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Centric and Non-centric Ca3Au∼7.5Ge∼3.5: Electron-Poor Derivatives of La3Al11. Syntheses, Structures, and Bonding Analyses

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journal contribution
posted on 2009-06-15, 00:00 authored by Qisheng Lin, John D. Corbett
Two La3Al11 type derivatives have been discovered in the Ca−Au−Ge system and structurally characterized by single-crystal X-ray diffraction. Compositions Ca3Au7.16(6)Ge3.84(6) (1) and Ca3Au7.43(9)Ge3.57(9) (2) lie within a non-centric Imm2 phase region with a ≈ 4.40 Å, b ≈ 13.06 Å, c ≈ 9.60 Å. The Au-richer and electron-poorer Ca3Au7.50(1)Ge3.50(1) (3) and Ca3Au8.01(1)Ge2.99(1) (4) occur within a centric Pnnm phase region with a ≈ 9.50 Å, b ≈ 13.20 Å, c ≈ 4.43 Å. Both phases contain complex [Au,Ge]116− polyanionic networks made up of hexagonal and pentagonal prisms that are filled with the electropositive Ca atoms. Both 3:11 phases represent opposed 1 × 3 × 1 superstructure distortions of CaAu2Ge2 (ThCr2Si2 type, I4/mmm), the structure of which has also been re-determined in this work. Linear muffin-tin-orbital (LMTO) calculations reveal that the symmetry variations induced by changes of the Au contents in the present 3:11 phases are consequences of bonding and structural optimizations. The hypothetical “CaAu2.33−2.67Ge1.33−1.000.33” compositions, which are close to those of 14, follow through creation and elimination of vacancies within the electronegative networks of CaAu2Ge2.