Electronic Structure of Lanthanum Hydrides with Switchable Optical   Properties
  Recent dramatic changes in the optical properties of LaH_{2+x} and YH_{2+x} films discovered by Huiberts et al. suggest their electronic structure is described best by a local model. Electron correlation is important in H^- -centers and in explaining the transparent insulating behavior of LaH_3. The metal-insulator transition at $x\sim 0.8$ takes place in a band of highly localized states centered on the $H$-vacancies in the LaH_3 structure. 
K.K. Ng, F.C. Zhang, V.I. Anisimov, and T. M. Rice