Hydration thermodynamics of pyrochlore structured oxides from TG and first principles calculations

In this work we discuss trends in the defect chemistry and hydration thermodynamics of rare-earth pyrochlore structured oxides, RE2X2O7 (RE = La-Lu and X = Ce, Zr, Sn, Ti). Density Functional Theory (DFT) calculations have been performed to elucidate trends in the general defect chemistry and hydration enthalpy for the abovementioned series. Further, to justify the use of such theoretical methods, the hydration properties of selected compositions were studied by means of thermogravimetric measurements. Both from DFT calculations and TG measurements, the hydration enthalpy becomes less exothermic with decreasing radii of RE-ions within the RE2X2O7 series (X=Ti, Sn, Zr, Ce). However, changing the X-site ion has smaller effect on the hydration enthalpies. The observed hydration trends are discussed in connection with trends in the stability of both protons and oxygen vacancies and changes in the electronic density of states and bonding environment through the series. Finally, the findings are discussed with respect to existing correlations for other binary and ternary rare-earth oxides.