Observation of the reversible H-induced structural transition in thin Y films via x-ray photoelectron diffraction

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 single-crystalline Y films grown by vapor deposition on W(110). Under a ${\mathrm{H}}_2$ partial pressure of $1\times {10}^{-5}$ mbar the hexagonal-closed- packed Y films convert to the face-centered-cubic Y dihydride. Unloading is accomplished by annealing the dihydride to 1000 K. No loss of crystallinity is observed during these martensitic transformations of the Y lattice. Moreover, we demonstrate a model to determine the H concentration in Y in situ.