Abstract
A first direct observation of the '50K' structural transition in beta -phase palladium hydride has been made using neutron scattering. This observation, of a superlattice reflection at (1/210), 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 indifferent 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 AA.