Isotropic ESR Line of theV−Center in MgO at Room Temperature

Neutron irradiation of MgO produces $V^{-}$ centers (holes trapped at cation vacancies). X irradiation is reported to give $V_{\mathrm{A}1}$ centers: ${\mathrm{O}}^{-}$-[=]-${\mathrm{O}}^{2-}$-${\mathrm{Al}}^{3+}$. Both give identical ESR spectra at 77 K. At 295 K the three lines of the anisotropic $V_{\mathrm{A}1}$ spectrum only broaden, whereas $V^{-}$ lines coalesce to an isotropic line. Since quenched, x-irradiated crystals show this line, the presence of isolated vacancies and not the type of radiation governs formation of $V^{-}$ centers.