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

Abstract

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 W͑110͒. Under a H 2 partial pressure of 1ϫ10 Ϫ5 mbar the hexagonal-closedpacked 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.The interaction of hydrogen with Y, La, and the rare-earth ͑RE͒ metals has been the subject of numerous investigations due to the interesting temperature-and concentrationdependent structures and properties observed in the solid solution ͑␣ phase͒ as well as in the stable dihydride ͑␤͒ and trihydride ͑␥͒ phases.1 An example is the recent observation of switchable optical properties of Y and La hydride films where shiny, metallic dihydride films become transparent semiconductors in the trihydride phase.2 It is obvious that the geometrical and electronic structure of RE hydrides are key quantities for the understanding of all these properties.For Y, the geometrical structure of the host-lattice and the hydrogen atom positions are well known over the entire range of H concentrations: Y crystallizes in the hexagonalclosed-packed ͑hcp͒ structure, while its dihydride transforms to a face-centered-cubic ͑fcc͒ CaF 2 -type structure.1 The insulating trihydride, finally, possesses a hcp unit cell ͑HoD 3 type͒. 3 The electronic structure of Y hydrides, however, is still a matter of dispute. 1Up until recently most work on hydrogen in Y has been done using polycrystalline bulk or powder samples. Existing photoemission spectroscopy data therefore only yields information on the occupied density of states or on charge transfer from Y to H via core-level shifts. [4][5][6][7] The reason for the absence of experimental band-structure data based on angleresolved ultraviolet photoemission spectroscopy ͑ARUPS͒ is due to the fact that loss of single crystallinity during the first transition ͑␣ to ␤͒ is difficult to avoid and that most bulk samples decompose into powder while going from ␤ to ␥. 1Recently an x-ray diffraction study demonstrated that in thin, monocrystalline Y films, the structural coherence is maintained during cycling ex situ between the dihydride and the trihydride phases. In this study we demonstrate that it is possible to transform thin, single-crystalline Y films into the dihydride phase and to unload them again without loss of order. We used x-ray photoelectron diffraction ͑XPD͒ to observe, in real space and near the surface, the changes occurring due to the H-induced structural transitions. In contrast to the previous study, 8 experiment, loading, and unloading are done in situ, and the films are not capped by a protective Pd layer, a most crucial prerequisite for future ARUPS studies on the electron...

Oxygen-segregation-controlled epitaxy of Y2O3 films on Nb(110)

“…3a) reveals six-fold symmetry. An identical pattern was observed for 150 Å Y deposited at RT at W(110) and was identified as a single-crystalline hc p(0001)-oriented Y film [8]. The O 1s photoelectron distribution (Fig.…”

“…Since for Y deposition on W(110) under equivalent deposition conditions no O contamination is detected [8], oxygen cannot originate from the residual gas. Obviously oxygen disolved in the Nb bulk [14] segregates to the surface and provides a natural source of very clean, atomic oxygen.…”

“…It is a powerful technique for surface structural investigations [7], and it has been shown that full hemispherical XPD patterns provide very direct information about the near-surface structure [8]. At photoelectron kinetic energies above about 500 eV, the strongly anisotropic scattering of photoelectrons by the ion cores leads to a forward-focusing of the electron flux along the emitterscatterer axis.…”

Fermi-surface topology of rare-earth dihydrides

“…Details of the structural analysis via x-ray photoelectron-spectroscopy and -diffraction are not presented here, the interested reader is referred to Ref. 21. The ARPES measurements were performed via sequential angle-scanning data acquisition.…”

ANGLE-SCANNED PHOTOEMISSION ON YbHx: RELEVANCE FOR SWITCHABLE MIRRORS

“…Recently, we were able to prepare thin, single crystalline Y films on W(110) and to load them with hydrogen. 8,9 The W(110) surface represents a quasihexagonal template inducing the growth of close-packed layers. Different sequences are possible (see Fig.…”