The optical absorption edge (indirect energy gap) for PbTe was determined in the temperature range of 80°–520°K. It was found that dEg/dT had a value of 4.1×10−4 eV/°K in the temperature range from 80°–350°K, while above 400°K, dEg/dT was approximately zero. The low-temperature linear portion extrapolated to 0.19 eV at 0°K while the high-temperature portion extrapolated to 0.36 eV. This behavior is explained by a two-valence band model, one active at low temperatures, the other in the high temperature range. A thermal energy gap of approximately 0.36eV was obtained from the temperature variation of resistivity and Hall coefficient in the intrinsic range, 500°–900°K. This value agrees with the high temperature optical data extrapolated to 0°K, thus correlating the optical and thermal energy gaps.

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The resistivity, Hall coefficient, and Seebeck coefficient of single crystals of PbTe were investigated in the range from 77° to 900 0 K using a capsuling arrangement which prevented tellurium loss from the specimens at elevated temperatures. The low temperature properties obtained agree with the data reported in the literature. The thermal energy gap obtained from the high temperature measurements cannot, however, be brought into agreement with the energy gap determined from room temperature absorption measurements by considering solely the excitation of electron-hole pairs across the energy gap at elevated temperatures. Rather, it is necessary to include the carriers generated by defect formation at elevated temperatures. Both Schottky-Wagner and Frenkel defects are present, the activation energies for formation of the two types of defects being related by the equation EF=!Es+0.55 ev~0.7 ev.

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