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, /spl sigma/ and /spl kappa/) of ternary PbTe/sub 1-x/Se/sub x/ (x = 0.1 and 0.3) and quaternary PbTe/sub 1-2x/Se/sub x/S/sub 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) /spl times/ 10/sup 18/ cm/sup -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 /spl sigma/(T) curve near 77 K were revealed in quaternary alloys with x /spl ges/ 0.1; for x = 0.15, unusual behavior of /spl 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 /spl sigma/(T) and /spl 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.