Improved Bulk Materials with Thermoelectric Figure‐of‐Merit Greater than 1: Tl 10– x Sn x Te 6 and Tl 10– x Pb x Te 6

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). Tl 4 SnTe 3 and Tl 4 PbTe 3 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 Te 6 and Tl 10– x Pb x Te 6 , 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 (Sn 2+ ) 2 (Te 2− ) 6 and (Tl + ) 8 (Pb 2+ ) 2 (Te 2− ) 6 (or: (Tl + ) 4 Pb 2+ (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 8.05 Sn 1.95 Te 6 with zT = 1.26 and Tl 8.10 Pb 1.90 Te 6 with zT = 1.46 around 685 K.