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Tl9BiTe6, a substitution variant of Tl5Te3, is one of the leading midtemperature thermoelectricsand is postulated to exceed ZT= 1 above 450 K whenprepared by zone-melting and reach ZT= 0.86 at 560K after hot-pressing. We have prepared the isostructural series Tl9Sb1–xTe6, Tl9–xSb1+xTe6, Tl9Bi1–xTe6, and Tl9–xBi1+xTe6, with xranging from 0 to 0.05, from the elements in the stoichiometricratios 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, accordingto (Tl+)9(Sb/Bi)3+(Te2–)6. The as-prepared samples of this 9-1-6 stoichiometry,however, exhibit relatively high electrical conductivity, which decreaseswith increasing temperature, indicative of the presence of extrinsiccharge carriers. The Seebeck coefficient is generally above +100 μVK–1. Decreasing the Sb and Bi content increasesthe hole carrier concentration and thus increases the electrical conductivitywhile decreasing the Seebeck coefficient. The best feature of thesethermoelectrics is their low thermal conductivity, which is consistentlywell below 0.7 W m–1K–1. In combinationwith reasonable electrical conductivity and a high Seebeck coefficient,high ZTvalues in excess of 1 can also be achievedvia simple hot-pressing after experimental optimization of the carrierconcentration via introducing deficiencies on the Bi site. Moreover,the variants with Sb instead of Bi exhibit similar thermoelectricperformance, a result of the combination of a better electrical performanceand higher thermal conductivity. [ABSTRACT FROM AUTHOR]

Thermoelectric properties of Cu 4 Mn 2 Te 4 , which is antiferromagnetic with a Néel temperature T N = 50 K and crystallizes in a spinel-related structure, have been investigated comprehensively here. The phase transition occurring at temperatures 463 and 723 K is studied by high-temperature X-ray diffraction (XRD) and differential scanning calorimetry (DSC), and its effect on thermoelectric properties is examined. Hypothetically Cu 4 Mn 2 Te 4 is semiconducting according to the formula (Cu + ) 4 (Mn 2+ ) 2 (Te 2- ) 4 , while experimentally it shows p-type metallic conduction behavior, exhibiting electrical conductivity σ = 2500 Ω -1 cm -1 and Seebeck coefficient α = 20 μV K -1 at 325 K. Herein, we show that the carrier concentration and thus the thermoelectric transport properties could be further optimized through adding electron donors such as excess Mn. Discussions are made on the physical parameters contributing to the low thermal conductivity, including Debye temperature, speed of sound, and the Grüneisen parameter. As a result of simultaneously boosted power factor and reduced thermal conductivity, a moderately high zT = 0.65 at 680 K is obtained in an excess Mn\In co-added sample, amounting to 5 times that of the pristine Cu 4 Mn 2 Te 4 . This value ( zT = 0.65) is the best result ever reported for spinel and spinel-related chalcogenides.