Tl9BiTe6, a substitution variant of Tl5Te3, is one of the leading midtemperature thermoelectrics and is postulated to exceed ZT = 1 above 450 K when prepared by zone-melting and reach ZT = 0.86 at 560 K after hot-pressing. We have prepared the isostructural series Tl9Sb1–x Te6, Tl9–x Sb1+x Te6, Tl9Bi1–x Te6, and Tl9–x Bi1+x Te6, with x ranging from 0 to 0.05, from the elements in the stoichiometric ratios and determined their thermoelectric properties after hot-pressing. In theory, these tellurides are narrow-gap semiconductors when x = 0, with all elements in common oxidation states, according to (Tl+)9(Sb/Bi)3+(Te2–)6. The as-prepared samples of this 9-1-6 stoichiometry, however, exhibit relatively high electrical conductivity, which decreases with increasing temperature, indicative of the presence of extrinsic charge carriers. The Seebeck coefficient is generally above +100 μV K–1. Decreasing the Sb and Bi content increases the hole carrier concentration and thus increases the electrical conductivity while decreasing the Seebeck coefficient. The best feature of these thermoelectrics is their low thermal conductivity, which is consistently well below 0.7 W m–1 K–1. In combination with reasonable electrical conductivity and a high Seebeck coefficient, high ZT values in excess of 1 can also be achieved via simple hot-pressing after experimental optimization of the carrier concentration via introducing deficiencies on the Bi site. Moreover, the variants with Sb instead of Bi exhibit similar thermoelectric performance, a result of the combination of a better electrical performance and higher thermal conductivity.

A variety of substitutions in Tl9BiTe6 and Tl9SbTe6 with Sn and Pb, amounting to 14 different samples, were performed by melting the stoichiometric amounts of elements at 923 K, followed by slow cooling. The pulverized powders were sintered using the hot-pressing technique. All samples were of single phase according to the powder X-ray diffraction patterns. Thermoelectric property measurements were performed to investigate the effects of Sn- and Pb-doping on the electrical conductivity, Seebeck coefficient, and thermal conductivity. Increasing the concentration of the dopants caused increases in electrical and thermal conductivity, while decreasing the Seebeck coefficient. Tl9Bi0.90Pb0.10Te6 and Tl9Bi0.85Pb0.15Te6 exhibited the highest power factor. The changes in lattice thermal conductivity were minor and did not follow a clear trend. Competitive ZT values were obtained for Tl9Bi0.95Sn0.05Te6, Tl9Bi0.95Pb0.05Te6, Tl9Sb0.97Sn0.03Te6, and Tl9Sb0.95Pb0.05Te6, namely 0.95, 0.94, 0.83, and 0.71 around 500 K, respectively. Higher dopant concentrations led to lower ZT values.