Electronic properties and bonding in ZrHx thin films investigated by valence-bandx-ray photoelectron spectroscopy

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Abstract

The electronic structure and chemical bonding in reactively magnetron sputtered ZrHx (x = 0.15, 0.30, 1.16)thin films with oxygen content as low as 0.2 at.% are investigated by 4d valence band, shallow 4p core-level,and 3d core-level x-ray photoelectron spectroscopy. With increasing hydrogen content, we observe significantreduction of the 4d valence states close to the Fermi level as a result of redistribution of intensity toward the H1s–Zr 4d hybridization region at ∼6 eV below the Fermi level. For low hydrogen content (x = 0.15, 0.30), thefilms consist of a superposition of hexagonal closest-packed metal (α phase) and understoichiometric δ-ZrHx(CaF2-type structure) phases, while for x = 1.16, the films form single-phase ZrHx that largely resembles thatof stoichiometric δ-ZrH2 phase. We show that the cubic δ-ZrHx phase is metastable as thin film up to x = 1.16,while for higher H contents the structure is predicted to be tetragonally distorted. For the investigated ZrH1.16film, we find chemical shifts of 0.68 and 0.51 eV toward higher binding energies for the Zr 4p3/2 and 3d5/2peak positions, respectively. Compared to the Zr metal binding energies of 27.26 and 178.87 eV, this signifiesa charge transfer from Zr to H atoms. The change in the electronic structure, spectral line shapes, and chemicalshifts as a function of hydrogen content is discussed in relation to the charge transfer from Zr to H that affectsthe conductivity by charge redistribution in the valence band. ; Funding agencies: Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009-00971]; Swedish Energy Research [43606-1]; Swedish Foundation for Strategic Research (SSF) through synergy grant FUNCASE [RMA11-0029]; Ca