The structure and dynamics of hydrogen in Mg2NiH4 studied by elastic and inelastic neutron scattering

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Abstract
The hydride Mg2NiH4 exhibits a phase transition from a low temperature to a high temperature phase at about 508 K. The dynamics of the hydrogen in the high temperature phase have been studied by cold neutron scattering experiments. According to these, the hydrogen atoms showed a high mobility, but the motion was mainly localized, in some way, on the six available octahedral positions around each nickel atom. The translational diffusion coefficient appeared simultaneously to be very small, within the instrumental resolution, implying values −6 cm2/s. Assuming a simple diffusive behavior in spherical symmetry for the localized part of the motion, relaxation times in the order of 10−11 s were obtained for the jumps of the hydrogen atoms. The low temperature phase of Mg2NiH4 was also studied indicating no other mobility of the hydrogen atoms than a vibrational motion, the low frequency part of which could be reproduced by a Debye spectrum with the temperature 150 K. In order to characterize the sample, a structure analysis was made on the high temperature phase of Mg2NiD4 using neutron diffraction. The structure data were in agreement with results recently reported, but anisotropic temperature parameters could be observed.