Hydrogenation-induced cerium valence change in CeNiZn

Polycrystalline CeNiZn (ZrNiAl type, P6¯2m, a = 715.6 (4), c = 388.4 (2) pm) forms a new hydride CeNiZnH1.2(1) under hydrogen exposure (10 bar) at 423 K. The resulting hydride crystallizes in a hexagonal structure (P6/mmm) with the unit cell parameters a = 433.19 (2) and c = 424.37 (2) pm. Thus, hydrogen atom insertion yields a structural transition from a ZrNiAl- to an AlB2-type structure with a high volume cell expansion of +20.1%. The magnetic, thermal and transport properties of CeNiZnH1.2(1) were investigated. Susceptibility and specific heat measurements do not reveal magnetic order down to 1.8 K but most likely the presence of ferromagnetic correlations due to the proximity of a ferromagnetic order or a nonmagnetic atom disorder spin-glass state. Moreover, the cerium valence is modified from an intermediate to a trivalent state, a change that results from the decrease of the hybridization strength between conduction and Ce (4f) electrons upon hydrogenation. Transport measurements also evidence a deviation from the typical resistivity behavior of metallic systems. Several scenarios are conceived to explain this change, in particular weak localization effect, knowing that it rarely occurs upon hydrogenation of intermetallics.