Group-III impurities in this wide-gap ionic crystal were examined. After heating in a reducing atmosphere, crystals containing these impurities acquired semiconducting properties which were imparted by electrons bound in H-like orbitals near to an impurity. As well as these donor states, non-transition impurities formed so-called deep states that were accompanied by strong lattice relaxation. That is, they were shifted markedly along the configuration coordinate. These states were the precise parallel of DX centers in covalent and ionic-covalent semiconductors. The difference in the behavior of non-transition impurities and that of transition and rare-earth impurities was analyzed. The difference was attributed to the manner of the filling of their valence shells by electrons. A multi-level analogy was drawn between the properties of deep centers in typical semiconductors, with an appreciable fraction of covalent bonding, and in the mainly ionic CdF2 with its semiconductor properties.

Donor Impurities and DX Centers in the Ionic Semiconductor CdF2. A.I.Ryskin, P.P.Fedorov: Physics of the Solid State, 1997, 39[6], 943-7