Three alkali-metal-gold-gallium systems. Ternary tunnel structures and some problems with poorly ordered cations

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.