An electron paramagnetic resonance study of the tetragonally distorted [Formula: see text] ion in [Formula: see text] and [Formula: see text

Chromium-doped Formula: see text] and Formula: see text] have been studied by both EPR and ENDOR spectroscopy. Formula: see text] ions enter the fluorite structure in distorted substitution cation sites. In both matrices the distortion observed is tetragonal. X- and Q-band EPR measurements at temperatures between 4 and 300 K allowed us to determine the ion symmetry and the following spin-Hamiltonian parameters: Formula: see text], Formula: see text], Formula: see text], Formula: see text] and Formula: see text] for Formula: see text]; and Formula: see text], Formula: see text], Formula: see text], Formula: see text], Formula: see text] and Formula: see text] for Formula: see text]. For Formula: see text], the weak superhyperfine interaction of Formula: see text] with the surrounding Formula: see text] ions has been studied by both EPR and ENDOR techniques for Formula: see text]. No ENDOR signals were detected for Formula: see text]. The results are tentatively explained in terms of a Jahn - Teller effect corresponding to Formula: see text] coupling strongly stabilized by lattice stresses, although other possible origins for the distortion cannot be completely ruled out.