Electronic Structure of Lanthanum Hydrides with Switchable Optical Properties

Recent dramatic changes in the optical properties of ${\mathrm{LaH}}_{2+x}$ and ${\mathrm{YH}}_{2+x}$ films discovered by Huiberts et al. [Nature (London) 380, 231 (1996)] suggest their electronic structure is described best by a local model. Electron correlation is important in ${\mathrm{H}}^{-}$ centers and in explaining the transparent insulating behavior of ${\mathrm{LaH}}_3$. The metal-insulator transition at $x\sim 0.8$ takes place in a band of highly localized states centered on the $\mathrm{H}$ vacancies in the ${\mathrm{LaH}}_3$ structure.