Local atomic order in the vicinity of Cu₂ dumbbells in TbCu₇-type YCu_6.576 studied by Bragg and total scattering techniques

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• 作者：Radovan Č ; erný ; Yaroslav Filinchuk ; Stefan Brü ; hne
• 关键词：A. Intermetallics ; miscellaneous ; B. Crystallography ; F. Diffraction ; B. Order/disorder transformations ; D. Site occupancy
• 刊名：Intermetallics
• 出版年：2009
• 出版时间：October 2009
• 年：2009
• 卷：17
• 期：10
• 页码：818-825
• 全文大小：523 K

文摘

The crystal structure of YCu_6.576, a hyperstoichiometric substitution variant of the CaCu₅-structure type, was shown to contain Cu₂ dumbbells replacing Y-atoms in a fraction s = 0.19 in the formula Y_1−sCu_5+2s. The structure type TbCu₇ is assigned to the disordered structure (P6/mmm, a = 4.96 Å, c = 4.15 Å). The local order in the vicinity of the Cu₂-dumbbells has been derived from total scattering X-ray synchrotron data via refinement of the atomic pair distribution function (PDF). The coordinating Cu₆-hexagon around the dumbbell site shows a shrinkage of 0.33(1) Å w.r.t. the equivalent environment of the Y-atom. No adjacent Y-atoms are substituted but a hexagonal arrangement of Cu₂-dumbbells, separated at least by the vectors √3a and √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 YCu_6.576 are consistent with the findings from local crystallography (total scattering data) answering a long-standing question in disordered CaCu₅-type based intermetallic structure variants.