Protonic and electronic defects in the 12R-type hexagonal perovskite Sr3LaNb3O12

The structural properties of the 12R-type hexagonal perovskite Sr3LaNb3O12 (space group R3¯) have been examined by neutron diffraction and 1H magic angle spinning-nuclear magnetic resonance (1H MAS–NMR). The latter technique supports the presence of protonic species in Sr3LaNb3O12 with bands at 5, 3.4 and 1.3ppm. The electrical-transport properties of the nominally stoichiometric and acceptor-doped phases Sr3LaNb3−xMxO12−δ (M=Ti, x=0.1; M=Zr, x=0.03) have been analysed in the temperature range 550–900°C by impedance spectroscopy in a selection of wet- and dry-gas atmospheres. Protons contribute increasingly to transport with decreasing temperature in wet air, as confirmed by an H+/D+ isotope effect, representing the first observation of proton conductivity in this structure type. The magnitude of the protonic transport increases with increasing acceptor-dopant concentration (x). A total conductivity value of 1.2×10−6Scm−1 is reached at 650°C for the Ti-doped phase in humidified air (pH2O≈0.03atm). The activation energy in wet air at 550–700°C is slightly lower for the doped compositions (0.93eV for the Zr-doped phase) in comparison to the nominally undoped material (1.13eV). The origin of proton transport in the latter may originate from “self doping” of a small amount of Sr on the La sites. Conductivity is dominated by n-type charge carriers in reducing conditions, as confirmed by the observance of a −1/4 power-law dependency for pO2≤~10−8atm. The activation energy of the title phase in the n-type region measured in 10% H2:90% N2 is 0.96eV. In dry oxidising atmospheres, p-type charge carriers predominate (Ea=1.42eV) with the p–n transition occurring at pO2~10−4atm. •Proton transport is observed in Sr3LaNb3O12 in wet oxidising conditions.•1H MAS-NMR and an H+/D+ conductive isotope effect support the presence of protons.•Proton transport increases with increasing acceptor-dopant concentration.•Electrical conductivity is n-type in reducing conditions.