Oxygen-associated holelike centers in calcium chlorapatite

Single crystals of calcium chlorapatite${\mathrm{Ca}}_5$ ${\left(\mathrm{P}{\mathrm{O}}_4\right)}_3$Cl grown from Ca${\mathrm{Cl}}_2$ flux in air show two holelike spin-1/2 defects by ESR after x irradiation at 300 K. Both defects are axially symmetric about the pseudohexagonal $c$ axis of the monoclinic structure. One defect exhibits partially resolved hyperfine interaction with the nuclei of three inequivalent chlorine ions. The hole is shared primarily by an ${\mathrm{O}}^{2-}$ impurity ion (substitutional for ${\mathrm{Cl}}^{-}$) and a nearest-neighbor ${\mathrm{Cl}}^{-}$ ion (site 1); it is shared weakly with the other nearest-neighbor ${\mathrm{Cl}}^{-}$ ion (site 2); and a next-nearest neighbor ${\mathrm{Cl}}^{-}$ion (site 3). The oxygen and chlorine ions lie in successive sites along the screw axis. Spin-Hamiltonian parameters obtained by fitting computer-simulated spectra to observed spectra are $g_{\parallel }=2.0031\mathrm{}$, $g_{\perp }=2.0255\mathrm{}$, for ${\mathrm{Cl}}^{35}$ in units of gauss ${\left|A_{\parallel }\right|}^{\mathrm{}\left(1\right)\mathrm{}}=30.3\mathrm{}$, ${\left|A_{\perp }\right|}^{\mathrm{}\left(1\right)\mathrm{}}=13.5\mathrm{}$, ${\left|A_{\parallel }\right|}^{\mathrm{}\left(2\right)\mathrm{}}=1.55\mathrm{}$, ${\left|A_{\perp }\right|}^{\mathrm{}\left(2\right)\mathrm{}}=0.80\mathrm{}$, ${\left|A_{\parallel }\right|}^{\mathrm{}\left(3\right)\mathrm{}}=0.94\mathrm{}$, and ${\left|A_{\perp }\right|}^{\mathrm{}\left(3\right)\mathrm{}}=0.40\mathrm{}$. An approximate wave function for this hole was constructed from a linear combination of valence orbitals of adjacent oxygen and chlorine ions using one adjustable parameter. Hyperfine interactions calculated with this wave function agree closely with the experimental values. The second defect shows a single line with $g_{\parallel }=2.0032\mathrm{}$ and $g_{\perp }=2.0386\mathrm{}$; an ${\mathrm{O}}^{-}$ion adjacent to a ${\mathrm{Cl}}^{-}$vacancy is suggested as a model.