High pressure optical studies of rare earth ions in CaF2 and other fluorides

High pressure optical absorption studies have been performed on a series of rare-earth ions Sm2+, Dy2+, Sm2+, and Tm2+ in CaF2, Tm2+ in SrF2, EuF2, and Gd3+ in CaF2. For the first five substances the transitions from the 4fn ground state to the 4fn − 1 5d configuration shift to lower energy with increasing pressure. This is consistent with the 4f→ 5d electronic transitions observed or postulated in a number of rare-earth metals at high pressure. The impurity center introduced by gadolinium in CaF2 has been shown to be a Gd3+ ion associated with a neighboring anion vacancy containing two trapped electrons. The shifts of the three peaks observed are consistent with the assignments of Alig. Two new absorptions are induced irreversibly by pressure. These behave very much like pressure-induced peaks in x-irradiated CaF2 which have been attributed to electrons trapped at lattice imperfections.