  Compositionally tunable vanadium oxyhydrides Sr2VO4-xHx (x = 0 - 1) without
considerable anion vacancy were synthesized by high-pressure solid state
reaction. The crystal structures and their properties were characterized by
powder neutron diffraction, synchrotron X-ray diffraction, thermal desorption
spectroscopy, and first-principles density functional theory (DFT)
calculations. The hydrogen anions selectively replaced equatorial oxygen sites
in the VO6 layers via statistical substitution of hydrogen in the low x region
(x < 0.2). A new orthorhombic phase (Immm) with an almost entirely
hydrogen-ordered structure formed from the K2NiF4-type tetragonal phase with x
> 0.7. Based on the DFT calculations, the degree of oxygen/hydrogen anion
ordering is strongly correlated with the bonding interaction between vanadium
and the ligands.
