New class of holographic materials based on CdF2 semiconductor crystals with bistable centers. I. Role of covalence in the formation of bistable centers

Abstract

Calculations of the electronic structure of In, Ga, and Al impurity centers in a CdF2 crystal in the cluster approximation using the method of scattered waves are made. The first two impurities form in additively colored crystals bistable centers having a ground two-electron (deep) state and a metastable hydrogen-like (shallow) state. A change in the nature of the chemical bond on doping a crystal with these impurities is traced, which consists in a considerable increase of its covalent component. A change for deep In and Ga centers is shown to be caused by the reconstruction of centers in their ground state, and a conclusion about the character of reconstruction is made. This conclusion agrees with recent calculations made for the center structure using the pseudopotential method. Conditions of formation of bistable centers in CdF 2 and their structure in different charge states are discussed.