All the essential switchable mirror properties of YHx exhibit a strong hysteresis that is related to the structural β-dihydride to γ-trihydride phase transition. This structural transition in YHx has so far been difficult to investigate since polycrystalline YHx films are optically homogeneous on the length scale of visible light wavelengths. However, in epitaxial YHx 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.@en

A detailed study is presented of the spectacular 'Manhattan switching', occurring during hydriding of epitaxial YH_x switchable mirrors. Manhattan switching involves block-wise domain switching, both in an optical and a structural sense, of discrete areas of a switchable mirror between its conductive metallic YH₂ state and its insulating YH₃ state. By means of in situ atomic force microscopy we link this domain-wise switching to the accompanying changes in resistivity and transmission of a switching mirror. We find that, relative to the optical transition and the resistivity, the structural switching is retarded compared to polycrystalline films. Further, the Manhattan effect is governed by the switching properties of the domain boundaries, that form a regular, connected network of ridges. This crucial role of the network can well be explained by assuming local variations of mechanical properties around each ridge. The findings stress the importance of local investigations on switchable mirrors.

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