Four Polyanionic Compounds in the K–Au–Ga System: A Case Study in Exploratory Synthesis and of the Art of Structural Analysis

Abstract

The K-Au-Ga system has been investigated at 350 °C for <50 at. % K. The potassium gold gallides K(0.55)Au(2)Ga(2), KAu(3)Ga(2), KAu(2)Ga(4) and the solid solution KAu(x)Ga(3-x) (x = 0-0.33) were synthesized directly from the elements via typical high-temperature reactions, and their crystal structures were determined by single crystal X-ray diffraction: K(0.55)Au(2)Ga(2) (I, I4/mcm, a = 8.860(3) Å, c = 4.834(2) Å, Z = 4), KAu(3)Ga(2) (II, Cmcm, a = 11.078(2) Å, b = 8.486(2) Å, c = 5.569(1) Å, Z = 4), KAu(2)Ga(4) (III, Immm, a = 4.4070(9) Å, b = 7.339(1) Å, c = 8.664(2) Å, Z = 2), KAu(0.33)Ga(2.67) (IV, I-4m2, a = 6.0900(9) Å, c = 15.450(3) Å, Z = 6). The first two compounds contain different kinds of tunnels built of puckered six- (II) or eight-membered (I) ordered Au/Ga rings with completely different cation placements: uniaxial in I and III but in novel 2D-zigzag chains in II. III contains only infinite chains of a potassium-centered 20-vertex polyhedron (K@Au(8)Ga(12)) built of ordered 6-8-6 planar Au/Ga rings. The main structural feature of IV is dodecahedral (Au/Ga)(8) clusters. Tight-binding electronic structure calculations by linear muffin-tin-orbital methods were performed for idealized models of I, II, and III to gain insights into their structure-bonding relationships. Density of states curves reveal metallic character for all compounds, and the overall crystal orbital Hamilton populations are dominated by polar covalent Au-Ga bonds. The relativistic effects of gold lead to formation of bonds of greater population with most post-transition elements or to itself, and these appear to be responsible for a variety of compounds, as in the K-Au-Ga system.