSn, and IrSn, RNlr (2) 4 Fig. 6 Coordination polyhedra and site symmetries in the stannides RhSn, and IrSn, the regular coordination of the transition-metal atoms on the one hand and the irregular coordinations of the tin atoms on the other, the latter with relatively large voids in their vicinity, as we have analysed in more detail for Os,Sn,,. We thank Dip1.-Ing. U. Rodewald, Dip1.-Chem. M. Gerdes and Mr. K. Wagner for the intensity data collection on the four-circle diffractometer, for the magnetic susceptibility measurements in the SQUID magnetometer and for the EDX investigation.Dr. R-D. Hoffmann contributed to this work in the early stages of the structure determinations and refinements. We are also indebted to Dr. G. Hofer (Heraeus Quarzschmelze, Hanau) for a generous gift of silica tubes. This work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. References 1 T. Wolpl and W. Jeitschko, 2. Anorg. Allg. Chem., 1994,620,467. 2 T. Wolpl and W. Jeitschko, J. Alloys Compd., 1994,210, 185. 3 A. Lang and W. Jeitschko, Z. Metallkde., 1996,in press. 4 K. Schubert, H. L. Lukas, H-G. Meissner and S. Bahn, Z. Metallkde., 1959,50, 534. 5 K. Schubert, Z. Naturforsch., Teil A, 1947,2,120. 6 P.Bussibe and K. Lazar, Hyperfine Interact., 1988,41,559. 7 P. Bussikre, M. Boge and K. 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Samson, in Structural Developments in Alloy Phases, ed. B. C. Giessen, Plenum Press, New York, 1969. 21 C. B. Shoemaker and D. P. Shoemaker, Monatsh. Chem., 1971, 102,1643. 22 W. B. Pearson, The Crystal Chemistry and Physics of Metals and Alloys, Wiley, New York, 1972. 23 P. Villars and L. D. Calvert, Pearson’s Handbook of Crystallographic Data for Intermetallic Phases, ASM International, Materials Park, Ohio, 2nd edn., 1991. 24 J. Donohue, The Structures of the Elements, Wiley, New York, 1974. 25 P. K. Panday and K. Schubert, J. Less-Common Met., 1969,18,175. Paper 6/04697E; Received 4th July, 1996 J. Muter. Chem., 1996,6( 12), 1897-1903 1903