New insights into the 6H-type hexagonal perovskite solid solution BaTiO3 − δ: Influence of acceptor and donor doping on crystal structure and electrical properties

Polycrystalline 6H-type hexagonal Barium Titanate BaTi0.5(Fe0.33M0.17)O3 − δ, where M = Mo or W was prepared by the solid state reaction method. The effects of the cationic substitution of Fe3 +, Mo6 + and W6 + for titanium in the B site for BaTiO3 perovskite lattice on the symmetry and the electrical properties were investigated. It was found that the substitution of Ti4 + by (Fe3 + Mo6 +) gives a hexagonal perovskite structure, whereas the substitution by (Fe3 + W6 +) cations results in a mixture of double cubic and hexagonal pervoskite phases. The difference in the average radius of cations in the B site (rB = 0.50 × rTi + 0.33 × rFe + 0.17 × rM, (M = Mo or W)) created the difference in cell parameters. The conductivity obtained for BaTi0.5(Fe0.33Mo0.17)O3-δ sample at a temperature of 200 °C was 2.3 × 10− 3 S cm− 1, which is much greater than that established by BaTi0.5(Fe0.33W0.17)O3-δ sample at a temperature of 328 °C (0.26 × 10− 3 S cm− 1) This means that a mixed ionic-electronic conductor (MIEC) character may be present in the first compound. Impedance Spectroscopy data collected for our samples over a range of temperatures and frequencies showed the appearance of a phenomenon of blocking at the grain boundary level, which means that the compounds are highly influenced by the oxygen stoichiometry. The correlation between the electrical properties of grain boundaries and their chemical composition is consistent with the interpretation in terms of the space charge model with a positive excess charge in the grain boundary core.