Thermoelectric Properties of TlGdQ2 (Q = Se, Te) and Tl9GdTe6

The ternary thallium chalcogenides TlGdQ(2) (Q = Se, Te), and Tl9GdTe6 were synthesized, and their thermoelectric properties were evaluated. The chalcogenides TlGdQ(2) are isostructural with TlSbQ(2) (space group ), adopting the alpha-NaFeO2 structure type, and Tl9GdTe6 is isostructural with Tl9BiTe6 (space group 4/). TlGdSe2 was found to be a wide-bandgap semiconductor with rather high Seebeck coefficient and low electrical conductivity. The corresponding telluride TlGdTe2 behaves like a doped semiconductor, and possesses very low thermal conductivity at room temperature on the order of 0.5 W m(-1) K-1, a property advantageous for thermoelectric applications. Tl9GdTe6 exhibits relatively high room-temperature electrical conductivity of around 850 Omega(-1) cm(-1) and a low Seebeck coefficient of 27 V K-1, yielding a low power factor. Of these three compounds, TlGdTe2 exhibits the best thermoelectric properties, with maximum dimensionless figure of merit in the measured temperature regime of 0.5 at 550 K.