Single crystals which were doped with Cr2O3 or CrF3 were studied by using continuous-wave electron paramagnetic resonance and electron spin echo envelope modulation techniques. In both cases, the Cr2+ ions entered the fluorite structure at substitutionally distorted cation sites. In the case of Cr2O3-doped crystals, X- and Q-band electron paramagnetic resonance measurements at temperatures of between 4 and 200K permitted the determination of the ion symmetry and of spin Hamiltonian parameters such as the g-factors and the zero-field splitting tensors, as well as the 53Cr2+ hyperfine tensor. Electron spin echoes were detected in that system, at temperatures below 20K. Analysis of the electron spin echo envelope modulation and continuous-wave electron paramagnetic resonance superhyperfine structure led to the proposal of a model that involved 2 nearest-neighbor fluoride ions in the (110) plane which contained the C2 axis, with those opposite being replaced by an O2- ion. In the case of CrF3-doped crystals, the distortion was again orthorhombic but the superhyperfine interaction involved 4 fluoride ions which were presumably located in the (110) plane perpendicular to the z defect axis. No electron spin echoes were detected for this system. A linewidth-narrowing effect was observed. This distortion was attributed to a dynamic Jahn-Teller effect which was due to a T2g(tg + eg) coupling that was stabilized by lattice stresses.

P.B.Oliete, V.M.Orera, P.J.Alonso: Physical Review B, 1996, 53[6], 3047-54