Model for the structural changes occurring at low temperatures in $\mathrm{Pd}{\mathrm{D}}_x$

A structural model for the short-range-ordered state in $\mathrm{Pd}{\mathrm{D}}_x$, with $0.7<x<~0.78$, observed at low temperatures, is proposed. The model describes the complicated diffuse intensity contours with "mixed" microdomains consisting of cells of the ${\mathrm{Ni}}_n\mathrm{Mo}$ type. A simple distortion, i.e., a relaxation of the D atoms towards vacant sites, is introduced in the "mixed" microdomain and accounts for the measured intensity asymmetry. Furthermore, a measurement of the intensities of the superlattice reflections in the long-range-ordered ${\mathrm{Ni}}_4$Mo structure is presented, and its result is reproduced by calculation, when the same relaxation towards the vacancy, is assumed. Finally an important Debye-Waller factor for the superlattice reflections is found.