Neutron powder diffraction study of deuterium ordering in β phase PdD at low temperatures

Abstract

We recently briefly reported the first observation by powder diffraction of a superlattice in β-PdD annealed at low temperatures. In this paper we give more details of our investigation by neutron powder diffraction of the low temperature phase diagram at several concentrations and temperatures. Two different superlattice were observed, at (1 (2n + 1)/2 0) positions in PdD0.62 and PdD0.65 annealed at 54 K, and at (h ±2/5h ±4/50) and equivalent positions in PdD0.77 annealed at 70 K. At the lower concentrations, contrary to the previous reports of a tetragonal I41/amd structure, a cubic superstructure in Pm3n with a doubled cell constant relative to the disordered structure was determined. The diffusion of D in 6d sites into neighbouring 6c sites generates the order, driven by a repulsive nearest-neighbour interaction. The ordering at [D]/[M] = 0.77 developed very slowly at 70 K and is described in an MoNi4 structure as in previous observations. No sign of ordering was found in PdD0.82 and PdD0.86 at temperatures down to 20 K. Enhanced anisotropic line broadening during ordering was observed in the Pm3n structure. It was successfully accounted for in the refinements of the cubic superlattice by assuming that it originates from strain along [001] which propagates anisotropically in other directions.