Formation kinetics and thermodynamic stability of the Aurivillius compounds in Bi₄Ti₃O₁₂–BiFeO₃ system

The Aurivillius compounds in the Bi₂O₃–Fe₂O₃–TiO₂ system, combining ferroelectric, semiconducting, and ferromagnetic properties, have attracted particular interest. Formation kinetics and thermodynamic stability are the fundamental knowledge needed for modeling and predicting the temporal microstructure and property evolution during materials processing but have not yet been addressed by quantitative experimental measurement. This article focuses on the Bi_n+1Fe_n–3Ti₃O_3n+3 Aurivillius compounds on the Bi₄Ti₃O₁₂–BiFeO₃ tie-line to elucidate the mechanisms and thermodynamic controls responsible for phase formation of compounds with various perovskite-like layers. Five high-purity Aurivillius compounds Bi₄Ti₃O₁₂, Bi₅FeTi₃O₁₅, Bi₆Fe₂Ti₃O₁₈, Bi₇Fe₃Ti₃O₂₁, and Bi₈Fe₄Ti₃O₂₄ with integer n = 3–7 values were synthesized and their phase transformation properties and enthalpies of formation were studied by X-ray diffraction in situ, high temperature differential scanning calorimetry, and high temperature oxide melt solution calorimetry. Thermodynamic stability of the compounds decreases with increasing n, and formation kinetics gradually slow down, demonstrating the inherent difficulty to synthesize pure Aurivillius compounds with n larger than 8. This difficulty was confirmed by an impurity phase coexisting with Bi₉Fe₅Ti₃O₂₇.