The strange case of enhancing the resistivity by extended sintering time in (Bi_0.5Na_0.5)TiO₃-BaTiO₃ solid solution

With the presence of volatile elements in the composition of a ceramic material, longer sintering time pre-sumably creates more vacancies. In the case of (Bi0.5Na0.5)TiO3-BaTiO3 lead-free piezoelectric ceramics and when batched and calcined with a Bi2O3 deficient stoichiometry, there can be a surprising increase in resistivity with increasing sintering time. It is deduced that the extended sintering time reduces the concentration of bis-muth vacancies (and the ionically compensating oxygen vacancies). This is also associated with a decrease of depolarization temperature, Td, and possibly a change in dielectric polarization phenomena from normal to relaxor ferroelectric. These observations do not occur in pure (Bi0.5Na0.5)TiO3, suggesting that they are actually facilitated by the Ba2+ content in the solid solution BNT-BT. Microstructural observations indicate the formation of BaTi2O5 secondary phase which considerably grows as sintering goes on. With more Ti4+ being incorporated to the secondary phase, the stoichiometry of the solid solution can be restored, so is the resistivity and the observed trends in decreased Td.