AN ELECTRON-PARAMAGNETIC-RESONANCE STUDY OF THE TETRAGONALLY DISTORTED CR2+ ION IN SRCL2 AND BAF2

Chromium-doped SrCl2 and BaF2 have been studied by both EPR and ENDOR spectroscopy. Cr2+ ions enter the fluorite structure in distorted subs titution cation sites. In both matrices the distortion observed is tet ragonal. 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: g(parallel to) = 1.974(3), g(perpendicular to) = 1.997(3), D - a = -2.114(5) cm(-1), \a\ = 0.018(3) cm(-1) and A(per pendicular to)(Cr-53) = 35 +/- 2 MHz for BaF2; and g(parallel to) = 1. 974(3), g(perpendicular to) = 1.996(4). D - a = -1.979(2) cm(-1), \a\ = 0.021(3) cm(-1), A(parallel to)(Cr-53) = 31.2 +/- 1.5 MHz and A(perp endicular to)(Cr-53) = 43.5 +/- 1.5 MHz for SrCl2. For BaF2, the weak superhyperfine interaction of Cr2+ with the surrounding F- ions has be en studied by both EPR and ENDOR techniques for B parallel to [100]. N o ENDOR signals were detected for SrCl2:Cr. The results are tentativel y explained in terms of a Jahn-Teller effect corresponding to T-2g x e (g) coupling strongly stabilized by lattice stresses, although other p ossible origins for the distortion cannot be completely ruled out.