Paramagnetic holelike defect in irradiated calcium hydroxyapatite single crystals

Electron-spin-resonance studies have been conducted at 9.2 and 23.1 GHz on synthetic calcium hydroxyapatite single crystals after exposure to ionizing radiation. The major paramagnetic defect is a spin-1/2 holelike center which is observable at 92 K but not at room temperature. It exhibits hyperfine interaction with a hydrogen nucleus as proved by deuterium substitution. The center is believed to be an O^- ion which results from removal of hydrogen from an OH^- ion. The hyperfine interaction is with the hydrogen of an adjacent OH^- ion. In first approximation the defect is axially symmetric with respect to the pseudohexagonal c axis; the spin-Hamiltonian parameters are g_∥=2.0018±0.0002, g_⊥=2.0683±0.0002, A_∥=-5.6±0.2 G, and A_⊥=+5.9±0.2 G. "Forbidden" transitions ΔMI=±1 cause complex unresolved spectra for orientations intermediate between →H∥→c and →H⊥→c. To fit computer-simulated spectra to the experimental spectra, it was necessary to postulate that the hyperfine interaction is with an OH^- ion tilted 6^∘{} or 7^∘{} relative to →c in six geometrically inequivalent directions. The isotropic hyperfine interaction was calculated using an approximate wave function constructed by a linear combination of orbitals from the O^- and OH^- ions. This establishes that the hole is strongly localized on the O^- and gives the absolute signs of the hyperfine components.