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 A0.55Au2Ga2, two others of AAu3Ga2 (A = Rb, Cs), and the related Na13Au41.2Ga30.3 were synthesized via typical high-temperature reactions
and their crystal structures determined by single-crystal X-ray diffraction
analysis: Na0.56(9)Au2Ga2 (I, I4/mcm, a = 8.718(1) Å, c = 4.857(1) Å, Z = 4), Rb0.56(1)Au2Ga2 (II, I4/mcm, a = 8.950(1) Å, c = 4.829(1) Å, Z = 4), Cs0.54(2)Au2Ga2 (III, I4/mcm, a = 9.077(1) Å, c = 4.815(1) Å, Z = 4), RbAu3Ga2 (IV, Pnma, a = 13.384(3) Å, b = 5.577(1) Å, c = 7.017(1) Å, Z = 4), CsAu3Ga2 (V, Pnma, a = 13.511(3) Å, b = 5.614(2) Å, c = 7.146(1) Å, Z = 4), Na13Au41.2(1)Ga30.3(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 K0.55Au2Ga2 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 AAu3Ga2 (IV, V) are ordered ternary Rb and Cs derivatives of the
SrZn5 type structure, demonstrating structural variability
within the AAu3Ga2 family. All attempts to prepare
an isotypic “NaAu3Ga2” were not
successful, but yielded only a similar composition Na13Au41.2Ga30.3 (NaAu3.17Ga2.33) (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 KAu3Ga2 and RbAu3Ga2 suggest a possible transformation from SrZn5- to BaZn5-types at high pressure.