Formation and stability ofV−andVAlcenters in MgO

$V^{-}$ and $V_{\mathrm{Al}}$ centers in MgO have been shown to possess identical electronic $g$ values at low temperatures and this paper reports on the distinction between the two centers. Although these two centers have almost identical EPR spectra, their electron-nuclear-double-resonance (ENDOR) spectra are clearly different. Optical-absorption measurements, supported by ENDOR and EPR, were used to study the formation and annihilation of these two centers. Trapped-hole centers have also been observed which are charge compensated by silicon and off-axis aluminum. The $V^{-}$ center cannot be produced in all MgO single crystals, and we attribute this to impurity dependence. The high cross section for $V^{-}$-center production by electron irradiation, ∼ ${10}^2$-${10}^3$ b (as compared to only a few barns for its antimorph, the negative-ion vacancy) indicates that the predominant formation mechanism is that of ionization rather than a knock-on process. This assertion is confirmed by the relative ease with which $V^{-}$ centers are formed by low-energy x rays. Irradiations with photons possessing energies of 1.25 MeV down to a few keV reveal a lack of energy dependence for $V^{-}$ center production.