Electron-Spin Resonances in Reactor-Irradiated Aluminum Oxide

Three prominent paramagnetic absorption systems have been detected in single crystals of $\alpha$ aluminum oxide (${\mathrm{Al}}_2$ ${\mathrm{O}}_3$) following reactor irradiation with and without subsequent low-temperature gamma-ray irradiation. The most prominent system has an asymmetric, slightly anisotropic absorption with $g\approx 2.00$, which appears to result from the superposition of components from several paramagnetic centers. A second system consisting of twelve equally intense lines corresponds to a spin-one center in six inequivalent sites whose $z$ axes are perpendicular to the crystal $c$ axis and nearly parallel to rows of ${\mathrm{O}}^{2-}$ ions. The spin-Hamiltonian parameters for this defect are: $g_z=2.0105\mathrm{}\pm 0.0020$, $g_x=g_y=2.0190\mathrm{}\pm 0.0020$, $S=1\mathrm{}$, $\mathrm{}\left|D\right|=0.0710\mathrm{}\pm 0.0002$ ${\mathrm{cm}}^{-1\mathrm{}}$, and $\mathrm{}\left|E\right|=0.0025\mathrm{}\pm 0.0005$ ${\mathrm{cm}}^{-1\mathrm{}}$, where $D$ and $E$ have the same sign. Its spectrum is attributed to an Al${\mathrm{O}}^{3-}$ molecular ion in a pair of oxygen sites, formed by a replacement collision. The third system is obtained when a reactor-irradiated crystal of ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ is gamma-ray irradiated at 77°K and measured prior to warming up. It is a three-line system which saturates with gamma-ray dose but whose saturation level increases with reactor dose. This system corresponds to a spin-$\frac{3}{2}$ center with a symmetry axis parallel to the $c$ axis; its spin-Hamiltonian parameters are: $g_{\parallel }=2.018\mathrm{}\pm 0.002$, $g_{\perp }=2.011\mathrm{}\pm 0.002$, $S=\frac{3}{2}$, and $\mathrm{}\left|D\right|=0.0373\mathrm{}\pm 0.0002$ ${\mathrm{cm}}^{-1\mathrm{}}$. It is attributed to interstitial ${\mathrm{O}}^{+}$ formed from interstitial ${\mathrm{O}}^0$ by ionization or hole trapping.