Phase Transformations in Calcium‐Substituted Lanthanum Ferrite

Abstract

Phase equilibria in the La1−xCaxFeO3−δ (LCF) system were assessed at temperatures below 1350°C in both simulated air and argon atmospheres using a combination of differential scanning calorimetry, thermogravimetric analysis, scanning electron microscopy, and high‐temperature X‐ray diffraction. The solubility limit of Ca in the perovskite structure was determined to be 38% A‐site substitution. A high‐temperature orthorhombic to rhombohedral transition was identified and the dependence on oxygen partial pressure and effect on thermal expansion were characterized. A partial, pseudobinary LaFeO3–CaFeO2.5 phase diagram is presented that is based on these analyses combined with data available in the open literature.

Thermal Expansion of Alkaline‐Doped Lanthanum Ferrite Near the Néel Temperature

“…Hudspeth et al . deduced, from neutron diffraction data, that the direction of the AFM moment is indiscernible between the two nearly equivalent pseudocubic lattice parameters, as was the case in our neutron diffraction data . However, the correlation between the Néel temperature and the calculated Fe–O1–Fe bond angle suggests that the superexchange interaction occurs along the Pbnm c ‐axis ( Pnma b ‐axis).…”

“…Consequently, it is likely that partial charge compensation by vacancies is more prevalent in the LSF and LBF systems than in the LCF system. Further assessment on the charge compensation mechanism has been reported by Price et al …”

Microstructural features of the La1−xCaxFeO3−δ solid solutions prepared via Pechini route

“…Method of polymer-salt precursors (Pechini route) appeared to be suitable for producing La 1Àx Ca x FeO 3Àd homogeneous solid solutions at 0 x 0.5 [19], 0 x 0.7 [20], but homogeneity of the solid solutions is still under the question.…”

Microdomain Formation, Oxidation, and Cation Ordering in LaCa2Fe3O8+y

“…This is due, in part, to limitations in the ability to adequately characterize these materials using diffraction techniques. The structural similarity of the multiple phases present in the system result in the convolution of peaks in diffraction patterns and an order–disorder transformation (ODT) in Ca‐rich specimens results in a quenched microstructure, which deceptively gives a cubic diffraction pattern …”

“…When Ca 2+ is substituted for La 3+ on the A site, a charge imbalance is introduced, which can be compensated by the formation of disordered oxygen vacancies or the increase in valance of iron from Fe 3+ to Fe 4+ . Both charge compensating methods have been shown to exist, allowing for variable nonstoichiometry of oxygen content in the material . When the concentration of Ca exceeds the solubility limit in the perovskite structure, a second phase (Grenier) is formed as a result of the ordering of oxygen vacancies and formation of tetrahedral layers.…”