Site-preference and valency for rare-earth sites in (R-Ce)2Fe14B magnets

Abstract

Rare-earth (R) permanent magnets of R2Fe14B have technological importance due to their high energy products, and they have two R-sites (Wyckoff 4f and 4g, with four-fold multiplicity) that affect chemistry and valence. Designing magnetic behavior and stability via alloying is technologically relevant to reduce critical (expensive) R-content while retaining key properties; cerium, an abundant (cheap) R-element, offers this potential. We calculate magnetic properties and Ce site preference in (R1−xCex) 2Fe14B [R = La,Nd] using density functional theory (DFT) methods—including a DFT + U scheme to treat localized 4f-electrons. Fe moments compare well with neutron data—almost unaffected by Hubbard U, and weakly affected by spin-orbit coupling. In La2Fe14B, Ce alloys for 0≤x≤1 and prefers smaller R(4f) sites, as observed, a trend we find unaffected by valence. Whereas, in Nd2Fe14B, Ce is predicted to have limited alloying (x≤0.3) with a preference for larger R(4g) sites, resulting in weak partial ordering and segregation. The Curie temperatures versus x for (Nd,Ce) were predicted for a typical sample processing and verified experimentally.

Mixed valency and site-preference chemistry for cerium and its compounds: A predictive density-functional theory study

“…As the most abundant RE, Ce is often considered as a RE replacement in permanent magnets [5]. The low-pressure γ phase of Ce exhibits a local magnetic moment, associated with a trivalent Ce 3+ configuration.…”

“…The mixed valency of Ce significantly affects its magnetic behavior. Due to the complex nature of a 2-14-1 structure [5], the Ce mixed valency is associated with Ce-site preferences. The 2-14-1 structure contains two inequivalent rare-earth (R) sites-R(4f ) and R(4g)-each with multiplicity 4 [5].…”

“…With different sized Ce 3+ and Ce 4+ ions, relaxations (ignored in previous studies) are crucial to predict reliable energetics and ground states. Localized Ce 3+ f electrons are addressed via a PBE+U approach [12] with a Hubbard U (set to 5 eV from previous work [5]) introduced in a screened Hartree-Fock manner. All of the other components (La, Ce 4+ , Fe, Co, and C) are addressed within PBE only, as the effect of Hubbard U on them is very small.…”

Effects of the Substitution of Mischmetal for PrNd on the Microstructures and Magnetic Properties of Rapidly Quenched (MM)PrNd-Fe-B Nanocrystalline Magnets

“…Developing the high abundant rare earth permanent magnets is requisite for both cost reduction and efficient utilization of rare earth resource. Thus, the research on substitution of PrNd by Ce or mischmetal alloy has been paid much attention [6][7][8][9][10][11][12][13][14][15][16][17][18][19]. Recently, Pathak et al [13] reported that the maximum energy product of the hot pressed and die upset (Nd 0.8 Ce 0.2 ) 2.4 Fe 12 Co 2 B alloy was 31 MGOe.…”

Intrinsic Magnetic Properties of Ce2Fe14B Modified by Al, Ni, or Si

“…The low cost and availability of Ce have driven interest in developing Ce-based permanent magnets to replace expensive Nd 2 Fe 14 B magnets in certain applications [7][8][9][10][11][12][13][14][15][16][17][18][19]. This has resulted in investigation of Ce 2 Fe 14 B and its alloys [20][21][22][23][24][25][26].…”