Thermoelectric figure of merit in solid solutions with phonon scattering by off-center impurities

The Seebeck coefficient and the electrical and thermal conductivities (S, sigma, and kappa) of ternary PbTe1-xSex (x=0.1 and 0.3) and quaternary PbTe1-2xSexSx (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) x 10(18) 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 sigma(T) curve near 77 K were revealed in quaternary alloys with xgreater than or similar to0.1; for x=0.15, unusual behavior of kappa(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 sigma(T) and kappa(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: at 300 K, Z(max)=2x10(-3) K-1 with a carrier density of similar to3x10(18) cm(-3); at 175 K, Z(max)=1.5x10(-3) K-1 with the density decreasing to 5x10(17) cm(-3). The obtained data indicate that the introduction of off-center impurities rises Z at Tless than or similar to300 K. (C) 2003 MAIK "Nauka/Interperiodica".