Anion vacancy-induced photochromism and lattice relaxation in yttrium oxyhydride

There is significant interest in understanding the mechanism behind the photochromic properties and light-induced structural and chemical alterations in yttrium oxyhydride, which remains unclear. This lack of knowledge hinders our ability to comprehend the causes of degradation, enhance photochromic performance, control color, improve durability, and fully utilize the material's potential applications. In this work, first-principles calculations have revealed that anion vacancies can cause the photochromic effect in yttrium oxyhydride. Such vacancies form deep localized energy levels in the band gap. Absorption of sunlight transfers electrons from the valence band to these defect levels and changes the charge state of the vacancies leading to the photochromic effect and lattice relaxation. This relaxation involves yttrium cations shifting outwards for positively charged vacancies and inward for negatively charged vacancies. These shifts cause lattice relaxation and affect the local environment around yttrium cations by inducing changes in the second coordination shell. Using extended X-ray absorption fine structure spectroscopy (EXAFS) on transparent and photodarkened yttrium oxyhydride films, we concluded that UV light illumination induces lattice relaxation in the second coordination shells of yttrium cations in agreement with the results of first-principles calculations.