Paramagnetic holelike defect in irradiated calcium hydroxyapatite single crystals

Abstract

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