Thermoelectric properties of Tl10-xLnxTe6, with Ln = Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho and Er, and 0.25 ≤ x ≤ 1.32

In the present study, thallium lanthanide tellurides, Tl(10-x)Ln(x)Te(6), Ln = Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho and Er; 0.25 <= x <= 1.32 were investigated. All these thallium lanthanide tellurides are isostructural to Tl9BiTe6, adopting the space group I4/mcm with mixed occupancy of Tl and Ln at the 4c site. The unit cell volume increases with increasing lanthanide content for a given Ln and decreases across the lanthanides series for a specific stoichiometry. Tl9CeTe6 has exceptionally small unit cell volume of 1019(1) angstrom(3), compared to, e. g., Tl9LaTe6 with 1047.2(1) angstrom(3) and Tl9NdTe6 with 1028(1) angstrom(3). The Seebeck coefficient values, S, increase as the lanthanide content increases for a given Ln. On the other hand, the electrical conductivity, sigma, and thermal conductivity, kappa, values decrease with increasing Ln content for a given Ln. The opposite trend (S decreases while sigma and kappa increase) is mostly observed across the lanthanide series for Tl(10-x)Ln(x)Te(6), x approximate to 1. For instance, S of Tl9.08Nd1.02(6)Te6, Tl9.0Gd1.0(1)Te6 and Tl9.05Er0.95(5) Te-6 are 81, 59, 49 mu V/K, respectively, at 500 K. Consequently, the dimensionless figure-of-merit values increase for a given Ln with increasing x (and temperature). The highest ZT value of similar to 0.20 was obtained for Tl8.98Nd1.02(6)Te6 and Tl8.99Sm1.01(7)Te6 at similar to 550 K. (C) 2012 Elsevier B.V. All rights reserved.