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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 ] - .

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Na–Ni–H Phase Formation at High Pressures and High Temperatures: Hydrido Complexes [NiH5]3– Versus the Perovskite NaNiH3