Analysis of the electronic structure of MF6-8 complexes (M = Al, Ga, In, Sc, Y) in this mainly ionic semiconductor revealed a significant increase in the covalency of the chemical bond for Al, Ga and In impurity centers; as compared with a normal crystal: that is, with a CdF86- complex and with an increase in the ionicity of the bond for Sc and Y. Consequently, an essential non centro-symmetrical distortion of the center was possible for the former centers upon capturing an extra electron. Additional requirements for the formation of such a (DX) state of the center were the presence of d-d interactions of the impurity ion and cations, and of filled atomic-type shells at the singly-ionized impurity ion. The former condition was not satisfied by Al. In the case of Sc and Y, breaking of the second condition and an increase in the ionicity of the bond upon doping inhibited the formation of DX centers. These ions produced only shallow donor levels.

Donor Impurities and DX Centers in the Ionic Semiconductor CdF2 - Influence of Covalency. D.E.Onopko, A.I.Ryskin: Physical Review B, 2000, 61[19], 12952–6