An MgO crystal was thermochemically reduced (TCR) under extreme reducing conditions such that the concentration of anion vacancies $(F$ centers) was exceptionally large, $6\times {10}^{18}{\mathrm{cm}}^{-3}.$ Optical absorption measurements demonstrate that in addition to F centers absorbing at $250\mathrm{nm},$ anion-vacancy clusters absorbing at $355,$ $406,$ $440,$ $480,$ and $975$ nm were observed. Upon thermal annealing in a reducing atmosphere, a broad extinction band at 345 nm with a full width at half maximum (FWHM) of 1.25 eV emerged. With further heat treatment the peak wavelength shifted toward 370 nm and the FWHM varied between $1.25$ and 2.25 eV. Transmission electron microscopy (TEM) showed that concomitantly there exist rectangular defects with typical dimensions of $3\mathrm{nm}.$ Microdiffraction, x-ray microanalysis, and high-resolution electron microscopy, in conjunction with Mie theory, indicate that these rectangular defects are nanocavities with their walls plated with magnesium. Therefore, both oxygen vacancies and magnesium-rich regions have been observed in a thermochemically reduced MgO crystal.

• Received 2 February 2000

DOI:https://doi.org/10.1103/PhysRevB.62.9299