Reversible loading of epitaxial Y(00.1) films with hydrogen

Yttrium can be loaded with hydrogen up to high concentrations causing dramatic structural and electronic changes of the host lattice. We report on the reversibility of hydrogen loading in thin, monocrystalline Y films grown by molecular beam epitaxy on Nb/sapphire substrates. During hydrogen loading, the Y film undergoes structural transitions from the cubic dihydrid phase to the hexagonal trihydrid phase, while the structural coherence is maintained. The transition from YH${}_2$ to YH${}_3$ occurs at room temperature at a hydrogen pressure of 10 mbar and is completely reversible.