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

Cerium and its technologically relevant compounds are examples of anomalous mixed valency, originating from two competing oxidation states---itinerant Ce⁴⁺ and localized Ce³⁺. Under applied stress, anomalous transitions are observed but not well understood. Here we treat mixed valency as an ``alloy'' problem involving two valences with competing and numerous site-occupancy configurations. We use density-functional theory with Hubbard U (i.e., DFT+U) to evaluate the effective valence and predict properties, including controlling the valence by pseudoternary 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 Ce³⁺, 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.