DEFECT ASSOCIATION, JAHN-TELLER OR OFF-CENTER EFFECT IN CAF2-COPPER, BAF2-SILVER

The results of a detailed optical and paramagnetic-resonance study per formed on copper in CaF2 and silver in BaF2 are presented. Two differe nt Ag+ centers were identified in BaF2. One is associated with an inte rstitial F- ion whereas the other one has a cubic surrounding. The Cu- 2+ ion in CaF2 was shown to reorient at 4.2 K between 6 equivalent min ima of D-2h symmetry. This fact is interpreted with the aid of a T2gX( T-2g+E-g) type Jahn-Teller effect. The nonlinear mixed coupling terms are shown to play an important role. The Cu+ impurity in CaF2 is presu mably off-center in the F- sublattice without associated defect or imp urity.