Band gap closure in yttrium hydride under high pressure

Trivalent rare-earth metals demonstrate spectacular change in electronic properties by hydrogenation. With increase in hydrogen concentration beyond $x \sim 2.7$, yttrium hydride, YH$_x$, shows metal-insulator phase transition with structural change from the fcc to hcp of yttrium metal lattice. Band gap opening due to orbital hybridization between $1s$ (H) and $4d$ (Y) has theoretically been proposed for the metal- insulator transition. Theoretical studies have also predicted that the volume reduction by applying hydrostatic pressure would lead to metallization in association with band gap closure. We have investigated structural properties of yttrium hydrides by means of x-ray diffraction and infrared absorption beyond a predicted metallization pressure of $\sim 18$~GPa. Hydride specimen was prepared by hydrogenation reaction of yttrium powder or foil with liquid hydrogen in a diamond anvil cell at room temperature. With increase in pressure beyond $\sim 10$~GPa, the hcp lattice of YH$_3$ transforms gradually to a fcc structure. Infrared spectra show peak position change in the hydrogen vibrational region of 450-1500 cm$^{-1}$ above $\sim 11$~GPa, corresponding to the x-ray diffraction results. The H-Y bonding state and expected metallization are discussed on the basis of the high pressure experimental results obtained x-ray diffraction and infrared absorption.