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

Abstract

Cerium and its technologically relevant compounds are examples of anomalous mixed valency, originating from two competing oxidation states -itinerant Ce 3+ and localized Ce 4+ . Under applied stress, anomalous transitions are observed but not well understood. Here we treat mixed valency as an "alloy" involving two valences with competing and numerous site-occupancy configurations, and we use density functional theory with Hubbard U (i.e., DFT+U) to evaluate the effective valence and predict properties, including controlling valence by pseudo-ternary alloying. For Ce and its compounds, such as (Ce-La)2(Fe-Co)14B permanent magnets, we find a stable mixed-valent α-state near the spectroscopic value of νs = 3.53. Ce valency in compounds depends on its steric volume and local chemistry; for La doping, Ce-valency shifts towards γ-like Ce 3+ , as expected from steric volume; for Co doping, valency depends on local Ce-site chemistry and steric volume. Our approach captures the key origins of anomalous valency and site-preference chemistry in complex compounds.

Manipulating Ce Valence in RE2Fe14B Tetragonal Compounds by La-Ce Co-doping: Resultant Crystallographic and Magnetic Anomaly

“…For instance, Ce 2 Fe 14 B exhibits lattice expansion after hydriding, meanwhile the Ce valence shifts towards the moment-carrying 4f1 (+3) state compared to the unhydrided parent2425. Calculations based on the density-functional theory also predict that La can function like interstitial hydrogen in the (La, Ce) 2 Fe 14 B compound to induce a favorable Ce 3+ state15. Thus, if La, with largest atomic radius among all REs, co-dope with Ce into the (Nd, Pr) 2 Fe 14 B lattice, may be able to produce a similar crystal lattice expansion and induce a preferable Ce 3+ configuration accordingly.…”

“…Since the La 2 Fe 14 B tetragonal structure is unstable121314, the most abundant and cheapest Ce has attracted considerable interest for decades151617181920212223. Different from Pr 3+ , Nd 3+ and La 3+ with stable valence, Ce generally exhibits a mixed valence of 3.44 due to the coexistence of trivalent 4f1 and tetravalent 4f 0 electronic states in Ce 2 Fe 14 B8.…”

Chemically Inhomogeneous RE-Fe-B Permanent Magnets with High Figure of Merit: Solution to Global Rare Earth Criticality

“…However, continually growing demand for Nd-Fe-B magnets and insecure supply for those closely-relied Nd/Pr/Dy/Tb due to geologic scarcity, extraction difficulties and political volatility has caused severe concerns about REs availability, which may exert spill-over effects on global economy, and security in the case of defense or military sectors26. Meanwhile, the large backlog of highly-abundant and inexpensive La/Ce (by-products of Nd/Pr/Dy/Tb during mineral extraction) due to the inferior magnetic properties of (La/Ce) 2 Fe 14 B to Nd 2 Fe 14 B111213141516171819202122 is a longstanding bottleneck for balanced utilization of RE resources41323.…”

Structure and properties of sintered MM–Fe–B magnets

“…Specifically, a part of positions of Nd 3+ were occupied by both Ce 3+ and Ce 4+ ions, and the radius of Ce 4+ ion is small compared with Nd 3+ , which led to the decrease of lattice constants. 11 Meanwhile, the theoretical density of MM 2 Fe 14 B magnet (ρ = 7.589 g/cm 3 ) was also obtained by XRD refinement. Fig.…”