Crystallographic evidence for chemical ordering inUCu5−xPdx

We report elastic neutron-diffraction measurements on ${\mathrm{UCu}}_{5-x}{\mathrm{Pd}}_x$ ($x=0.65,$ 1.0, and 1.5) using the High-Intensity Powder Diffractometer instrument at the Los Alamos Neutron Science Center. Data from six detector banks were simultaneously refined using Rietveld analysis. From the refinements of the data, we find that for Pd concentrations $x<\mathrm{}1,$ Pd atoms preferentially occupy the minority Cu/Pd $4c$ sites, and Cu atoms fully occupy the majority Cu/Pd $16e$ sites. For Pd concentrations $x>\mathrm{}1,$ Pd atoms fully occupy the minority sites and a mixture of Cu and Pd atoms occupy the $16e$ sites. At the special concentration $x=1,$ we find that the Pd and Cu atoms occupy separate crystallographic sites. This arrangement of atoms is indicative of chemical ordering, although no superlattice peaks were observed. The implications of chemical ordering in ${\mathrm{UCu}}_{5-x}{\mathrm{Pd}}_x$ on disorder-driven models of non-Fermi-liquid behavior will be discussed.