Six new intermetallic compounds have been characterized in the alkali metal (A = Na, Rb, Cs)-gold-gallium systems. Three isostructural compounds with the general composition A(0.55)Au(2)Ga(2), two others of AAu(3)Ga(2) (A = Rb, Cs), and the related Na(13)Au(41.2)Ga(30.3) were synthesized via typical high-temperature reactions and their crystal structures determined by single-crystal X-ray diffraction analysis: Na(0.56(9))Au(2)Ga(2) (I, I4/mcm, a = 8.718(1) Å, c = 4.857(1) Å, Z = 4), Rb(0.56(1))Au(2)Ga(2) (II, I4/mcm, a = 8.950(1) Å, c = 4.829(1) Å, Z = 4), Cs(0.54(2))Au(2)Ga(2) (III, I4/mcm, a = 9.077(1) Å, c = 4.815(1) Å, Z = 4), RbAu(3)Ga(2) (IV, Pnma, a = 13.384(3) Å, b = 5.577(1) Å, c = 7.017(1) Å, Z = 4), CsAu(3)Ga(2) (V, Pnma, a = 13.511(3) Å, b = 5.614(2) Å, c = 7.146(1) Å, Z = 4), Na(13)Au(41.2(1))Ga(30.3(1)) (VI, P6 mmm, a = 19.550(3) Å, c = 8.990(2) Å, Z = 2). The first three compounds (I-III) are isostructural with tetragonal K(0.55)Au(2)Ga(2) and likewise contain planar eight-member Au/Ga rings that stack along c to generate tunnels and that contain varying degrees of disordered Na-Cs cations. The cation dispositions are much more clearly and reasonably defined by electron density mapping than through least-squares refinements with conventional anisotropic ellipsoids. Orthorhombic AAu(3)Ga(2) (IV, V) are ordered ternary Rb and Cs derivatives of the SrZn(5) type structure, demonstrating structural variability within the AAu(3)Ga(2) family. All attempts to prepare an isotypic "NaAu(3)Ga(2)" were not successful, but yielded only a similar composition Na(13)Au(41.2)Ga(30.3) (NaAu(3.17)Ga(2.33)) (VI) in a very different structure with two types of cation sites. Crystal orbital Hamilton population (COHP) analysis obtained from tight-binding electronic structure calculations for idealized I-IV via linear muffin-tin-orbital (LMTO) methods emphasized the major contributions of heteroatomic Au-Ga bonding to the structural stability of these compounds. The relative minima (pseudogaps) in the DOS curves for IV correspond well with the valence electron counts of known representatives of this structure type and, thereby, reveal some magic numbers to guide the search for new isotypic compounds. Theoretical calculation of total energies vs volumes obtained by VASP (Vienna Ab initio Simulation Package) calculations for KAu(3)Ga(2) and RbAu(3)Ga(2) suggest a possible transformation from SrZn(5)- to BaZn(5)-types at high pressure.

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.

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