Single crystals of MgO have been additively colored by heating them in Mg vapor at temperatures from 1600 to 1800\ifmmode^\circ\else\textdegree\fi{}C and pressures up to 4000 Torr. The ${F}^{+}$ and ${F}^{\ensuremath{'}}$ optical absorption bands (due to anion vacancies containing one and two electrons, respectively) were observed to be extremely similar but distinguishable, and were observed at liquid-nitrogen temperature to peak at 4.96 and 5.03 eV, respectively. The oscillator strength for the ${F}^{\ensuremath{'}}$ center is estimated to be 0.70\ifmmode\pm\else\textpm\fi{}0.05. A photoconversion process ${F}^{+}\ensuremath{\leftrightarrow}{F}^{\ensuremath{'}}$ was studied. The luminescence bands for these centers were observed at 3.1 and 2.4 eV, respectively. The density of ${F}^{\ensuremath{'}}$ centers ${N}_{{F}^{\ensuremath{'}}}$ produced by the additive coloring process was observed to be related to the density of atoms in the vapor ${N}_{\mathrm{Mg}}$ by the equation ${N}_{{F}^{\ensuremath{'}}}=1270{N}_{\mathrm{Mg}}{e}^{\ensuremath{-}\frac{Q}{\mathrm{kT}}}$, where $Q=1.53$ eV. The temperature dependence of these two bands was measured and is discussed.

F + and F ′ Centers in Magnesium Oxide

Defects in the alkaline earth oxides

A study of the production and annealing of defect cluster centers in MgO single crystals has been undertaken. A comparison of the annealing of single negative-ion vacancies in neutron-irradiated, electron-irradiated, and Mg-additively colored samples is made. The results indicate that isolated negative-ion vacancies are not mobile below 900\ifmmode^\circ\else\textdegree\fi{}C, and the annealing of these defects at lower temperatures in irradiated crystals is due to interstitial migration. The broad absorption bands at 352, 573, and 975 nm and the zero-phonon lines observed in neutron-irradiated crystals, previously proposed as due to $F$-aggregate centers, are not observed in electron-irradiated and additively colored samples even after annealing. Therefore, there is some uncertainity whether these lines and/or bands are due to $F$-aggregate centers.

Defect Cluster Centers in MgO

A luminescence band with a half-width of about 0.6 eV at liquid-nitrogen temperature, which can be excited by uv light and x rays, has been observed in irradiated and additively colored MgO crystals at 3.13 eV. By combining ESR and optical studies, this emission band has been identified as due to ${F}^{+}$ centers (oxygen-ion vacancies each occupied by one electron). Furthermore, after resolving the ${F}^{+}$- and the $F$-band absorption in the vicinity of 5.0 eV, an analysis of the bands indicates that the oscillator strength of the $F$ center is about 1.25, when the oscillator strength of the ${F}^{+}$ center is taken to be 0.8.

Luminescence of the F + Center in MgO

Thermally generated defects in ultrathin MgO films on Mo(100) have been investigated using high-resolution electron-energy-loss spectroscopy (HREELS) in the 0--9-eV spectral region. The results indicate that the MgO films are thermally stable and nearly free from defects up to a temperature of 1100 K, above which defects in MgO are generated. The three distinct loss features observed at 1.15, 3.58, and 5.33 eV are due to electronic transitions associated with surface F centers, F aggregates, and F (or F{sup +}) centers, respectively. The addition of lithium into the MgO films produces (Li{sup +}O{sup {minus}}) centers and promotes the production of F-type defects upon high-temperature treatment. The formation of the (Li{sup +}O{sup +}) centers is likely a consequence of the substitution of Li{sup +} for Mg{sup 2+} in the magnesium oxide lattice. The present studies have demonstrated the capabilities of HREELS for studies of electronic transitions associated with a variety of defects in the near surface region.

/pdf/electron-energy-loss-spectroscopy-studies-of-thermally-3kac6khona.pdf

Electron-energy-loss-spectroscopy studies of thermally generated defects in pure and lithium-doped MgO(100) films on Mo(100).

Negative-ion vacancies in irradiated MgO

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Negative-ion vacancies in irradiated MgO