Significant ion conduction in Cu acceptor-substituted bismuth titanate polycrystalline ceramics

This paper reports that Cu acceptor substitution at the Ti site has significant effects on structure and electrical properties of Aurivillius-phase Bi₄Ti₃O₁₂ polycrystalline ceramics. Phase purity, microstructure and defect chemistry of Bi₄Ti_3−xCu_xO_12−x (BiT–100xCu, x = 0–0.12) are characterized by XRD, SEM, EDS and EPR measurements. Neutron diffraction refinement indicates that BiT–8Cu has an orthorhombic symmetry [space group: B2cb; lattice parameters: a = 5.4116(1) Å, b = 32.833(1) Å, c = 5.4479(1) Å and V = 967.98(5) Å³]. Impedance spectra of BiT–100xCu were measured under variable oxygen partial pressure (pO₂). Bi₄Ti₃O₁₂ shows mixed electronic (hole) and ionic conduction owing to the existence of oxygen vacancies (VO··) arising from the loss of Bi₂O₃ during sintering. Interestingly, bulk conductivity in Cu-substituted Bi₄Ti₃O₁₂ ceramics is predominately ionic due to the fact that there are considerably additional oxygen vacancies introduced into the perovskite lattices. The optimum composition of BiT–8Cu shows high ionic conductivity in the bulk with a value of ~ 0.007 S/cm at 650 °C.