Hydration Entropy and Enthalpy of a Perovskite Oxide from Oxygen Tracer Diffusion Experiments

Water incorporation into perovskite oxides generates protonic defects in the form of hydroxide ions. In this study, an indirect method to probe the thermodynamics of water incorporation is demonstrated. Acceptor-doped single-crystal samples of SrTiO₃ were subjected to H₂¹⁸O/H₂¹⁶O exchange annealing at temperatures of 723 < T/K < 1023 at a water partial pressure of pH₂O = 0.1 bar; from ¹⁸O diffusion profiles, measured by secondary ion mass spectrometry, oxygen tracer diffusion coefficients DO* were obtained. The decreased values of DO* for wet (relative to dry) conditions yielded Δ_hydH = -(73 ± 15) kJ mol^-1 and Δ_hydS = -(148 ± 18) J mol^-1 K^-1 as the hydration enthalpy and entropy of SrTiO₃. For T < 1023 K and this pH₂O, the experiments also indicate that oxygen exchange from H₂O(g) is faster than that from O₂(g) (with a lower activation enthalpy) and that the surface space-charge potential is decreased under wet conditions.