Oligomerization versus polymerization of Te(x)(n-) in the polytelluride compound BaBiTe₃. Structural characterization, electronic structure, and thermoelectric properties

The compound BaBiTe₃ was prepared by the reaction of Ba/Bi/Te at over 700°C in either K₂Te₄ or BaTe₃ flux and was recrystallized in a Ba/BaTe₃ flux. The black rod-shaped polycrystalline material crystallizes in the orthorhombic space group R2₁2₁2₁ (no. 19) with a = 4.6077(2) Å, b = 17.0437(8) Å, c = 18.2997(8) Å, V = 1437.1 (2) ų, Z = 8, and d(calc) = 6.74 g/cm³. Number with F(o)² > 3σ(F(o))² 3373, number of variables 92, and sin (θ/λ) < 0.7. The final R/R(w) = 4.55/5.61%. The structure is BaBiSe₃ type and may be described as layers made up from interdigitating columnar [Bi₄Te₁₀(Te₂)](∞) 'herring-bone' shaped segments. Ba²⁺ ions are in distorted tri-capped trigonal prismatic sites between the layers. From band structure calculations, a formal charge distribution taking into account the occurrence of short-bonding Te···Te contacts in the structure can be written as Ba²⁺₄Bi³⁺₄Te^2-₈Te^1-₄. The electrical conductivity, thermopower, thermal lattice conductivity, and infrared absorption properties of this material suggest that it is a narrow gap semiconductor. These results are discussed in the context of the band scheme.