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

Several single-phase cobaltites (5R0.2)CoO3-δ with five different rare-earth elements R in the A-sublattice were synthesized via the glycerol-nitrate technique. The X-ray diffraction (XRD) results confirmed the orthorhombic (Pnma) perovskite-like structure of (5R0.2)CoO3-δ, and the energy-dispersive microanalysis (EDS) showed the uniform cation distribution in these oxides. The heat capacity of (5R0.2)CoO3-δ was studied by means of differential scanning calorimetry, and the enthalpy of formation – by reduction reaction calorimetry and drop solution calorimetry. The shapes of the heat capacity, dominated above room temperature by the broad peak from the cobalt spin state transition, are very similar to those of the “simple” rare-earth cobaltites RCoO3-δ. Likewise, the trends in the enthalpies of reduction in hydrogen and the enthalpies of formation from binary oxides (R2O3 and CoO) are basically the same for (5R0.2)CoO3-δ and RCoO3-δ, indicating increased stability for the oxides with larger average radius of the rare-earth ions (or, in other words, the Goldschmidt tolerance factor closer to 1). The calculated enthalpies of mixing in the A-sublattice of (5R0.2)CoO3-δ are close to zero, so these solid solutions can be regarded as close to the ideal ones. This also means that there is no significant enthalpic destabilization in the (5R0.2)CoO3-δ oxide systems, unveiling the possibility of obtaining multiple thermodynamically stable—in wide temperature ranges, down to room temperature—perovskite cobaltites with finely variable, by changing the complex composition, A-site ion sizes and dependent properties.