Local atomic order in the vicinity of Cu2 dumbbells in TbCu7-type YCu6.576 studied by Bragg and total scattering techniques

The crystal structure of YCu6.576, a hyperstoichiometric substitution variant of the CaCu5-structure type, was shown to contain Cu-2 dumbbells replacing Y-atoms in a fraction s = 0.19 in the formula Y1-sCu5+2s. The structure type TbCu7 is assigned to the disordered structure (P6/mmm, a = 4.96 angstrom, c = 4.15 angstrom). The local order in the vicinity of the Cu-2-dumbbells has been derived from total scattering X-ray synchrotron data via refinement of the atomic pair distribution function (PDF). The coordinating Cu-6-hexagon around the dumbbell site shows a shrinkage of 0.33(1) angstrom w.r.t. the equivalent environment of the Y-atom. No adjacent Y-atoms are substituted but a hexagonal arrangement of Cu-2-dumbbells, separated at least by the vectors root 3a and root 3b exists in layers perpendicular to c. The stacking along [001] is random and can be modeled locally from the PDF data consistent with both ab- and abc-type stacking. Therefore, the local order is comparable to that in Ni17Th2- and Th2Zn17-structure types. X-ray single crystal analysis and Rietveld refinements of neutron- and X-ray powder data (Bragg diffraction data) of YCu6.576 are consistent with the findings from local crystallography (total scattering data) answering a long-standing question in disordered CaCu5-type based intermetallic structure variants. (C) 2009 Elsevier Ltd. All rights reserved.