Enhancing electronic conductivity in strontium titanates through correlated A and B-site doping

A doping strategy aimed at improving n-type conductivity in perovskite oxides by enhancing bulk oxide ion mobility is proposed and discussed. To justify and further assess this premise, the oxygen deficiency (δ) and conductivity (σ) of reduced La_0.4Sr_0.4Ga _xTi_1-xO_3-x/2-δ (0 ≥ x ≥ 0.15) samples were studied as a function of Ga doping (x). Both δ and σ were found to increase significantly with Ga doping. After a typical reduction process carried out at 1000 °C in 5%H₂/Ar the samples showed an increase in δ from 0.035 to 0.060 with doping. The corresponding conductivity also increased with doping showing a maximum of ∼50 S cm ^-1 (measured at 880 °C in 5%H₂/Ar on ∼62% dense pellet) for x = 0.05. Conductivity data suggests Ga doping promotes fast reduction and significantly improves the stability of the reduced phase in oxidizing conditions. Although the studied compositions preserved their cubic crystal structure throughout testing and showed no sign of degradation of the microstructure, some Ga loss was shown to take place at low pO₂. However, after a first redox cycle, no further change in Ga stoichiometry was observed. Two mechanisms that account for the manner in which Ga is lost from the perovskite lattice are proposed and discussed.