EPR STUDY OF HYDROGEN-IONS IN STRESSED CAO CRYSTALS

The H2- ion in CaO is paramagnetic and has an isotropic electron param agnetic resonance spectrum above 13 K. Below 13 K, the defect symmetry is tetragonal with [001] as the symmetry axis. In this present work, the EPR spectrum of H2- in a stressed sample at 13 K has the same symm etry and spin-Hamiltonian parameters as of unstressed samples at 4 K. The strain results in a preferential alignment of the centers along th e stretch axis-the [001]. The superhyperfine interaction with the magn etic Ca-43 isotopes in the [001] plane yields \A perpendicular to\ = 1 1.0+/-0.1 MHz, \A parallel to\ = 9.1+/-0.1 MHz with the symmetry axis along the [100] or [010]. A model consistent with the results is a T-1 u ground state with the degeneracy lifted by a static tetragonal Jahn- Teller distortion of the lattice that becomes apparent below 13 K.