Isotope Effects in Switchable Metal-Hydride Mirrors

Measurements of optical reflectance, transmittance, and electrical resistivity on the switchable mirror systems ${\mathrm{YH}}_x$ and ${\mathrm{YD}}_x$ show that the absorption of hydrogen induces the same variations as that of deuterium. In both cases there is a weak transparency window for the metallic dihydride (dideuteride) phase and a yellowish transparency in the insulating trihydride (trideuteride) phase. The slightly higher electrical resistivity of the deuterides is related to the lower energy of their optical phonons. The absence of significant isotope effects in the optical properties of ${\mathrm{YH}}_x\left({\mathrm{YD}}_x\right)$ is at variance with Peierls-like theoretical models. It is, however, compatible with strong electron correlation models.