The structure of molten calcium ferrite under various redox conditions

Laser-heated melts based on the 43CaO-57Fe₂O_3- eutectic, close to the calcium ferrite (CF) composition, were measured with high-energy X-ray diffraction using aerodynamic levitation over a range of redox states controlled by CO/CO₂ gas atmospheres. The iron-oxygen coordination number was found to rise from 4.4 ± 0.3 at 15% Fe³⁺ to 5.3 ± 0.3 at 87% Fe³⁺. Empirical potential structure refinement modelling was used to obtain the ferric and ferrous partial pair distribution functions. It was found that the Fe²⁺ iron-oxygen coordination number is consistently approximately 10% higher in CF than in pure iron oxide, while Fe³⁺ is essentially identical in all but the most oxygen-rich environments (where it is higher in CF compared with FeO). The model also shows calcium octahedra to be the dominant species across all redox environments, although the population of CaO₇ increases with the availability of oxygen at the expense of CaO₄ and CaO₅. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 1)'.