Interaction of hydrogen with aluminum vacancies in the Y $$_3$$ 3 Al $$_5$$ 5 O $$_{12}$$ 12 garnet and effects on positron trapping

The introduction of hydrogen in the yttrium aluminum garnet, Y $$_3$$ 3 Al $$_5$$ 5 O $$_{12}$$ 12 (YAG), has been known to affect the optical and luminescence properties of this material. This makes it imperative to examine the nature of hydrogen impurities in YAG and also to understand how hydrogen interacts with native defects. Recent studies based on positron-annihilation lifetime spectroscopy (PALS) provided strong evidence on the presence of hydrogen inside the YAG lattice that eventually led to strong reduction of the positron lifetimes attributed to cation-vacancy defects. The present study reports first-principles calculations that determine the character of isolated hydrogen states in the YAG solid as well as the interaction and binding of hydrogen to the aluminum monovacancies. A hybrid functional approach that incorporates exact electron-exchange interactions is employed to determine the defect association of aluminum vacancies with hydrogen and the charge-transition levels of the resulting vacancy-hydrogen complexes. The effects of hydrogen towards passivation were studied by means of two-component density-functional theory where the positron trapping and corresponding lifetimes of the vacancy defects were calculated as a function of the number hydrogen atoms bound to each vacancy. The final results are also discussed in connection with the experimental PALS data. Graphic Abstract