Thermoelectric properties of hot-pressed Tl9LnTe6 (Ln = La, Ce, Pr, Nd, Sm, Gd, Tb) and Tl10−xLaxTe6 (0.90 ⩽ x ⩽ 1.05)

Abstract

In the first part of this work, thallium lanthanide tellurides with composition Tl9LnTe6 (Ln=La, Ce, Pr, Nd, Sm, Gd, Tb) have been prepared and their high temperature electrical and thermal transport properties investigated. Nearly single phase samples were obtained. Rietveld refinement results showed that Tl9CeTe6 had an exceptionally small unit cell volume of 1033Å3. The electrical conductivity and thermal conductivity increased across the lanthanide series from La to Tb. On the other hand, the Seebeck coefficient values decreased with Ln varying from La to Tb. Tl9SmTe6 or Tl9GdTe6 constituted an exception, as the Seebeck coefficient of Tl9SmTe6 was smaller than that of Tl9GdTe6. Tl9LaTe6 has the largest figure-of-merit of this series.Thereafter, Tl10−xLaxTe6 samples were prepared with x=0.90, 0.95, 1.00, 1.05, 1.10, aiming toward higher zT through composition optimization and expanding of the measurement temperature range. With the increasing La content, the unit cell volume increased, while the electrical conductivity, thermal conductivity and lattice thermal conductivity decreased, and the Seebeck coefficient increased. The highest zT=0.57 was observed for Tl9LaTe6 at 600K.