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 × 10¹⁸ cm^-3. Optical absorption measurements demonstrate that in addition to F centers absorbing at 250 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 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.