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Substitution Patterns Understood through Chemical Pressure Analysis: Atom/Dumbbell and Ru/Co Ordering in Derivatives of YCo5

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journal contribution
posted on 21.02.2017 by Katerina P. Hilleke, Rie T. Fredrickson, Anastasiya I. Vinokur, Daniel C. Fredrickson
Interstitials, mixed occupancy, and partial substitution of one geometrical motif for another are frequently encountered in the structure refinements of intermetallic compounds as disorder or the formation of superstructures. In this article, we illustrate how such phenomena can serve as mechanisms for chemical pressure (CP) release in variants of the CaCu5 type. We begin by comparing the density functional theory CP schemes of YCo5, an f-element free analogue of the permanent magnet SmCo5, and its superstructure variant Y2Co17 = [Y2(Co2)1]­Co15 (Th2Zn17-type) in which one-third of the Y atoms are replaced by Co2 dumbbells. The CP scheme of the original YCo5 structure reveals intensely anisotropic pressures acting on the Y atoms (similar to CP schemes of other CaCu5-type phases). The Y atoms experience large negative pressures along the length of the hexagonal channels they occupy while being simultaneously squeezed by the channel walls. Moving to the Y2Co17 structure provides significant relief to this CP scheme: the inserted Co2 pairs densify the atomic packing along the hexagonal channels while providing space for the bulging of the walls to better accommodate the remaining Y atoms. This Y/Co2 substitution pattern thus yields a much smoother CP scheme, but residual pressures remain. The experimental relevance of these remaining stresses is investigated through a structural refinement of a Ru-substituted variant of Y2Co17 using single crystal X-ray diffraction. A comparison of the Y2Co17 CP scheme with the observed Ru/Co ordering reveals that Ru preferentially substitutes for Co atoms whose net CPs are most negative, in accord with the larger size of the Ru atoms. These results hint that a wider variety of elemental site preferences may be understandable from the viewpoint of CP relief.