Crystal structure of two pentastrontium trisilicidogermanides, Sr5S1.7Ge1.3 and Sr5Si2.87Ge0.13

Abstract

Gei.3oSii.7oSr5, tetragonal, Wmcm (No. 140), a = 8.107(6) A, c = 15.80(1) A, V= 1038.4 A, Z = 4, R$t(F) = 0.026, wR(F) = 0.059, T= 298 K. Discussion SrsSii.7Gei.3 and Sr5Si2.87Geo.13 crystallise with the O5B3 structure type, like Sr5Si3 and SrsGe3 [1, 2], They contain dumb-bells and isolated X 4 Zintl anions (X = Si, Ge). S r sS iuGeo and Sr5Si2.87Geo.13 can be described, according to the Z in t l -Klemm concep t , by the fo rmula t ion (Sr)5[X2][X]. These two compounds are very interesting examples for investigating the effect of the electronegativity of the semimetal on the structure of Zintl phases. According to Allred and Rochow, germanium (2.0) is slightly more electronegative than silicon (1.7). Therefore, germanium is expected to occupy the positions with a higher formal negative charge. In full agreement with the Zintl-Klemm concept, in Sr5Sii.7Gei.3 and Sr5Si2.87Geo.13 germanium preferably occupies the isolated X positions (79% and 13%, respectively), while silicon the X2 dumb-bells positions (75% and 100%, respectively). As expected, the bond length in the dumb-bell in Sr5SiijGei.3 amounts 2.53 A and is intermediate to the bond lengths in SrsSis and SrsGcs (2.47 A and 2.59 A, respectively). Since the dumb-bell in Sr5Si2.87Geo.13 are exclusively occupied by silicon, the bond length is exactly the same as in SrsSi3. Our investigations have shown that SrsSi3 and SrsGe3 form a solid solution Sr5Si3-xGex. Geo.i3Si2.87Sr5, tetragonal, IMmcm (No. 140), a = 8.079(6) A, c = 15.72(1) A, V= 1026.0 A, Z = 4, Rgt(F) = 0.026, wR(F) = 0.069, Γ =298 Κ. Source of material Sr5Sii.7Gei.3 and Sr5Si2.87Geo.13 are prepared by heating at 1363 Κ a mixture of the elements with the Sr/Si/Ge ratios of 5:1:2 and 5:2:1, respectively (12 h, cooling down within 5 h). Both compounds form grey-black, plate-like crystals with a metallic lustre. They are exceptionally air and moisture sensitive. The Si/Gel and Si/Ge2 sites are occupied by different amounts of silicon and germanium. The corresponding occupancy factors are also refined. 1. Pentastrontium trisilicidogermanide, SrsS iuGeo Table 1. Data collection and handling. Crystal: Wavelength: μ: Diffractometer, scan mode: 20max: N(hkl)measured, N(hkl)anique: Criterion for /0bs, N(hkl)^: N(param)rcfintd'· Programs: black plate, size 0.05 χ 0.15 χ 0.20 mm Mo K a radiation (0.71073 A) 293.25 cm Stoe IPDS, 101 exposures, Δφ = 1.5° 48.06° 2243, 241 7obs > 2 af/obs), 222 18 SHELXS-96 [3], SHELXL-96 [4], COLTURE [5] Si/Ge1 * Correspondence author (e-mail: nesper@inorg.chem.ethz.ch) 10.1515/ncrs-1999-0403 Downloaded from PubFactory at 09/02/2016 06:04:02AM via free access 414 Sr5Sii.7Gei.3 and Sr5Si2.87Geo.13 Table 2. Atomic coordinates and displacement parameters (in A). Atom Site Occ. X y ζ Un U22 C/33 Un Un U23 Sr(l) 4 c 0 0 0 0.0214(6) Un 0.0524(9) 0 0 0 Sr(2) 16/ 0.17826(6) jc+1/2 0.14102(4) 0.0249(4) Un 0.0254(5) -0.0005(3) -0.0023(2) Un Si(l) 4 a 0.21(1) 0 0 1/4 0.0267(9) U π 0.029(1) 0 0 0 Ge(l) 4 a 0.79 0 0 1/4 0.0267 Uu 0.029 0 0 0 Si(2) 8 h 0.746(9) 0.3897(2) λ+1/2 0 0.0204(9) υ Π 0.023(1) 0.0016(8) 0 0 Ge(2) 8 h 0.254 0.3897 *+1/2 0 0.0204 Un 0.023 0.0016 0 0 2. Pentastrontium trisilicidogermanide, Sr5Si2.g7Geo.13 Table 3. Data collection and handling. Crystal: Wavelength: μ: Diffractometer, scan mode: 2 6 m a x : AYM/jmeasured, N(hkl)unique: Criterion for I0bs, N(hkl)gt: N(param),e fined: Programs: black plate, size 0.10 χ 0.20 χ 0.20 mm Mo Ka radiation (0.71073 A) 264.42 cm Stoe IPDS, 181 exposures, Δφ = 1.0° 48.14° 2700, 235 U s > 2 a(7obsj, 231 17 SHELXS-96 [3], SHELXL-96 [4], COLTURE [5] Table 4. Atomic coordinates and displacement parameters (in A). Atom Site Occ. χ y ζ Un U22 U33 U[2 Un 1/23 Sr(l) Ac 0 0 0 0.0233(6) Un 0.053(1) 0 0 0 Sr(2) 16/ 0.17856(7) *+l/2 0.14133(5) 0.0259(5) Un 0.0262(6) -0.0002(3) -0.0021(2) Un Si(l) 4 a 0.87(1) 0 0 1/4 0.027(2) Un 0.025(2) 0 0 0 Ge(I) 4 a 0.13 0 0 1/4 0.027 Un 0.025 0 0 0 Si(2) 8 h 0.3919(2) *+l/2 0 0.019(1) Un 0.023(1) 0.000(1) 0 0 Acknowledgment. This work was supported by the Swiss National Foundation under project no. 2000-050675.97