Formation of Nets of Corner-Shared Bicapped Gold Squares in SrAu₃Ge: How a BaAl₄-Type Derivative Reconciles Fewer Valence Electrons and the Origin of Its Uniaxial Negative Thermal Expansion

Abstract

SrAu3Ge was synthesized by direct fusion of the mixed elements at high temperature followed by annealing treatments, and its structure was determined by single crystal X-ray diffraction means in space group (Pearson symbol: tP10) P4/nmm, a = 6.264(1) Å, c = 5.5082(9) Å, Z = 2 at room temperature. The structure of SrAu3Ge, a reapportioned √2 × √2 × 1 superstructure of CeMg2Si2 (P4/mmm), exhibits checkerboard nets of corner-shared bicapped Au squares (or corner-shared Au­(Au4/2)Ge octahedra), in which the apical Au–Ge pairs in adjoining nets are strongly interbonded in the c direction. This motif contrasts with that of the common BaAl4 (I4/mmm) prototype in which Al squares in comparable layers are alternately monocapped by Al from the top or the bottom. Typical examples show valence electron counts (vec) between 12 and 16 for the BaAl4 type and that for CeMg2Si2 is similar, 15. The special stability of SrAu3Ge, with vec = 9, derives from significant relativistic contribution of the Au 5d10 states to the Au–Ge and Au–Au bonding. These factors are also recognized in the marked redistribution of Au and Ge site occupancies from those in CeMg2Si2. SrAu3Ge exhibits a pronounced uniaxial negative thermal expansion along c, with a coefficient of −1.57 versus 2.16 × 10–5 K–1 in a and b. The reticulated Au5Ge octahedral layers expand in the ab plane on heating, whereas the strong, interlayer Au–Ge bonds remain fixed.

Phase Relations and Crystal Structures in the Ternary Systems Sr‐{Ag, Au}‐{Si, Ge}

“…This lower occupancy significantly improved the final refinement (R F value lower by about 1 % and smaller ADP's in all sites) and leads to the refined composition SrAu 1.65(4) Ge 2.15 ᮀ 0.2 . As the ThCr 2 Si 2 structure type has been reported for stoichiometric SrAu 2 Ge 2 by May et al [26] and by Lin et al [27] also at 700°C, another alloy richer in gold (nominal composition Sr 22 Au 33 Ge 45 ) closer to the 1:2:2 composition was investigated by XPD after annealing at 700°C. Indeed, no additional reflections were observed on the X-ray powder patterns and the Rietveld refinement endorses the ThCr 2 Si 2 structure type (see Figure 11) for τ 2 .…”

“…Afterwards, the samples were cut into several pieces. One of them was used for characterization of the as cast state, the others were vacuumsealed in quartz tubes, and annealed at various temperatures between 650°and 900°C for 7-10 d. Furthermore, all samples were water [27] a) This work.…”

“…Hitherto, only a few ternary phases from these Sr systems have been reported in the literature. Common to all these is a phase with 122 stoichiometry and ThCr 2 Si 2 structure type (space group I4/mmm): SrAg 2 Ge 2 , [25] SrAg 2 Si 2 , [26] SrAu 2 Ge 2 , [26,27] and SrAu 2 Si 2 . [28] The off-stoichiometric compounds SrAu 1.6 Ge 2.4 (CeBe 2 Ge 2 type, space group P4/nmm) and SrAu 1.9 Ge 2.1 (BaCu 2 Sb 2 type, space group I4/mmm) have been reported in our paper on ternary compounds AET x X 4-x .…”

“…[24] SrAu 3 Ge (BaAu 3 Ge type, space group P4/nmm) has recently been described by Lin and Corbett. [27] Merlo et al [29] investigated RTX compounds with R = alkaline earth, Eu or Yb, T = Ag, and X = Si, Ge, Sn, or Pb and among them characterized the compounds SrAg 0.9 Ge 1.1 (AlB 2 type, space group P6/mmm), Sr(Ag 0.5 Ge 0.5 ) 2 [30] from X-ray single crystal investigations.…”

Electron-Poor Polar Intermetallics: Complex Structures, Novel Clusters, and Intriguing Bonding with Pronounced Electron Delocalization

“…SrAu 3 Ge can be obtained by fusion of stoichiometric mixtures of elements at 700 o C for 6h, followed by slow cooling to 400 o C at which it is annealed for 6 days, and then quenched to room temperature. SrAu 3 Ge is a line compound, stable in air at room temperature.The structure of SrAu 3 Ge (P4/nmm; a = 6.274(1) Å, c = 5.4944(9) Å),40 Figure 3a, features layers of corner-shared Au 5 Ge octahedra stacked along the c-axis with the cavities sandwiched by uncapped Au 4 squares and filled with Sr atoms. This structure is an ordered derivative of CeMg 2 Si 2 (P4/mmm) in which the apical atoms capping above and below the square nets are identical (Figure 3b).…”