Theory for metal hydrides with switchable optical properties

Recently it has been discovered that lanthanum, yttrium, and other metal hy dride films show dramatic changes in the optical properties at the metal-in sulator transition. Such changes on a high-energy scale suggest the electro nic structure is best described by a local model based on negatively charge d hydrogen (H-) ions. We develop a many-body theory for the strong correlat ion in a H- ion lattice. The metal hydride is described by a large U limit of an Anderson lattice model. We use lanthanum hydride as a prototype of th ese compounds, and find that LaH3 is an insulator with a substantial gap co nsistent with experiments. It may be viewed either as a Kondo insulator or a band insulator due to strong electron correlation. A H vacancy state in L aH3 is found to be highly localized due to the strong bonding between the e lectron orbitals of hydrogen and metal atoms. Unlike the impurity states in the usual semiconductors, there is only weak internal optical transitions within the vacancy. The metal-insulator transition takes place: in a band o f these vacancy states.