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 degrees 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 P2(1)2(1)2(1) (no. 19) With a = 4.6077(2) Angstrom, b = 17.0437(8) Angstrom, c = 18.2997(8) Angstrom, V = 1437.1(2) Angstrom(3), Z = 8, and d(calc) = 6.74 g/cm(3). Number with F-o(2) > 3 sigma(F-o)(2) 3373, number of variables 92, and sin (theta/lambda) < 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(Te-2)](infinity), ''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 Ba(4)(2+)Bi(4)(3+)Te(2-)gTe(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.