CALPHAD-guided phase stability and defect chemistry of pseudobrookite synthesized from natural ilmenite

Pseudobrookite is an iron–titanium oxide whose phase formation and defect chemistry are governed by composition, oxygen activity, and impurity content. In this work, pseudobrookite is synthesized directly from natural ilmenite concentrate using a combined CALPHAD-based thermodynamic and experimental approach. Unlike previous ilmenite-based approaches that rely on hydrometallurgical or chemically modified routes, the present study employs a single-step oxidative calcination under ambient atmosphere, enabling direct conversion without acid leaching, solvent-based processing, or precursor purification. Particular emphasis is placed on the role of mineral-derived impurities as intrinsic variables influencing phase equilibria and defect-related behavior. CALPHAD-based thermodynamic analysis is used to elucidate phase stability, impurity solubility, and nonstoichiometry under oxidizing conditions. These findings are supported experimentally, with X-ray diffraction confirming pseudobrookite formation after calcination at 1200 °C and electron microscopy revealing a mesoporous microstructure. X-ray photoelectron spectroscopy indicates mixed Fe²⁺/Fe³⁺ states and defect-related oxygen species, while UV–Vis diffuse reflectance spectroscopy shows strong visible-light absorption with direct band gaps of 2.1–2.2 eV. As functional validation, the material exhibits visible-light-driven methylene blue degradation, achieving ∼39% removal after 240 min.