Ordering of deuterium in PdD0.65at 54 K

Neutron powder diffraction studies on PdD 0.65 demonstrate that the 50 K anomaly is due to an order-disorder transition of deuterium within octahedral 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 crystal structure remains cubic, and is accurately described in space group Pm3n by doubling the cell constant relative to the disordered structure (space group Fm3m).