Enhanced Thermoelectric Properties of Variants of Tl9SbTe6 and Tl9BiTe6

Tl 9 BiTe 6, a substitution variant of Tl 5 Te 3, 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 Tl 9 Sb 1– x Te 6, Tl 9– x Sb 1+ x Te 6, Tl 9 Bi 1– x Te 6, and Tl 9– x Bi 1+ x Te 6, 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+ (Te 2– ) 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.