Na–Ni–H Phase Formation at High Pressures and High Temperatures: Hydrido Complexes [NiH(5)](3–)Versus the Perovskite NaNiH(3)

[Image: see text] The Na–Ni–H system was investigated by in situ synchrotron diffraction studies of reaction mixtures NaH–Ni–H(2) at around 5, 10, and 12 GPa. The existence of ternary hydrogen-rich hydrides with compositions Na(3)NiH(5) and NaNiH(3), where Ni attains the oxidation state II, is demonstrated. Upon heating at ∼5 GPa, face-centered cubic (fcc) Na(3)NiH(5) forms above 430 °C. Upon cooling, it undergoes a rapid and reversible phase transition at 330 °C to an orthorhombic (Cmcm) form. Upon pressure release, Na(3)NiH(5) further transforms into its recoverable Pnma form whose structure was elucidated from synchrotron powder diffraction data, aided by first-principles density functional theory (DFT) calculations. Na(3)NiH(5) features previously unknown square pyramidal 18-electron complexes NiH(5)(3–). In the high temperature fcc form, metal atoms are arranged as in the Heusler structure, and ab initio molecular dynamics simulations suggest that the complexes are dynamically disordered. The Heusler-type metal partial structure is essentially maintained in the low temperature Cmcm form, in which NiH(5)(3–) complexes are ordered. It is considerably rearranged in the low pressure Pnma form. Experiments at 10 GPa showed an initial formation of fcc Na(3)NiH(5) followed by the addition of the perovskite hydride NaNiH(3), in which Ni(II) attains an octahedral environment by H atoms. NaNiH(3) is recoverable at ambient pressures and represents the sole product of 12 GPa experiments. DFT calculations show that the decomposition of Na(3)NiH(5) = NaNiH(3) + 2 NaH is enthalpically favored at all pressures, suggesting that Na(3)NiH(5) is metastable and its formation is kinetically favored. Ni–H bonding in metallic NaNiH(3) is considered covalent, as in electron precise Na(3)NiH(5), but delocalized in the polyanion [NiH(3)](−).