We have measured water uptake and hydration enthalpy in 50% La and Nd doped CeO2, also to be taken as compositions in the series La2−xNdxCe2O7 (x = 0.0, 0.5, 1.0 and 2.0) using combined thermogravimetry (TG) and differential scanning calorimetry (DSC), TG-DSC. The TG-DSC data unambiguously yield standard molar hydration enthalpies of ~−74 kJ/mol independent of water uptake. The interpretation of the TG results, however, does not fit a classical model of hydration of all oxygen vacancies. Instead, the hydration appears to be limited to a small fraction of the free vacancies. Hydration further decreases as the Nd content (x) and long-range order increases and regions of disorder decrease. We propose a new model explaining why hydration occurs only in a small fraction of the nominally free vacancies: The higher basicity of La/Nd compared to Ce promotes protonation at oxide ion sites with high coordination to La/Nd, and the observed water uptake and modelling suggests that mainly oxide ions fully coordinated to 4 La/Nd neighbours become protonated. The statistical variation of coordination around oxygen sites in a disordered fluorite oxide creates a limited number of such oxide ions sites which results in limited hydration. The model matches well the experimental results and DFT calculations of proton trapping at the fully La-coordinated sites for 50% La-doped CeO2, and also rationalizes conductivity data.
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