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.