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 ${\mathrm{O}}^{-}$ ion which results from removal of hydrogen from an O${\mathrm{H}}^{-}$ ion. The hyperfine interaction is with the hydrogen of an adjacent O${\mathrm{H}}^{-}$ ion. In first approximation the defect is axially symmetric with respect to the pseudohexagonal $c$ axis; the spin-Hamiltonian parameters are $g_{\parallel }=2.0018\mathrm{}\pm 0.0002$, $g_{\perp }=2.0683\mathrm{}\pm 0.0002$, $A_{\parallel }=-5.6\mathrm{}\pm 0.2$ G, and $A_{\perp }=+5.9\mathrm{}\pm 0.2$ G. "Forbidden" transitions $\Delta M_I=\pm 1$ cause complex unresolved spectra for orientations intermediate between $\overrightarrow{\mathrm{H}}\parallel \overrightarrow{\mathrm{c}}$ and $\overrightarrow{\mathrm{H}}\perp \overrightarrow{\mathrm{c}}$. To fit computer-simulated spectra to the experimental spectra, it was necessary to postulate that the hyperfine interaction is with an O${\mathrm{H}}^{-}$ ion tilted 6° or 7° relative to $\overrightarrow{\mathrm{c}}$ 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 ${\mathrm{O}}^{-}$ and O${\mathrm{H}}^{-}$ ions. This establishes that the hole is strongly localized on the ${\mathrm{O}}^{-}$ and gives the absolute signs of the hyperfine components.