  It was shown that the divalent rare earth ions (La, Ce, Gd, Tb, Lu, and Y) in
cubic sites in alkaline earth fluorides are unstable with respect to electron
autodetachment since its d1(eg) ground state is located in the conduction band
which is consistent with the general tendency of these ions in various
compounds. The localization of doubly degenerate d1(eg) level in the conduction
band creates a configuration instability around the divalent rare earth ion
that leading to the formation of anion vacancy in the nearest neighborhood, as
was reported in the previous paper [Journal of Physics and Chemistry of Solids
74 (2013) 530-534]. Thus, the formation of the stable divalent ions as La, Ce,
Gd, Tb, Lu, and Y (PC+ centers) in CaF2 and SrF2 crystals during x-ray
irradiation occurs via the formation of charged anion vacancies near divalent
ions (Re2+va), which lower the ground state of the divalent ion relative to the
conductivity band. Photochromic effect occurs under thermally or optically
stimulated electron transition from the divalent rare earth ion to the
neighboring anion vacancy and reverse under ultraviolet light irradiation.
