THE ORDERED STRUCTURE OF PDD0.78 AT 70-75 K

The ordered structure of beta-PdD0.78 at 70-75 K has been studied by t ime-resolved neutron powder diffraction. The ordered superlattice has a MoNi4 structure with approximately four full D sites and one vacant D site in successive (420) planes, which is consistent with previous o bservations. Several superlattice lines not reported in previous singl e-crystal studies were observed. Quantitative information about the or dered structure was obtained by careful ; Rietveld analysis of the dat a, including anisotropic line broadening and diffuse peak background c orrections. The structure refinement confirms a relaxation of D atoms towards the vacant planes and reveals a related second-order displacem ent of the D atoms in the octahedral sites towards a preferred vacant nearest-neighbour site. The thermal vibration along the c-axis is anis otropic as a result of the preferential relaxation.