Theoretical Investigation on the Phase Stability and Site Preference of R(Co,T)12 and R(Co,T)12Nx(R=Y, Ce, Pr, Nd, Sm, Gd, Tb, Ho, Er, Dy, T=Mo, Mn, Ni)

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.

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The isothermal section of the Tb–Co–Cr ternary system at 873 K

Yao

Lan

Zhou

et al. 2011

Phase Transitions

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The isothermal section of the phase diagram of the Tb-Co-Cr ternary system at 873 K was investigated by means of X-ray diffraction, metallography, and scanning electron microscopy equipped with energy dispersive X-ray spectroscopy. Ternary phases and their lattice parameters as a function of composition of solid solution were systematically studied. The existence of one ternary phase reported previously was confirmed. The ternary compound Tb(Co, Cr) 12 crystallizes with ThMn 12 -type structure, space group I4/mmm. The linear homogeneity range along the line of 7.69 at.% Tb in TbCo 12Àx Cr x was found to be about x ¼ 3.6-5.7, i.e., 27.7-43.8 at.% Cr. The lattice parameters are a ¼ 0.8326-0.8352 nm and c ¼ 0.4709-0.4740 nm. The maximum solid solubilities of Cr in Tb 2 Co 17 , TbCo 3 , and TbCo 2 are about 20.0, 8.2, and 7.9 at.%, respectively.Keywords: Tb-Co-Cr ternary systems; phase diagrams; X-ray diffraction; scanning electron microscopy 1. Introduction Due to its high efficiency, energy saving and environmental awareness, magnetic refrigeration (MR) is becoming a promising technology to replace the conventional gascompression refrigeration [1,2]. Much attention has been paid to the properties of the R-Co (R stands for a rare earth element) system, while the RCo 5 and R 2 Co 17 -type compounds show outstanding magnetic properties, some RCo 2 -type alloys show a large magnetocaloric effect in a low magnetic field and their Curie temperature T c can vary in a very large temperature range [3][4][5]. For improving some magnetic properties, the substitution for Co in the R-Co system compounds by other transition element also has been extensively studied [6,7]. The phase diagram can provide important information for developing new materials. To further understand the relations between formation of the compounds and their properties, the investigation of the phase diagram of the relevant systems is useful. The phase diagrams of Er-Co-V [8], Gd-Co-V [9], Dy-Co-Ti [10], and Dy-Co-V [11], etc. ternary systems have been reported in the literature. So far, no phase diagram of the Tb-Co-Cr ternary system is described. In this study, we investigated the phase equilibria in the Tb-Co-Cr ternary system at 873 K.

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Theoretical Investigation on the Phase Stability and Site Preference of R(Co,T)₁₂ and R(Co,T)₁₂N_x(R=Y, Ce, Pr, Nd, Sm, Gd, Tb, Ho, Er, Dy, T=Mo, Mn, Ni)

Cited by 3 publications

References 17 publications

Experimental Determination of Phase Equilibria in the Ce-Co-Zr Ternary System

Experimental Determination of Phase Equilibria in the Ce-Co-Zr Ternary System

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

The isothermal section of the Tb–Co–Cr ternary system at 873 K

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Theoretical Investigation on the Phase Stability and Site Preference of R(Co,T)12 and R(Co,T)12Nx(R=Y, Ce, Pr, Nd, Sm, Gd, Tb, Ho, Er, Dy, T=Mo, Mn, Ni)

Cited by 3 publications

References 17 publications

Experimental Determination of Phase Equilibria in the Ce-Co-Zr Ternary System

Experimental Determination of Phase Equilibria in the Ce-Co-Zr Ternary System

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

The isothermal section of the Tb–Co–Cr ternary system at 873 K

Contact Info

Product

Resources

About

Theoretical Investigation on the Phase Stability and Site Preference of R(Co,T)₁₂ and R(Co,T)₁₂N_x(R=Y, Ce, Pr, Nd, Sm, Gd, Tb, Ho, Er, Dy, T=Mo, Mn, Ni)

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