Stabilization of high-temperature disorder of fluorine sublattice by quenching in calcium fluoride crystals

Abstract

The superionic state of crystals with the fluorite structure caused by disordering of anion sublattice is accompanied by the formation of short-lived clusters due to the correlation between anti-Frenkel defects at the increase of their concentration. The existence of clusters comprising anion vacancies as well as interstitial and displaced anions at high temperatures was deduced from the analysis of neutron scattering. The quenched CaF2 crystals show intense ultraviolet absorption with a long-wavelength tail that can be ascribed to the clusters thus testifying the opportunity of a high-temperature disorder freezing. It was found that the fluorine vacancies introduced into the crystal during preheating in the reduction atmosphere of calcium vapor trigger and intensify the process of cluster formation. The model that takes into account a strong electron-vibration coupling gives rise to the reconstruction of both the valence and conduction bands in the cluster region thereby clarifying the nature of absorption spectra of quenched crystals.