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Early transition metal-based intermetallic compounds, such as ZrV 2 , ZrMn 2 , and ZrCr 2 , can reversibly absorb large amounts of hydrogen and are of great interest for hydrogen storage. The dissociative hydrogen chemisorption can proceed easily on the surface, allowing the rapid formation of intermetallic hydrides. In this study, we present the results of initial NH 3 activity tests conducted on various Zr-based AB 2 -type intermetallic compounds, free of precious transition metals and rare earth metals. A dissociative hydrogen chemisorption together with nitrogen storage proceeds on AB 2 -type Laves phase intermetallic compounds accompanied by a lattice expansion. The formed intermetallic nitride hydride ZrCr 2 N 0.4 H 1.4 exhibits stable catalytic activity as an ammonia synthesis catalyst. In this study, we found a dissociative hydrogen chemisorption together with nitrogen storage proceed on AB 2 -type Laves phase intermetallic compounds, allowing the rapid formation of intermetallic nitride hydrides. The cell volume expansion upon hydrogenation facilitates nitrogen adsorption and dissociation. The formed intermetallic nitride hydride ZrCr 2 N 0.4 H 1.4 exhibits stable catalytic activity as an ammonia synthesis catalyst. [Display omitted] • A hydrogen adsorption together with nitrogen storage in Laves phase intermetallic compounds. • The lattice expansion accompanied by H intercalation could help the insertion of N. • The formed intermetallic nitride hydride exhibits stable catalytic activity for ammonia synthesis. [ABSTRACT FROM AUTHOR]

In this study, we investigate the phase behavior in Ln HO (Ln = Nd, Gd, and Er) at external pressures up to 5 GPa. In contrast to LaHO, we observed a direct phase transformation from the fluorite-type structure to the anti-Fe 2 P-type structure in all the compounds examined. In the high-pressure anti-Fe 2 P-type structure above 3 GPa, the H− and O2− anions are coordinated four-fold and five-fold, respectively, as observed in LaHO. The absence of the PbCl 2 -type structure by chemical pressure (i.e., smaller Ln 3+ substitution) is supported by DFT calculations. We further found a tendency for the fluorite-type structure to stabilize toward the higher-pressure side as the Ln ion becomes smaller; in ErHO and GdHO, the fluorite-type structure survives at the secondary phase even at 5 GPa. This suggests that chemical pressure plays a different role in the structural stabilization than physical pressure. The unexpected observation is interpreted in terms of entropy stabilization since, unlike LaHO and NdHO, the fluorite-type ErHO and GdHO compounds are anion disordered. We have performed high-pressure synthesis of Ln HO (Ln = Nd, Gd, and Er), and observed that the fluorite-structure directly transforms to the anti-Fe 2 P-type structure, without forming the PbCl 2 -type structure as in LaHO. The unexpected expansion of the fluorite phase toward high pressure for smaller Ln 3+ is derived from entropy stabilization. [Display omitted] • A phase diagram for the synthesis pressure of Ln HO is obtained. • NdHO, GdHO, and ErHO transform directly from the fluorite structure to anti-Fe 2 P structure under pressure. • The fluorite phase space is extended to higher pressures due to entropy stabilization. • Anion order/disorder gives a crucial influence on the phase transformation. [ABSTRACT FROM AUTHOR]