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

Abstract

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.

Structural Plasticity in the Frank‐Kasper Realm: Chemical Pressure Roles of the μ‐ and χ‐Phase Units in the Mo−Fe−Cr System

“…As shown in the Supporting Information, the CP scheme shows little change from the non‐spin polarized result (though the volume of the unit cell expands significantly). As such, the formation of local magnetic moments does not appear to influence the tensions between the negative Mo−Fe and positive Fe−Fe CPs within the structure, a situation similar to what we saw earlier for YCo 5 [56] . Based on these results, we will focus on the remainder of this work on non‐spin‐polarized models of the structures.…”

“…In moving from Mo 6 Fe 7 to Mo 5 Fe 8 , there are three potential points for Fe to substitute for Mo: the dumbbell positions, the honeycomb net in the Zr 4 Al 3 ‐type layer, or the Mo atoms at the centers of the Friauf polyhedra in the MoFe 2 slabs. Following recent applications of the CP method to predict site preferences in the Y−Co−Ru [56] and Ca−Cu−Al [57] systems, we expect the placement of the smaller Fe atoms would be most favorable at points where the original Mo atoms are effectively too big for their coordination environments, i. e. where their net atomic pressures are most positive. In the CP scheme of Mo 6 Fe 7 (Figure 4a), each of the Mo sites displays both negative and positive CP features, but the differing balance between these effects leads to different net CPs.…”

Tutorial on Chemical Pressure Analysis: How Atomic Packing Drives Laves/Zintl Intergrowth in K3Au5Tl

“…This method is based on the recognition that non-optimal interatomic distances are detectable in the local pressures that surround the atoms of a solid-state lattice, as constructed from the output of DFT calculations. In the study of a variety of intermetallic phases, CP analysis has provided explanations for diverse structural phenomena, such as the insertion of interfaces into simple structures [19,20,24,25], the adoption of local icosahedral symmetry in quasicrystal approximants [26][27][28], the emergence in incommensurability [29][30][31][32][33][34], the stabilizing effects of various kinds of single point substitutions [23][24][25]35,36], and the formation of intergrowth structures [27,35,[37][38][39]. The CP maps generated in the process can also be used to analyze the forces involved in chemical bonding and molecular structure [40][41][42][43].…”

High-throughput design of magnetic materials

“…For all the derived structures, the Kagome layers formed by the Co atoms on the 3g sites of SmCo 5 are slightly distorted, while the changes occur mostly in the SmCo 2 layers due to large chemical pressure. 189 Such a plenty of phases offer an arena to understand the structure-property relationship and to help us to design new materials, particularly the mechanical understanding based on the local atomic structures. For instance, Sm 2 Co 17 has uniaxial anisotropy while all the other early RE-Co 2:17 compounds show a planar behaviour.…”