Thermal stability of thermoelectric properties for nondoped PbTe

The undoped PbTe compounds were prepared by the vertical Bridgman method. Hall coefficient R/sub H/ and /spl rho/ were measured by a d.c. four-terminal method. The temperature dependence of /spl rho/ and R/sub H/ were measured in the temperature range from 700 to 300K in an Ar atmosphere. PbTe showed p-type conduction at 300K and was transformed to n-type conduction at about 500K. No thermal hysteresis was observed in the temperature dependence of /spl rho/ for the heating temperature below 500K at slower cooling rates than 8.88/spl times/10/sup -2/K/s and at cooling rates above 6.45/spl times/10/sup -1/ K/s from 700K. The thermal hysteresis in the temperature dependence of /spl rho/ was confirmed at a slower cooling rate than 8.88/spl times/10/sup -2/K/s at temperatures higher than 600K. The Hall concentration of a specimen which showed thermal hysteresis in the /spl rho/-curve at the slow cooling rate of 8.88/spl times/10/sup -2/K/s from 700K was found to be 1.25/spl times/10/sup 24//m/sup 3/ smaller, 1/3 of that with no thermal hysteresis.