  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 significant reduction of the 4d valence states close to the Fermi
level as a result of redistribution of intensity towards the H 1s - Zr 4d
hybridization region at about 6 eV below the Fermi level. For low hydrogen
content (x=0.15, 0.30), the films consist of a superposition of hexagonal
closest packed metal (alpha-phase)and understoichiometric delta-ZrHx (CaF2-type
structure) phases, while for x=1.16, the film form single phase ZrHx that
largely resembles that of stoichiometric delta-ZrH2 phase. We show that the
cubic delta-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 ZrH1.16 film, we find chemical shifts of 0.68 and 0.51 eV towards
higher binding energies for the Zr 4p3/2 and 3d5/2 peak positions,
respectively. Compared to the Zr metal binding energies of 27.26 and 178.87 eV,
this signifies a charge-transfer from Zr to H atoms. The change in the
electronic structure, spectral line shapes, and chemical shifts as function of
hydrogen content is discussed in relation to the charge-transfer from Zr to H
that affects the conductivity by charge redistribution in the valence band.
