Local switching in epitaxial YH_x switchable mirrors

All the essential switchable mirror properties of YH_x exhibit a strong hysteresis that is related to the structural β-dihydride to γ-trihydride phase transition. This structural transition in YH_x has so far been difficult to investigate since polycrystalline YH_x films are optically homogeneous on the length scale of visible light wavelengths. However, in epitaxial YH_x films the relation between local structural changes and local optical transmission can be investigated using in situ atomic force microscopy and optical microscopy, in hysteretic yttriumhydride mirrors exhibiting the earlier reported, domainwise pixel switching. We find widely varying local switching kinetics during the whole hydrogenation process. As a result, an epitaxial mirror forms essentially an ensemble of individual microcolumns, each of which can be in three regimes: (i) a single-phase regime of the β dihydride phase, where the optical transmission lowers with increasing hydrogen concentration, (ii) a layered two phase configuration in which the optical transmission rises with the fraction of high-concentration γ phase, and (iii) a single-γ-phase regime in which the transmission still increases significantly. The erect of a wide distribution of local properties on the global mirror properties are discussed within a simple model.