Faculté des sciences

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

Hayoz, J. ; Pillo, T. ; Bovet, M. ; Guthrie, St. ; Pastore, G. ; Schlapbach, Louis ; Aebi, Philipp

In: Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2000, vol. 18, no. 5, p. 2417-2431

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≤x≤2.9). The films have been characterized in situ combining angle-resolved photoelectron spectroscopy (ARPES) and low energy electron diffraction. Direct Y dihydride... Plus

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    Summary
    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≤x≤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 H2 partial pressures of ≈5×10–6 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≈0) up to the lower boundary of the pure trihydride phase (x≈2.9). Upon annealing Y dihydride films the desired H concentration can be adjusted within the α-phase or the (α+β) 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(111) domains rotated by 180° with respect to each other are observed. In the α- and γ-phase the Y atoms form a hexagonal-close-packed(0001) oriented lattice. Furthermore, the previously established model for in situ H concentration estimation in Y [J. Hayoz et al., Phys. Rev. B 58, R4270 (1998)] is extended successfully from the α to β to the to γ-phase transition. Ultraviolet photoemission spectroscopy data unequivocally reveal the opening of a gap extending as far as 1 eV below EF 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 H2 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.