Defect chemistry and high-temperature thermodynamics of PrBaCo2O6-δ

Highlights: • The model function log(pO₂) = f(δ, T) can be extrapolated to lower temperatures. • The value of $\Delta {\stackrel{-}{h}}_O$ was determined by direct oxidation of a sample in the calorimeter. • Both calculated and independently determined values of $\Delta {\stackrel{-}{h}}_O$ coincide with each other. • Oxygen exchange impact on drop calorimetry measurements was assessed. • The values of $C_p$(T),${\Delta }_{298.15}^T{H}^o$ and ${\Delta }_{298.15}^T{S}^o$ for PrBaCo₂O_5.77 were calculated. Using the drop calorimetry and differential scanning calorimetry (DSC), the enthalpy increments of pure PrBaCo₂O_6-δ double perovskite in air in the temperature range from 365.3 K to 1272.8 K were obtained. As-measured enthalpy increments corresponded to the constant pO₂, but variable oxygen content in PrBaCo₂O_6-δ (isodynamical conditions). The defect structure analysis yielded the analytical expressions for the equilibrium oxygen partial pressure, log(pO₂) = f(δ, T), and the partial molar enthalpy of oxygen, $\Delta {\overline{h}}_O$ = f(T, δ) in PrBaCo₂O_6-δ, both functions being valid in relatively wide temperature range. The value of $\Delta {\overline{h}}_O$ at 1273 K was assessed independently by means of direct oxidation of PrBaCo₂O_6-δ in the DSC setup, and was found to agree well with $\Delta {\overline{h}}_O$ calculated from the defect structure model and pO₂ – T – δ diagram. The knowledge of $\Delta {\overline{h}}_O$ = f(T, δ) allowed recalculating the enthalpy increments for the sample with constant oxygen content, ${\Delta }_{298.15}^T{H}^{\circ }$(PrBaCo₂O_5.77), which were fitted with a modified Einstein function. The smoothed values of ${\Delta }_{298.15}^T{H}^{\circ }$, heat capacity C_p(T), and entropy increments ${\Delta }_{298.15}^T{S}^{\circ }$ were evaluated and tabulated for PrBaCo₂O_5.77 in the temperature range 350 ≤ T/K ≤ 1300. The results obtained can be used for calculating ${\Delta }_{298.15}^T{H}^{\circ }$, C_p(T), and ${\Delta }_{298.15}^T{S}^{\circ }$ for PrBaCo₂O_6-δ with an arbitrary constant oxygen content.