Defect Properties,\nAnion Ordering, and Photochromic\nMechanism in Yttrium Oxyhydride

Yttrium oxyhydride (YHO) undergoes a reversible photochromic\ntransition\nwhen exposed to ultraviolet light. However, the mechanism for this\ntransformation is not fully understood, and the structure and precise\nchemical composition of YHO remain under debate. Here, we use first-principles\ndensity functional theory calculations with a hybrid functional to\nstudy the structure, chemical stability, and point defect properties\nof YHO. As experiments have shown, we find that YHO prefers a cubic\nstructure, with H and O anions present in equal concentrations and\nlocated on tetrahedral sites. Stoichiometric and ordered YHO is chemically\nstable, but it has a wide band gap of 5.01 eV, considerably larger\nthan that measured in experiments (2.4–3.8 eV). On the other\nhand, Y<sub>4</sub>H<sub>10</sub>O has a smaller band gap of 2.97\neV and also has a region of chemical stability; thus, the actual material\nmay include some fraction of this H-rich structure. The defect chemistry\nof YHO is dominated by anionic antisite species (H<sub>O</sub> and\nO<sub>H</sub>), with hydrogen interstitials (H<sub><i>i</i></sub>) and vacancies (<i>V</i><sub>H</sub>) also present\nin reasonably high concentrations. We show that antisite disorder\nlowers the band gap relative to the perfectly ordered structure, bringing\nthe magnitude of the gap into closer agreement with experiment. Based\non our calculations of defect migration and the positions of defect\nstates relative to the band edges, we link the onset of photochromic\nbehavior to the reaction H<sub>O</sub><sup>–</sup> → <i>V</i><sub>O</sub><sup>0</sup> + H<sub><i>i</i></sub><sup>–</sup>, which follows photoexcitation of a H<sub>O</sub><sup>+</sup> defect. H<sub><i>i</i></sub><sup>–</sup> can subsequently\nmigrate away and be trapped by additional H<sub>O</sub><sup>+</sup> defects, contributing to the persistence\nof the reaction, while the resultant oxygen vacancy, <i>V</i><sub>O</sub><sup>0</sup>, introduces\nan occupied defect state that leads to optical absorption at visible\nwavelengths. Our results can explain reported discrepancies between\nexperimental and computational results for YHO, and they allow us\nto propose specific atomic-scale processes that can lead to photochromism.\nUnderstanding these mechanisms is key for unlocking YHO’s application\nin devices ranging from smart windows and optoelectronics to electrochemical\nsynapses for neural networks.