constant of 2 X 10 -5 cm2/sec is calculated from the solution of the appropriate diffusion boundary value problem.
The time required for nearly stoichiometric crystals 2 mm thick to come into approximate equilibrium with their vapor at 500°C was calculated from this diffusion coefficient and was found to be about 20 hours. Several natural crystals of PbS were exposed to various fixed pressures of sulfur vapor under these conditions and then quenched to room temperature. The pressures used corresponded to the vapor pressure of sulfur in the range from 50°C to 160°C. Room temperature Hall effect and resistivity data on these crystals are shown in Fig. .
Hall effect and resistivity data were also taken from --190°C to +200°C. Both p-type and n-type crystals were made by this method with carrier concentrations between 1015 and 1019/cc. The intrinsic activation energy for PbS is 0.34 eV and extrapblated data indicate that" intrinsic material has a resistivity of about 3Qcm at room temperature.
While the samples studied ranged in impurity concentration from 1016/cc to 1019/cc, no e~cidence for impurity scattering was observed from --190°C to + 200°C.