Kinetics of the photochromic effect in oxygen-containing yttrium hydrides

The kinetics of the photochromic reaction of oxygen-containing rare-earth hydrides is commonly described by an exponential function assuming a single-step process. In this paper, we elaborate on the origin of the photochromic effect in oxygen-containing rare-earth metal hydrides, considering the kinetics of the reaction as a two-step process. We show that the fit to the experimental data is improved drastically when two processes that cause the photodarkening are assumed: a fast reaction rate-limited—for example, electronic or local—process and a slow, e.g., diffusion-limited process. Our model helps to better understand the photochromic process and pave the way for tuning the photochromic response to the need of applications by controlling the degree and response time of darkening and bleaching.