Phonon-generated microfields and the high-temperature shift of the absorption edge in cubic ZnS

The temperature shift of the exponential absorption edge of cubic Zns was measured up to 750 \ifmmode^\circ\else\textdegree\fi{}K. The shift was found to be mainly due to the phonon-generated electrical microfields. The weak-field approximation of the Franz equation for exponential edges was found to hold only up to 350 \ifmmode^\circ\else\textdegree\fi{}K. Above this temperature a more general form of the field-effect relations has to be introduced, and this was found to fit the experimental results well.