Site exchange in hexagonal Ce(Ni,Cu)6 intermetallics

The crystal structures of CeNi3Cu3 and CeNi4Cu2 intermetallics have been investigated by time-of-flight neutron powder diffraction on the ROTAX diffractometer at the ISIS spallation neutron source. Both compounds are found to crystallize in the TbCu7 structure which is a disordered defect variant of the hexagonal CaCu5 (P6/mmm) structure. A Rietveld structure refinement has shown the presence of a partial substitution of Ce atoms by dumb bell pairs of transition metal atoms at 2e sites which change the local uniaxial environment of the Ce atom. This type of substitution is found to largely conserve the CaCu5 structure. The large contrast in neutron scattering lengths for Ni and Cu has additionally allowed for a distinction to be made between the three possible types of dumbbell substitutions, Cu-Cu, Ni-Cu, Ni-Ni as well as the occupancies of the 2c and 3g sites. (orig.)