An EPR study of neutron-irradiated CaO. Observation of the molecular ion OH2-

Two previously unreported EPR spectra have been observed from defects in neutron-irradiated CaO. The first spectrum consists of a single isotropic line with no apparent structure at g=2.0051. The second spectrum is described using spin Hamiltonians with S= 1 / 2 and I= 1 / 2 . At room temperature the spin Hamiltonian has axial symmetry, but at 77K it is orthorhombic. By considering the behaviour of the spectrum at intermediate temperatures it is shown that the higher-temperature spectrum is due to a defect which is moving in the lattice, thereby averaging an otherwise orthorhombic Hamiltonian. The activation energy for this reorientation is found to be approximately 0.13 eV. A detailed model is presented to explain the observed spectra, involving the molecular ion OH 2- at an anion site.