High oxide-ion conductivity in acceptor-doped Bi-based perovskites at modest doping levels.

A combination of impedance spectroscopy, time-of-flight secondary ion mass spectrometry and literature data are used to show that, (i) the bulk oxide ion conductivity of A-site, alkaline earth-doped BiFeO3 (BF) is independent of the ionic radius of the alkaline earth ion (Ca, Sr, Ba) and, (ii) despite very different A-site environments in (Na1/2Bi1/2)TiO3 and BF, similar high levels and optimisation of bulk oxide ion conductivity in these Bi-based tilted perovskites is achieved at modest acceptor doping levels of ∼1-10%. These results clearly demonstrate that optimisation of oxide ion conductivity in these materials requires concepts beyond a simple crystallochemical approach based on matching the ionic radii of acceptor dopant and host lattice ions.

The usefulness of molecular-dynamics simulations in clarifying the activation enthalpy of oxygen-vacancy migration in the perovskite oxide BaTiO3.

We employed molecular-dynamics simulations with interatomic pair-potentials to examine oxygen-vacancy diffusion in the cubic phase of perovskite BaTiO3 as a function of temperature. By comparing the absolute rate of vacancy diffusion as well as its temperature dependence with experimental data, we are able to narrow down the activation enthalpy of migration to 0.70-0.76 eV.

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 ΔhydH = -(73 ± 15) kJ mol-1 and ΔhydS = -(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.