Hydrogen and deuterium in epitaxial Y(0001) films: Structural properties and isotope exchange

Hydrogen in yttrium is of fundamental interest as a model system for drivin g metal-insulator transitions including switchable optical properties from reflecting to transparent in the visible region. We report on the structura l properties of hydrogenated and deuterated thin, monocrystalline Y(0001) f ilms grown by molecular beam epitaxy on Nb/Al2O3 substrates. X-ray diffract ion reveals the response of the host metal lattice upon hydrogen loading. T he structural coherence in all three spatial directions as well as the epit axial relation to the substrate are maintained, though the sample undergoes structural phase transitions between the different hydride phases. With ne utron reflectivity measurements we have determined the hydrogen and deuteri um content since the critical angle for total reflection depends on their c oncentration in the sample. Measurements on hydrogenated and deuterated fil ms show that H and D are completely interchangeable within the trihydride p hase. Neutron scattering also allows us to determine the position of the de uterium atoms within the yttrium matrix. All structural information gained on thin films is in agreement with the space group P (3) over bar c1 which was previously determined from powder samples.[S0163-1829(99)02810-6].