Preferential Co and Fe atom occupancy in R2(Fe1-xCo )14B intermetallic compounds (R = Nd, Y and Ce)

Abstract

A powder neutron scattering study of atom substitution in Nd 2 (Fe 1− Co ) 14 B, Y 2 (Fe 1− Co ) 14 B and Ce 2 (Fe 1− Co ) 14 B compounds is presented. Compared to previous studies, this one benefits from the considerable progress made in neutron diffraction measurements and their analysis during the last 15 years. The normalization of the occupancy values to those expected from a random distribution permits to reveal small deviations from random distribution. On all crystallographic sites some preferential occupancy is evidenced, the extent of which depends on the Co versus Fe content. A statistical model quantitatively describing site occupancies is proposed, which allows us to study the extreme cases of very dilute substitutions. Steric considerations primarily determine atom substitution, more particularly Fe-substitution in Co-rich compounds. The enthalpy of mixing must also be considered to describe Co preferential occupancies more exactly.

Site Occupancy Preference and Magnetic Properties in Nd2(Fe,Co)14B

“…This indicates that in the Co-rich Nd 2 (Fe,Co) 14 B phase, Fe prefers to enter the 8j 2 site and has the least preference for the 4e site. The result agrees with the experimental reports that in Nd 2 (Fe,Co) 14 B, Co and Fe prefer to avoid and occupy the 8j 2 site, respectively [25][26][27][28]. The phase stability of the 2:14:1 system is less affected by doping with Co.…”

“…Similarly, the value will be much lower than one if the Co atom prefers to avoid the site. A similar method was applied to analyze the site occupancy preference from neutron diffraction data in the 2:14:1 system [28]. As shown in Figure 1b, the relative occupancy P i for the 8j 1 site has the largest value (2.8 at 1400 K), while that at the 8j 2 site has the lowest value (0.03 at 1400 K).…”

“…van Noort and Buschow [26] found that Fe atoms strongly prefer occupying the 8j 2 and 16k 2 sites from Mössbauer spectroscopy. Similarly, using Mössbauer spectroscopy, Ryan et al [27] concluded that Fe prefers the 8j 2 sites, the other sites being randomly occupied by Co in Nd 2 Fe 14−x Co x B. Eslava et al reported that the Co occupancy preference depends on the Co content in Nd 2 Fe 14−x Co x B [28]. Despite these labor-, time-, and cost-intensive experimental approaches, the first-principles DFT (density function theory) calculations can predict the site occupancy and its effect on magnetic properties in intermetallic compounds such as the 2:14:1 system.…”

A two-sublattice model for extracting rare-earth anisotropy constants from measurements on (Nd,Ce)2(Fe,Co)14B single crystals

“…As mentioned above, in Co substituted samples, Co shows a preference to occupy 4c, 8j1 and 16k2 sites [10,11]. The 4c and 8j1 sites are the most influenced by R atoms, as they are surrounded by 4 and 3 of them, respectively.…”

“…The partial substitution of Co for Fe in (Nd,Ce)-Fe-B increases the Curie temperature, due to enhanced exchange interactions. On the other hand, references [10,11] report that in a limited substitutional range, Co preferentially occupies the 4c and 8j1 M sites, which show a stronger affinity with the R elements than the other M sites. DFT calculations of Co-doped Ce2Fe14B predict that, by altering the coordination conditions, the valance state of Ce shifts towards 3+ [12], and in a sweet spot concentration range of Ce and Co, improved magnetic properties can be obtained [13].…”