Charge-transfer bands in crystals of alkaline earth fluorides with Eu3+ and Yb3+1

The absorption, luminescence, and excitation spectra of CaF2, SrF2, and BaF2 crystals with EuF3 or YbF3 impurity have been investigated in the range 1-12 eV. In all cases, strong wide absorption bands (denoted as CT1) were observed at energies below the 4f(n)-4f(n-1)5d absorption threshold of impurity ions. Weaker absorption bands (denoted as CT2) with energies 1.5-2 eV lower than those of the CT1 bands have been found in the spectra of CaF2 and SrF2 crystals with EuF3 or YbF3 impurities. The fine structure of the luminescence spectra of CaF2 crystals with EuF3 impurities has been investigated under excitation in the CT bands. Under excitation in the CT1 band, several Eu centers were observed in the following luminescence spectra: C-4v , O (h) , and R aggregates. Excitation in the CT2 bands revealed luminescence of only C-4v defects.