Formation of anion-vacancy clusters and nanocavities in thermochemically reduced MgO single crystals

An MgO crystal was thermochemically reduced (TCR) under extreme reducing conditions such that the concentration of anion vacancies $(F$ centers) was exceptionally large, 6×10¹⁸cm^-3. Optical absorption measurements demonstrate that in addition to F centers absorbing at 250nm, 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 3nm. 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.