Substitution of Au or Hg into BaTl₂ and BaIn₂. New Ternary Examples of Smaller CeCu₂-Type Intermetallic Phases

The compounds BaAu_0.40(2)Tl_1.60(7) (1), BaAu_0.36(4)In_1.64(4) (2), and BaHg_0.92(2)In_1.08(2) (3) have been prepared by high-temperature techniques. Single-crystal X-ray diffraction shows that these have the orthorhombic CeCu₂-type structure, Imma, Z = 4 (a = 5.140(1), 5.104(1), 5.145(1) Å; b = 8.317(2), 8.461(2), 8.373(2) Å; c = 8.809(2), 8.580(2), 8.715(2) Å, respectively). The structure consists of a four-linked honeycomblike polyanion (4²6³8) of [Tr₂]^2- (Tr = In or Tl) with encapsulated Ba²⁺ cations. The Au or Hg randomly replace Tr in a single type of site. The two gold phases exhibit appreciable nonstoichiometry ranges. Band calculations (EHTB) demonstrate that the three compounds are electron-poor and metallic, and the latter has been confirmed for 1 through resistivity and magnetic susceptibility measurements. The orthorhombic structure of 1 contrasts with the hexagonal structure of BaTl₂ (CaIn₂-type, P6₃/mmc), a change that appears to be driven by substitution of the smaller Au atoms into the polyanion network. Relativistic effects for the heavier Au and Hg are evidently responsible for decreases in lattice parameters and bond lengths from BaIn₂ to those in isostructural 2 and 3.