Gold Network Structures in Rhombohedral and Monoclinic Sr2Au6(Au,T)3 (T = Zn, Ga). A Transition via Relaxation

Abstract

Quantitative syntheses, structure determinations and interpretations, and band calculations are reported for the nonstoichiometric rhombohedral (R3c) and monoclinic (C2/c) Sr2Au6(Au(3-x)T(x)) (T = Zn, Ga) compounds. Several different compositions of the two Sr phases were similarly refined from single crystal X-ray diffraction data as R3c: a ≈ 8.43 Å, c ≈ 21.85 Å, Z = 6 and C2/c: a ≈ 14.70 Å, b ≈ 8.47 Å, c ≈ 8.70 Å, β ≈ 123.2°, Z = 4. The R3c Zn phase is stable in the composition region x ∼ 2.5-2.9 whereas its C2/c neighbor is the major product at x ∼ 2.2-2.3. Gallium versions of both were also identified. Both R3c and C2/c structural types contain hexagonal-diamond-like gold superlattices stuffed with strings of interstitial Sr and disordered triangular (Au,T)3 units. The latter space group is a maximal, nonisomorphic subgroup of the former, and the decrease in interstitial radius from Ba to Sr (∼0.08 Å experimentally) evidently drives the symmetry reduction, relaxation, and small distortions, principally around the Sr sites. Au-Au bonding among the Au hexagons in the host lattices and with gold components in the triangular interstitials is dominant and reflected in their tight packing and short interatomic separations.