ORDERING OF DEUTERIUM IN PDD0.65 AT 54-K

Neutron powder diffraction studies on PdD0.65 demonstrate that the 50 K anomaly is due to an order-disorder transition of deuterium within o ctahedral interstitial sites. The slow transition to the ordered phase involves diffusion of deuterium from the nearest-neighbour deuterium positions to the second-nearest-neighbour positions. The ordered cryst al structure remains cubic, and is accurately described in space group Pm (3) over bar n by doubling the cell constant relative to the disor dered structure (space group Fm3m).