Thermal Ionization of Impurities in Polar Crystals. II. Application to Interstitials in Cubic ZnS

The general theory of thermal ionization of impurities in a polar crystal is applied to interstitials. The interaction energy of the trapped electron with the optical vibrational modes is included in the electronic wave equation and the normal coordinate dependence of the electronic wave function and energy is determined through Simpson's method. This leads to an evaluation of the displacements, $c$, and the configurational constant, $S$, occurring in the general theory. The numerical evaluation of the frequency factor indicates that it has the form $s(E, T)$ at moderately high temperatures, $T$, but depends only on the trap depth, $E$, at low temperatures. The theory is applied to interstitials in the cubic form of zinc sulfide, leading to an activation energy of 0.61 ev, a frequency factor of ${10}^8$ ${\sec }^{-1\mathrm{}}$ at low temperatures and of ${10}^{8.6}$ ${\sec }^{-1\mathrm{}}$ at 320°K, at which temperature a glow peak is known to occur. These results are in reasonable agreement with experiment.