Thermoelectrical figure of merit in PbTe-based solid solutions with phonon scattering by off-center impurities

The Seebeck coefficient and the electrical and thermal conductivities (S, σ and κ) of ternary PbTe_1-xSe_x (x = 0.1 and 0.3) and quaternary PbTe_1-2xSe_xS_x (x = 0.025, 0.05, 0.1, and 0.15) solid solutions have been studied. Polycrystalline samples with an electron density of (0.5-5.0) × 10¹⁸ cm^-3 were used; their quality was monitored by comparing the measured and calculated mobility values at 85 K. A considerable decrease in mobility and an anomalous trend in the σ(T) curve near 77 K were revealed in quaternary alloys with x ≥ 0.1; for x = 0.15, unusual behavior of κ(T) was also found. According to estimates, the lattice thermal conductivity of this material is temperature-independent in the 80- to 300-K temperature range. This means that a reduction in phonon-phonon scattering with an increase in temperature is completely compensated by an increase in the scattering on impurities. The observed anomalies of σ(T) and κ(T) are considered assuming the possible of off-center location of sulfur atoms at the lattice sites. The thermoelectric figure of merit Z of the studied alloys has been determined in the range 80-300 K. In spite of decreasing mobility, the maximum Z was obtained in a quaternary compound with x = 0.1.