Na-Ni-H Phase Formation at High Pressures and High Temperatures: Hydrido Complexes [NiH] the Perovskite NaNiH

The Na-Ni-H system was investigated by synchrotron diffraction studies of reaction mixtures NaH-Ni-H at around 5, 10, and 12 GPa. The existence of ternary hydrogen-rich hydrides with compositions NaNiH and NaNiH, where Ni attains the oxidation state II, is demonstrated. Upon heating at ∼5 GPa, face-centered cubic () NaNiH forms above 430 °C. Upon cooling, it undergoes a rapid and reversible phase transition at 330 °C to an orthorhombic () form. Upon pressure release, NaNiH further transforms into its recoverable form whose structure was elucidated from synchrotron powder diffraction data, aided by first-principles density functional theory (DFT) calculations. NaNiH features previously unknown square pyramidal 18-electron complexes NiH . In the high temperature form, metal atoms are arranged as in the Heusler structure, and molecular dynamics simulations suggest that the complexes are dynamically disordered. The Heusler-type metal partial structure is essentially maintained in the low temperature form, in which NiH complexes are ordered. It is considerably rearranged in the low pressure form. Experiments at 10 GPa showed an initial formation of NaNiH followed by the addition of the perovskite hydride NaNiH, in which Ni(II) attains an octahedral environment by H atoms. NaNiH is recoverable at ambient pressures and represents the sole product of 12 GPa experiments. DFT calculations show that the decomposition of NaNiH = NaNiH + 2 NaH is enthalpically favored at all pressures, suggesting that NaNiH is metastable and its formation is kinetically favored. Ni-H bonding in metallic NaNiH is considered covalent, as in electron precise NaNiH, but delocalized in the polyanion [NiH].