Defect association, Jahn-Teller or off-center effect in CaF₂:Copper, BaF₂:Silver ?

Dépt. de Chimie Physique, Université de Genève
30, quai E. Ansermet, CH-1211 Geneve 4, Suisse

The results of a detailed optical and paramagnetic-resonance study performed on copper in CaF₂ and silver in BaF₂ are presented. Two different Ag⁺ centers were identified in BaF₂. One is associated with an interstitial F^- ion whereas the other one has a cubic surrounding. The Cu²⁺ ion in CaF₂ was shown to reorient at 4.2 K between 6 equivalent minima of D_2h symetry. This fact is interpreted with the aid of a T_2gx(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 presumably off-center in the F^- sublattice without associated defect or impurity.