Crystal structures and thermoelectric properties of the series Tl10−xLaxTe6 with 0.2 ≤ x ≤ 1.15

Abstract

The tellurides Tl_10−xLa_xTe₆ were synthesized from the elements in stoichiometric ratios at 873 K, followed by slow cooling. These materials are substitution variants of Tl₅Te₃, crystallizing in space groupI4/mcm, with lattice dimensions of a = 8.9220(4) Å, c = 13.156(1) Å, V = 1047.2(1) ų, for x = 1 (Z = 2). Increasing the La content occurs with an increase in the unit cell volume and the c axis, but a decrease of the a axis. Tl₅Te₃ is a metallic compound, while Tl₉LaTe₆ was calculated to be semiconducting. Correspondingly, the Seebeck coefficient increases with increasing x, while the electrical and thermal conductivity both decrease. The highest thermoelectric figure-of-merit determined thus far is 0.21 at 581 K for cold-pressed Tl₉LaTe₆.