Hydrogen Ordering and New Polymorph of Layered Perovskite Oxyhydrides: Sr₂VO_4–<i>x</i>H_<i>x</i>

Compositionally tunable vanadium oxyhydrides Sr₂VO_4–<i>x</i>H_<i>x</i> (0 ≤ <i>x</i> ≤ 1.01) 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 <i>x</i> region (<i>x</i> < 0.2). A new orthorhombic phase (<i>Immm</i>) with an almost entirely hydrogen-ordered structure formed from the K₂NiF₄-type tetragonal phase with <i>x</i> > 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.