Continuous-wave and pulsed EPR studies of Cr2+ defects in CaF2.

Abstract

${\mathrm{Cr}}_{2}$${\mathrm{O}}_{3}$ and ${\mathrm{CrF}}_{3}$ doped ${\mathrm{CaF}}_{2}$ single crystals have been studied by cw-EPR and electron spin echo envelope modulation (ESEEM) techniques. In both cases ${\mathrm{Cr}}^{2+}$ ions enter the fluorite structure at substitutionally distorted cation sites. For ${\mathrm{Cr}}_{2}$${\mathrm{O}}_{3}$ doped crystals, X and Q band EPR measurements at temperatures between 4 and 200 K allowed us to determine the ion symmetry and spin Hamiltonian parameters, such as the g factors and the zero-field splitting tensors, as well as the $^{53}\mathrm{Cr}^{2+}$ hyperfine tensor. Electron spin echoes (ESE) were detected on that system at temperatures below 20 K. The analysis of the ESEEM and cw-EPR superhyperfine (shf) structure lead us to propose a model composed of two nearest-neighbor fluoride ions placed in the (110) plane containing the ${\mathit{C}}_{2}$ axis with the ones opposite substituted by an ${\mathrm{O}}^{2\mathrm{\ensuremath{-}}}$ ion. For ${\mathrm{CrF}}_{3}$ doped crystals the distortion is also orthorhombic but the shf interaction is 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 dynamical Jahn-Teller effect corresponding to ${\mathit{T}}_{2\mathit{g}}$\ensuremath{\bigotimes}(${\mathit{t}}_{\mathit{g}}$+${\mathit{e}}_{\mathit{g}}$) coupling stabilized by lattice stresses. \textcopyright{} 1996 The American Physical Society.