Improved Bulk Materials with Thermoelectric Figure‐of‐Merit Greater than 1: Tl10–xSnxTe6 and Tl10–xPbxTe6

Noting the steadily worsening problem of depleted fossil fuel sources, alternate energy sources have become increasingly important; these include thermoelectrics, which may use waste heat to generate electricity. To be economically viable, the thermoelectric figure‐of‐merit, zT, which is related to the energy conversion efficiency, needs to be in excess of unity (zT > 1). Tl₄SnTe₃ and Tl₄PbTe₃ were reported to attain a thermoelectric figure‐of‐merit zT _max = 0.74 and 0.71, respectively, at 673 K. Here, the thermoelectric properties of both materials are presented as a function of x in Tl_10–x Sn _x Te₆ and Tl_10–x Pb _x Te₆, with x varying between 1.9 and 2.05, culminating in zT values in excess of 1.2. These materials are charge balanced when x = 2, according to (Tl⁺)₈(Sn²⁺)₂(Te^2?)₆ and (Tl⁺)₈(Pb²⁺)₂(Te^2?)₆ (or: (Tl⁺)₄Pb²⁺(Te^2?)₃). Increasing x causes an increase in valence electrons, and thus a decrease in the dominating p‐type charge carriers. Larger x values occur with a smaller electrical conductivity and a larger Seebeck coefficient. In each case, the lattice thermal conductivity remains under 0.5 W m^?1 K^?1, resulting in several samples attaining the desired zT _max > 1. The highest values thus far are exhibited by Tl_8.05Sn_1.95Te₆ with zT = 1.26 and Tl_8.10Pb_1.90Te₆ with zT = 1.46 around 685 K.