Direct evidence of A-site-deficient strontium bismuth tantalate and its enhanced ferroelectric properties
The Rietveld refinement of synchrotron-radiation diffraction has been performed to investigate the displacive-type ferroelectricity and crystal chemistry for Sr-deficient and Bi-excess strontium bismuth tantalate. The analysis of Sr0.73Bi2.18Ta2O9 demonstrates that excess Bi (Bi0.18) is substituted at the perovskite A site (Sr site) as trivalent ions and that the charge neutrality in the crystal is satisfied through the introduction of cation vacancies at the A site. The substitution of Bi with the A-site vacancies led to a marked increase in Curie temperature (TC), and resulted in a twofold remanent polarization without sacrificing the polarization switching properties. The structure refinement revealed that the rotation of TaO6 octahedra in the a-b plane accompanied with the whole shift of the octahedra along the a axis is largely enhanced by the Bi substitution with A-site vacancies, which is responsible for the higher TC and giant polarization.