New insights from Raman and IR spectroscopy into the metal-insulator transition of YHx switchable mirrors

The important recent contributions of elastic and inelastic light scattering to our understanding of the electronic structure and the metal-insulator transition of the switchable-mirror prototype material YHx will be reviewed. In particular, we will discuss the striking broadening with increasing temperature of one single Raman line, corresponding to a breathing mode of Y atoms surrounding a hydrogen site. It is shown that the extra broadening is due to electron-phonon coupling and that it follows an Arrhenius law with an activation energy of 0.09eV. A close inspection of our IR spectra shows at the same energy an absorption line with an anomalous temperature dependence, which results from the overlap of a phonon and an electron excitation. This energy is assigned to the activation of electrons trapped in hydrogen vacancies, corroborating the correlated electron models for the electronic structure.