In situ neutron powder diffraction measurements of the absorption and desorption of hydrogen (deuterium) in (La,Ce)Ni 2 compounds: Amorphization and recrystallization

Abstract

The structural transformation of RNi 2 Laves phases (≡ La, Ce) on absorption of hydrogen and deuterium was studied using X-ray powder diffraction at room temperature, 90 and 120 °C and using time-resolved neutron powder diffraction at room temperature. For the intermetallic compounds, superlattice lines indexed in a double cubic cell are observed in addition to the diffraction peaks of the cubic C-15-type structure. In the case of in situ absorption measurements, small increases in the lattice parameters of the crystalline phases LaNi 2.18 and CeNi 2.16 are observed and the formation of an amorphous phase is detected from the first stage of the absorption. Under equilibrium conditions at room temperature, the lattice parameter of the crystalline phase LaNi 2.18 increases as previously, whereas that of CeNi 2.16 is found to decrease with increasing deuterium content. A mechanism for the segregation of the cerium atoms from CeNi 2.16 leading to a compound rich in cerium vacancies and cerium hydride is proposed on the basis of the results of the high temperature (90 and 120 °C) hydrogen absorption experiments. The thermal desorption of the amorphous hydride CeNi 2.16H 3.65 (studied by neutron diffraction) leads to a recrystallization of the intermetallic compound CeNi 2 + y after the formation of intermediate CeNi x phases.