Phase transformations in processing of bismuth titanate
Doping bismuth titanate with sodium resulted in a reduction in the electrical conductivity. This is in apparent contradiction to the established conduction mechanism by holes. The lower conductivity enabled the application of a higher electric field across the sample while poling while keeping the leakage current low. This resulted in a higher piezoelectric constant d 33 and a higher relative permittivity. Sodium doping also results in a phase transformation from Bi4Ti3O12 to the Na0.5Bi4.5Ti4O15 phase. This transformation proceeds through the intermediate Na0.5Bi8.5 Ti7O27 phase which has a structure resembling half each of Bi4Ti3O12 and Na0.5Bi 4.5Ti4O15 unit cells.
High resolution TEM was done to study the structure of bismuth titanate. The mechanism of the transformation was studied by observing the transformation front and the crystallographic index of the transformation front was calculated from the Moiré fringes that were observed in the high-resolution TEM image. From this, a mechanism has been proposed for the phase transformation. A model has also been developed to relate the phase fractions with the conductivity of the multi-phase samples. A model has been developed to explain the reduction in the conductivity with sodium doping. It is also been shown that the interface between the phases can offer a significantly large resistance and possibly plays an important role in reducing the conductivity of the multiphase, sodium doped bismuth titanate.