The 50 K transitions in β-phase palladium deuteride observed by neutron scattering

A first direct observation of the '50K' structural transition in β-phase palladium hydride has been made using neutron scattering. This observation, of a superlattice reflection at (1/2 10) allows us to conclude that the transition is essentially an order-disorder transition involving a local rearrangement of deuterium atoms and vacancies between the eight different interpenetrating FCC sublattices having twice the original lattice parameter. The low intensity of the reflection corresponds to a small value for the long-range order parameter and its dimensions in reciprocal space indicate that the long-range order only extends to about 25 A. (author)