Preparation and characterization of clean, single-crystalline YHx films (0 ≤ x ≤2.9) on W(110)

Yttrium can be loaded with hydrogen up to high concentrations causing dramatic structural and electronic changes of the host lattice. We report on the preparation of clean, single-crystalline YHx films (0 less than or equal to x less than or equal to 2.9). The films have been characterized in situ combining angle-resolved photoelectron spectroscopy (ARPES) and low energy electron diffraction. Direct Y dihydride growth, i.e., Y evaporation under a H-2 partial pressures of approximate to 5 x 10(-6) mbar at 500 K on W(110), is the most convenient starting point for the preparation of clean single-crystalline Y hydride films covering H concentrations from the "clean metal" (x approximate to 0) up to the lower boundary of the pure trihydride phase (x approximate to 2.9). Upon annealing Y dihydride films the desired H concentration can be adjusted within the alpha-phase or the (alpha+beta) two-phase regime. On the other hand, the extension of our photoelectron spectrometer with an homemade ultrahigh vacuum (UHV) compatible hydrogenation system allows to induce the transition from Y dihydride to Y trihydride within a few minutes. The hydrogenation system combines a high-pressure reaction cell with hydrogen permeation through a Pd-24%Ag tube. The overall design is such that the sample never gets in contact with non-UHV compartments. For direct Y dihydride growth on W(110) two equally populated face-centered-cubic(lll) domains rotated by 180 degrees with respect to each other are observed. In the alpha- and gamma-phase the Y atoms form a hexagonal-close-packed(0001) oriented lattice. Furthermore, the previously established model for in situ I-I concentration estimation in Y [J. Hayoz et al., Phys: Rev. B 58, R4270 (1998)] is extended successfully from the alpha to beta to the beta to gamma-phase transition. Ultraviolet photoemission spectroscopy data unequivocally reveal the opening of a gap extending as far as 1 eV below E-F for normal electron emission upon the phase-transformation from Y dihydride to Y trihydride. It also appears that the H absorption rate strongly depends on the H-2 purity. Our experimental results demonstrate the capability of this setup for in situ preparation and investigations on the geometrical and electronic structure of Y hydride films and, more generally, rare-earth hydride films using ARPES. (C) 2000 American Vacuum Society. [S0734-2101(00)01805-4].