Thermodynamics of compositionally complex (“high-entropy”) oxides – A-site mixed (5R0.2)CoO3-δ (R = rare-earth element) perovskite-type cobaltites

Abstract

Due to their unique magnetic and electronic properties, materials based on rare-earth perovskite-type cobaltites are promising for electronic and energy conversion applications. In this study, several single-phase cobaltites (5R_0.2)CoO_3-δ with five different rare-earth elements R in the A-sublattice were synthesized via the glycerol-nitrate technique. X-ray diffraction (XRD) confirmed the orthorhombic (Pnma) perovskite structure of (5R_0.2)CoO_3-δ, and energy-dispersive microanalysis showed a uniform cation distribution in these oxides. The heat capacity of (5R_0.2)CoO_3-δ was investigated using differential scanning calorimetry, and the enthalpy of formation was determined via reduction reaction calorimetry and drop solution calorimetry. The heat capacity curves, dominated above room temperature by the broad peak associated with the cobalt spin state transition, closely resemble those of the “simple” rare-earth cobaltites RCoO_3-δ. Similarly, trends in the enthalpies of reduction in hydrogen and the enthalpies of formation from binary oxides (R₂O₃ and CoO) are identical for (5R_0.2)CoO_3-δ and RCoO_3-δ. This indicates enhanced stability for the oxides with larger average radius of the rare-earth ions (i.e., the Goldschmidt tolerance factor closer to 1). The calculated enthalpies of mixing in the A-sublattice of (5R_0.2)CoO_3-δ are close to zero, suggesting that these solid solutions can be regarded as near-ideal ones. The absence of significant enthalpic destabilization in the (5R_0.2)CoO_3-δ oxide systems indicates the feasibility of synthesizing multiple perovskite cobaltites that are thermodynamically stable in wide temperature ranges, down to room temperature. This offers the potential to finely tune A-site ion sizes and dependent properties by varying the complex composition.