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

The ternary thallium chalcogenides TlGdQ₂ (Q = Se, Te), and Tl₉GdTe₆ were synthesized, and their thermoelectric properties were evaluated. The chalcogenides TlGdQ₂ are isostructural with TlSbQ₂ (space group $R\bar{3}m$), adopting the α-NaFeO₂ structure type, and Tl₉GdTe₆ is isostructural with Tl₉BiTe₆ (space group I4/mcm). 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 Ω⁻¹ cm⁻¹ and a low Seebeck coefficient of 27 μV 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.