Empirical Analysis of Stability of An+1BnO3n+1 Ruddlesden–Popper Phases Using Reciprocal n-Values

Layered An+1BnO3n+1 (n = 1…∞) Ruddlesden–Popper (RP) phases are a promising system for a variety of applications. Within the RP family, the thermodynamic properties of the phases are essentially additive with variation in the n value, but at present, there are no general approaches that would allow one to evaluate the individual stability of the RP phases and the possibility of their interconversion. The aim of this paper is to present a novel concept for performing a thermodynamic analysis of RP phases using the reciprocal values of the index n. We present an empirical equation ΔG1/n = ΔGP + B1/n + B2/n2, where ΔG1/n and ΔGP are the molar Gibbs energies of formation of the Ruddlesden–Popper (RP) phase (AO)1/nABO3 and the parent ABO3 perovskite, respectively, and n is a stoichiometry index of An+1BnO3n+1 RP phase. The correlation was validated using available thermodynamic data for the systems Sr-Ti-O, Ca-Ti-O, Sr-Zr-O, La-Ni-O, and La-Co-O. For all A-B combinations, the equation quantitatively describes the Gibbs energy of RP phase formation. Predicted values for the non-linear approximation lie within the experimental uncertainty in determining ΔG1/n. The proposed correlation was used to analyze the relative stability of the RP phases and to determine the feasibility of synthesizing new compounds.