CONTINUOUS-WAVE AND PULSED EPR STUDIES OF CR2+ DEFECTS IN CAF2

Cr2O3 and CrF3 doped CaF2 single crystals have been studied by cw-EPR and electron spin echo envelope modulation (ESEEM) techniques. In both cases Cr2+ ions enter the fluorite structure at substitutionally dist orted cation sites. For Cr2O3 doped crystals, X and Q band EPR measure ments at temperatures between 4 and 200 K allowed us to determine the ion symmetry and spin Hamiltonian parameters, such as the g factors an d the zero-field splitting tensors, as well as the Cr-53(2+) hyperfine tenser. Electron spin echoes (ESE) were detected on that system at te mperatures below 20 K. The analysis of the ESEEM and cw-EPR superhyper fine (shf) structure lead us to propose a model composed of two neares t-neighbor fluoride ions placed in the (110) plane containing the C-2 axis with the ones opposite substituted by an O2- ion. For CrF3 doped crystals the distortion is also orthorhombic but the shf interaction i s with four fluoride ions presumably in the (110) plane perpendicular to the z defect axis. No ESE was detected for this system. A linewidth narrowing effect was observed. We assign this distortion to a dynamic al Jahn-Teller effect corresponding to T(2g)x(t(g) + e(g)) coupling st abilized by lattice stresses.