Coloration Induced in MgO by MeVNe+20Bombardment

The coloration produced near room temperature in MgO by 1.0- to 4.8-MeV ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}_{}^{+}{}_{}{}^{}$ bombardment has been investigated. Energetic ${\mathrm{Ne}}^{+}$ ions incident upon crystalline MgO induce point defects on the anion sublattice primarily through atomic collisions. An electron(s) trapped in the anion vacancy forms an $F$-type center producing a characteristic, intense optical-absorption band near 5 eV. This point-defect lattice damage is restricted primarily to a thin layer of material approximately 1 to 2 μ thick located 2 to 4 μ beneath the bombarded surface. Average $F$-type-center volume concentrations in this thin layer range from 4×${10}^{18}$ to 5×${10}^{19}$ ${\mathrm{cm}}^{-3\mathrm{}}$ resulting from a ${\mathrm{Ne}}^{+}$ dose range of ${10}^{13}$ to 3×${10}^{16}$ ions/${\mathrm{cm}}^{-2\mathrm{}}$. $F$-type-center production is relatively inefficient; it is estimated that 5 keV are expended per center produced, in the low-dose range, assuming centers are created only by atomic collision processes. The peak $F$-type-center volume concentration saturates near ${10}^{20}$ ${\mathrm{cm}}^{-3\mathrm{}}$ resulting from a dose of ${10}^{16}$ ions/${\mathrm{cm}}^2$ at 3-MeV energy. This concentration level is followed by a decrease upon subsequent bombardment. A similar saturation effect is observed for a 574-nm absorption band. A quadratic dependence between the growth of the intensity of the $F$-type center and that of the 574-nm band suggests the latter is associated with a divacancy. Volume-concentration saturation levels for both bands are independent of many impurities commonly present in this material. Isochronal annealing results demonstrate that both bands have annealed out at 600 °C. Comparison is made with previously reported fast-neutron irradiations and the present work. Point defects on the cation sublattice, such as the $V^{-}$ center, were sought for but determined not to be present in sufficient concentration to be detected by the presence of the well-known 2.3-eV optical-obsorption band.