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

Abstract

Two La 3 Al 11 type derivatives have been discovered in the Ca-Au-Ge system and structurally characterized by single-crystal X-ray diffraction. Compositions Ca 3 Au 7.16(6) Ge 3.84(6) (1) and Ca 3 Au 7.43(9) Ge 3.57(9) (2) lie within a non-centric Imm2 phase region with a ≈ 4.40 A ˚, b ≈ 13.06 A ˚, c ≈ 9.60 A ˚. The Au-richer and electron-poorer Ca 3 Au 7.50(1) Ge 3.50(1) (3) and Ca 3 Au 8.01(1) Ge 2.99(1) (4) occur within a centric Pnnm phase region with a ≈ 9.50 A ˚, b ≈ 13.20 A ˚, c ≈ 4.43 A ˚. Both phases contain complex [Au,Ge] 11 6polyanionic networks made up of hexagonal and pentagonal prisms that are filled with the electropositive Ca atoms. Both 3:11 phases represent opposed 1 Â 3 Â 1 superstructure distortions of CaAu 2 Ge 2 (ThCr 2 Si 2 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 2.33-2.67 Ge 1.33-1.00 0 0.33 " compositions, which are close to those of 1-4, follow through creation and elimination of vacancies within the electronegative networks of CaAu 2 Ge 2 .

Lu5Pd4Ge8 and Lu3Pd4Ge4: Two More Germanides among Polar Intermetallics

“…Focusing on the total DOS, a difference between the two compounds at the Fermi energy (E F ) is evident: for Lu 5 Pd 4 Ge 8 a pseudo-gap is visible just above E F , instead for Lu 3 Pd 4 Ge 4 the Fermi level corresponds to a local maximum of the DOS, indicating a potential electronic instability. This might be a sign of particular physical properties (e.g., superconductivity or magnetic ordering) [36] or of small structural adjustments (e.g., off-stoichiometry due to statistical mixture or increase of vacancy concentration) [37] which, adequately modelled, would shift the E F towards a local minimum. Even if EDXS elementary composition is compatible with a slightly off-stoichiometry, there is no strong indication of this coming from XRD data, so, the stoichiometric model was considered here.…”

New representatives with BaAl4, La3Al11 and BaHg11 structure types from the R–Zn–Ga systems (R = Y, Lu, Gd–Tm)

“…Depending on the nature of d-and p-elements (the atomic size and the relative electronegativities) formation of close ordered or/ and deformed structures to the BaAl 4 -type (or La 3 Al 11 -type) are also typical for above mentioned systems. Up to our best knowledge, there are A(BC) 4 [13,21,22]. It should be noted that, among the ternary ReMeGa (Med-metal) systems, intermetallic compounds with BaAl 4 , La 3 Al 11 and related structures have been found to exist in the systems with d-elements from Ia, IIa and VIIIa subgroups of periodic table [23,24].…”

Disorder–Order Structural Transformation in Electron-Poor Sr3Au8Sn3 Driven by Chemical Bonding Optimization

“…The influences of subtle changes of size and vec in La 3 Al 11 -type and related structures are particularly manifested in electron-poor (vec/atom < 2.2; one valence electron for Au) Au-dominated systems, Ca 3 Au x Ge 11−x (x ≈ 7−8). 13 Recently, we established that the Au-richer phase Ca 3 Au x Ge 11−x (x > 7.5) forms with centric Pnnm symmetry, but a slight decrease of the Au/Ge ratio (or increase of vec) results in formation of the Au-poorer 3:11 phase Ca 3 Au x Ge 11−x (x < 7.5) with noncentric Imm2 symmetry. However, size factors are also prominent for this Au-rich phase, as evidenced by the experimental observations that replacement of Ca by Sr results in formation of SrAu 3 Ge (P4/nmm), 20 replacement of Ge by Ga leads to Ca 3 Au 6.6 Ga 4.4 (P6 3 /mmc), 21 neither being an La 3 Al 11 type, and replacement of Ge by Sn yields Ca 14 Au 45 Sn 6 (P6/m), 22 a Gd 14 Ag 51 -type product.…”

“…In this work, we report the synthesis, structures, and bonding of Sr 3 Au 8 Sn 3 phases in the Sr−Au−Sn system, motivated by the biphasic separation of Ca 3 Au x Ge 11−x at different Au/Ge ratios. 13 Also, there are only two known ternary compounds in the Sr−Au−Sn system, i.e., the CaAuGe-type SrAuSn 23 (Pnma) and the ThCr 2 Si 2 -type SrAu 1.35 Sn 2.65 (I4/mmm). 15 Interestingly, the structure of Sr 3 Au 8 Sn 3 does not accommodate varied compositions, in contrast to Ca 3 Au x Ge 11−x ; 13 rather, a disorder−order transformation driven by chemical bonding optimization occurs as a function of temperature.…”

“…Crystallographic Data for HT and LT Forms of Sr 3 Au 8 Sn 3 Al 11 -type and derivative structures,13,16 the present two structures may be viewed as vacancy ordered and structurally optimized 1 × 3 × 1 superstructures of the BaAl 4type structure. Structural relationships between BaAl 4 and La 3 Al 11 -type structures have been well documented in the literature,35 so only differences between HT and LT forms of Sr 3 Au 8 Sn 3 are discussed in the following.…”