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 SrTiO 3 were subjected to H 2 18 O/H 2 16 O exchange annealing at temperatures of 723 < T /K < 1023 at a water partial pressure of p H 2 O = 0.1 bar; from 18 O diffusion profiles, measured by secondary ion mass spectrometry, oxygen tracer diffusion coefficients D O * were obtained. The decreased values of D O * for wet (relative to dry) conditions yielded Δ hyd H = -(73 ± 15) kJ mol -1 and Δ hyd S = -(148 ± 18) J mol -1 K -1 as the hydration enthalpy and entropy of SrTiO 3 . For T < 1023 K and this p H 2 O, the experiments also indicate that oxygen exchange from H 2 O(g) is faster than that from O 2 (g) (with a lower activation enthalpy) and that the surface space-charge potential is decreased under wet conditions.