Direct evidence of<i>A</i>-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 ${\mathrm{Sr}}_{0.73}{\mathrm{Bi}}_{2.18}{\mathrm{Ta}}_{2}{\mathrm{O}}_{9}$ demonstrates that excess Bi $({\mathrm{Bi}}_{0.18})$ 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 ${(T}_{C}),$ and resulted in a twofold remanent polarization without sacrificing the polarization switching properties. The structure refinement revealed that the rotation of ${\mathrm{TaO}}_{6}$ octahedra in the $a\ensuremath{-}b$ 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 ${T}_{C}$ and giant polarization.