Oxide ion conductors find important technical applications in electrochemical devices such as solid-oxide fuel cells (SOFCs), oxygen separation membranes and sensors. Na0.5Bi0.5TiO3 (NBT) is a well-known lead-free piezoelectric material; however, it is often reported to possess high leakage conductivity that is problematic for its piezo- and ferroelectric applications. Here we report this high leakage to be oxide ion conduction due to Bi-deficiency and oxygen vacancies induced during materials processing. Mg-doping on the Ti-site increases the ionic conductivity to ~0.01 S cm(-1) at 600 °C, improves the electrolyte stability in reducing atmospheres and lowers the sintering temperature. This study not only demonstrates how to adjust the nominal NBT composition for dielectric-based applications, but also, more importantly, gives NBT-based materials an unexpected role as a completely new family of oxide ion conductors with potential applications in intermediate-temperature SOFCs and opens up a new direction to design oxide ion conductors in perovskite oxides.

The thermodynamics of space-charge formation at grain boundaries in acceptor-doped SrTiO₃ is examined. Thermodynamic models of varying complexity are developed, which predict the space-charge potential as a function of thermodynamic variables, such as dopant concentration, temperature and oxygen partial pressure. Based on the results, limits to the space-charge potential that can arise at a grain boundary and strategies for tuning the space-charge potential are discussed. With literature equations linking the space-charge potential to electrical properties, one specific thermodynamic model is subsequently applied to electrical impedance data reported in the literature for tilt bicrystal samples of Fe-doped SrTiO₃. The thermodynamic driving energies for space-charge formation obtained from the analysis are examined as a function of tilt misorientation angle, in order to explore the relationship between driving energy and interface atomistic structure. In addition, the capabilities and deficiencies of the entire approach (from driving energies via space-charge potentials to electrical properties), with regard to predicting experimental behaviour, are demonstrated.

18O/16O exchange and subsequent time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis was employed to investigate the transport of oxygen, and thus the behavior of oxygen vacancies, in [nominally undoped, (100) oriented] single-crystal SrTiO3 substrates. Isotope exchange anneals were performed as a function of temperature, 948 < T/K < 1123, at an oxygen activity aO2 = 0.50 and as a function of oxygen activity, 0.01 < aO2 < 0.70, at T = 1073 K. All isotope profiles show the same characteristic form: an initial drop over tens of nanometers close to the surface, which is attributed to an equilibrium space-charge layer depleted of oxygen vacancies, followed by a profile extending several microns into the solid, which is attributed to diffusion in a homogeneous bulk phase. The entire isotope profile can be described quantitatively by a numerical solution to the diffusion equation with a position-dependent diffusion coefficient; the description yields the tracer diffusion coefficient in the bulk D*($\infty${}), the surface exchange coefficient ks*, and the space-charge potential $\Phi${}0. All D*($\infty${}) data are consistent with nominally undoped SrTiO3 substrates being weakly acceptor doped; the activation enthalpy for the migration of oxygen vacancies in bulk SrTiO3 is found to be $\Delta${}Hmig,V $\approx${} 0.6 eV. The surface termination of the SrTiO3 substrates was seen to affect significantly the surface exchange coefficient ks*. Values of $\Phi${}0 obtained as a function of T and aO2 are approximately 0.5 V, indicating strong depletion of oxygen vacancies within the equilibrium surface space-charge layers. Thermodynamic modeling indicates that space-charge formation at the TiO2-terminated (100) surface is driven by the Gibbs formation energy of oxygen vacancies at the interface being lower than in the bulk.