Correlations aimed at linking the hydration thermodynamics to materials parameters can be of vital importance for further development of proton conducting oxides. However, the currently proposed correlations are to a large degree troubled by scattering limiting their predictive power. As such, the present contribution has investigated Sc-doped CaSnO3 and CaZrO3 in an attempt to further elucidate the thermodynamic trends of hydration in perovskites.
The defect structure and conductivity of polycrystalline samples of CaSn0.95Sc0.05O3-δ and CaSn0.9Sc0.1O3-δ has been studied by AC impedance measurements in the temperature range 150-1100 °C. The majority defects were characterised by measuring the conductivity as a function of p(H2O) and p(O2). These indicated that both samples are predominantly oxygen ion conducting with a protonic contribution under wet conditions at 300-400 °C for CaSn0.9Sc0.1O3-δ. Under oxidising conditions at elevated temperatures of 1000 °C and above, the conductivity is dominated by electron holes. The p(H2O) dependencies at temperatures of 300-500 °C for CaSn0.9Sc0.1O3-δ were modelled based on a simplified defect structure resulting in an extracted hydration enthalpy and entropy of -41 ± 5 kJ mol-1 and -107 ± 7 J K-1 mol-1, respectively.
Impedance spectroscopy was conducted in the temperature range 150-700 °C in order to study the hydration processes in the grain boundaries of Sc-doped CaSnO3. It was demonstrated that the grain boundary conductivity exhibited a larger contribution from protons than the bulk at temperatures below 400 °C. These differences were suggested to be due to the presence of inherent space charge layers depleting the oxygen vacancies. The depletion of oxygen vacancies was more severe for CaSn0.95Sc0.05O3-δ than CaSn0.9Sc0.1O3-δ, which is consistent with a smaller segregation of acceptors compensating the positively charged grain boundary core.
Simultaneous thermogravimetry (TG) and differential scanning calorimetry (DSC), TG-DSC, was applied to measure the standard molar hydration enthalpies and entropies of CaSn1-xScxO3-δ and CaZr1-xScxO3-δ as a function of the concentration of Sc. It was demonstrated that both the hydration enthalpy and entropy become increasingly negative with increasing Sc substitution. The thermodynamic parameters were in agreement with the values from literature and the results from conductivity measurements and TG. The values were also demonstrated to be consistent with respect to temperature (500-900 °C) and p(H2O) (0.1-1 atm). This suggests that TG-DSC can be used for the evaluation of hydration thermodynamics in oxides.
The obtained thermodynamic parameters of hydration from TG-DSC were compared with available literature values of other perovskites in an attempt to correlate the values to materials properties. While the hydration enthalpy was found to be in good agreement with the basicity of the oxide, the hydration entropy exhibited a dependence to the vibrational wavenumber of the OH stretching mode (νOH). As the two are interrelated properties, it suggests a correlation between the thermodynamic parameters of hydration, as was demonstrated in both materials studied.