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 CeCu(2)-type structure, Imma, Z = 4 (a = 5.140(1), 5.104(1), 5.145(1) A; b = 8.317(2), 8.461(2), 8.373(2) A; c = 8.809(2), 8.580(2), 8.715(2) A, respectively). The structure consists of a four-linked honeycomblike polyanion (4(2)6(3)8) of infinity3[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, P6(3)/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.