Epitaxial switchable yttrium-hydride mirrors

By means of x-ray scattering and scanning probe microscopy it is shown that high-quality epitaxial Y films can be deposited on $\text{(111)-}{\mathrm{CaF}}_{\mathrm{2}}$ substrates. The films can reversibly be switched from metallic ${\mathrm{YH}}_{\mathrm{2}}$ to transparent insulating ${\mathrm{YH}}_{\mathrm{3-\delta }}.$ Although hydrogen absorption involves an expansion of the lattice and a symmetry change from hcp to fcc, the epitaxiality of the film remains intact during the switching process. The transparency and the insulating nature of the substrate opens unique possibilities to investigate electrically and optically these switchable mirror films in the single crystalline state.