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Magnesium oxide (MgO), which is an insulator ceramic with high significance in geology and earth sciences, is not generally considered as a photocatalyst due to its wide bandgap as ~7.8 eV. Experimental studies show that nanocrystalline MgO can have optical bandgaps down to 5.25–5.95 eV and density functional theory calculations suggest that the introduction of oxygen vacancies can reduce the optical bandgap below 5 eV. In this study, oxygen monovacancies and divacancies (F, F⁺ and F₂⁺ centers) are experimentally introduced in nanocrystalline MgO by severe plastic deformation via the high-pressure torsion (HPT) method. The overall optical bandgap is successfully reduced to 3.9 eV after HPT processing. The HPT-processed samples exhibit photocatalytic dye degradation, while the degradation rate improves with increasing the imposed strain. These results not only confirm that the simultaneous …