Electrical conductivity study of B-site Ga doped non-stoichiometric sodium bismuth titanate ceramics

The present investigation looks at the effect of Ga3+ doping for B-site Ti4+ on the phase stability and oxygen-ion conductivity of Na0.54Bi0.46TiO3-δ for the possible electrolyte application in low-temperature solid oxide fuel cells. Dense bulk samples were prepared using the conventional solid-state route. Although XRD revealed the presence of a single rhombohedral perovskite phase, SEM study indicated the existence of impurity phase in the form of fibers in the as-sintered samples. The local chemical compositional analysis showed Na/Bi molar ratio to be > 1 in the perovskite phase of all the tested compositions and the existence of Na2Ti6O13 compound in the impurity phase. Impedance spectroscopy studies suggested a monotonous decrease in bulk conductivity on Ga3+ doping on the B-site of Na0.54Bi0.46TiO3-δ. The bulk conductivity of 7.8 mS/cm measured in Na0.54Bi0.46TiO3-δ at 600 °C declined to 5.8 mS/cm on 2 at.% Ga3+ doping. The conductivity reduction was attributed to the formation of local defect clusters which act as sinks, and thus leads to decrease in the concentration of free oxygen vacancy. Although not much change in initial bulk conductivity of Na0.54Bi0.46Ti0.99Ga0.01O3-δ was found under various atmospheres from 500 °C to 600 °C, the annealing study carried out at 600 °C for 12 h under 5% H2-95% N2 condition showed the decomposition of perovskite phase to form metallic Bi. Both pure and Ga-doped samples showed a considerable conductivity degradation of 12% after 100 h ageing in air at 600 °C. The phenomenon related to the formation of local defect clusters over time was proposed to be a possible reason for the observed conductivity decay.