Centric and Non-centric Ca₃Au_∼7.5Ge_∼3.5: Electron-Poor Derivatives of La₃Al₁₁. Syntheses, Structures, and Bonding Analyses

Two La₃Al₁₁ type derivatives have been discovered in the Ca−Au−Ge system and structurally characterized by single-crystal X-ray diffraction. Compositions Ca₃Au_7.16(6)Ge_3.84(6) (1) and Ca₃Au_7.43(9)Ge_3.57(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 Ca₃Au_7.50(1)Ge_3.50(1) (3) and Ca₃Au_8.01(1)Ge_2.99(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 “CaAu_2.33−2.67Ge_1.33−1.00◻_0.33” compositions, which are close to those of 1−4, follow through creation and elimination of vacancies within the electronegative networks of CaAu₂Ge₂.