Temperature dependence of the electronic band gap and exciton states of superionic PbF2

The optical properties of cubic $\beta$-Pb${\mathrm{F}}_2$ in the vicinity of the fundamental absorption edge at temperatures between 298—800 K were obtained from near-normal-incidence reflectance spectra. A direct band gap of 5.84 eV at 298 K was obtained from detailed analysis of the exciton line shape. At 550 K, a type of "phase transition" occurs, leading to a 10% decrease in the real part of the dielectric constant and anomalous behavior of the exciton linewidth and binding energy. Anomalous behavior was also observed in the temperature coefficient of the band gap near the superionic phase transition temperature 703 K.