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 ferroelectric perovskite Na_0.5Bi_0.5TiO₃, NBT, can exhibit three types of electrical behaviour, i.e. oxide-ion conduction (type I), mixed ionic–electronic conduction (type II) and insulating/dielectric (type III) based on various defect mechanisms. Here we review how to tune the electrical properties of NBT via several mechanisms, including A-site Na or Bi non-stoichiometry, isovalent substitution, and acceptor- and donor-doping. The diversity of the electrical behaviour in the NBT lattice is attributed to the high level of oxide-ion conductivity originating from highly mobile oxygen ions which can be fine-tuned to optimise or suppress ionic conduction. High oxide-ion conductivity can be obtained by manipulating the starting Na/Bi ≥1 and by acceptor-doping to make NBT a potential electrolyte material for intermediate temperature solid oxide fuel cells (IT-SOFCs). In contrast, the oxide-ion conduction can be partially or fully suppressed by having a starting (nominal) composition with Na/Bi <1, donor-doping, or utilising the trapping effect between oxygen vacancies and some B-site acceptor dopants. This significantly reduces the dielectric loss and makes NBT-based materials excellent candidates as high-temperature dielectrics for capacitor applications.

Understanding the molecular mechanism of proton conduction is crucial for the design of new materials with improved conductivity. Quasi-elastic neutron scattering (QENS) has been used to probe the mechanism of proton diffusion within a new phosphonate-based metal–organic framework (MOF) material, MFM-500(Ni). QENS suggests that the proton conductivity (4.5 × 10^–4 S/cm at 98% relative humidity and 25 °C) of MFM-500(Ni) is mediated by intrinsic “free diffusion inside a sphere”, representing the first example of such a mechanism observed in MOFs.

${\text{CaCu}}_3{\text{Ti}}_4{\text{O}}_{12}$ ceramics with a range of resistivities have been prepared using both conventional sintering and spark plasma sintering. For all cases, the high effective permittivity is associated primarily with an internal barrier layer capacitor mechanism. Additional polarization associated with the electrode-sample interface may appear but its visibility depends on the grain boundary resistivity $\left({\text{R}}_{\text{gb}}\right)$ of the ceramic. If the ${\text{R}}_{\text{gb}}$ is large, the electrode polarization is obscured by sample-related effects; if the ${\text{R}}_{\text{gb}}$ is small, a separate impedance associated with the electrode polarization may be seen. Discrepancies in the literature regarding the magnitude and origin of the high effective permittivity are attributed to a combination of differences in processing conditions, electrode contact material and measuring frequency range.