Crystal structures and ferroelectric properties of ${\mathrm{SrBi}}_{\mathrm{2}}{\mathrm{Ta}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{9}}$ and ${\mathrm{Sr}}_{\mathrm{0.8}}{\mathrm{Bi}}_{\mathrm{2.2}}{\mathrm{Ta}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{9}}$

Crystal structures of ferroelectric materials of stoichiometric ${\mathrm{SrBi}}_{\mathrm{2}}{\mathrm{Ta}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{9}}{\mathrm{ (T}}_{\mathrm{C}}\text{=300\hspace{0.05em}°}\mathrm{C})$ and Sr-deficient-and-Bi-excess ${\mathrm{Sr}}_{\mathrm{0.8}}{\mathrm{Bi}}_{\mathrm{2.2}}{\mathrm{Ta}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{9}}{\mathrm{ (T}}_{\mathrm{C}}\text{=400\hspace{0.05em}°}\mathrm{C})$ were refined by neutron powder diffraction. ${\mathrm{Bi}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{2}}$ layer and ${\mathrm{TaO}}_{\mathrm{6}}$ octahedra are considerably distorted and atomic displacements along the a axis cause ferroelectric spontaneous polarization. In ${\mathrm{Sr}}_{\mathrm{0.8}}{\mathrm{Bi}}_{\mathrm{2.2}}{\mathrm{Ta}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{9}},$ both Bi substitution and cation vacancies at the Sr site were revealed and a chemical composition of ${\mathrm{(Sr}}_{\mathrm{0.82}}{\mathrm{Bi}}_{\mathrm{0.12}}{\mathrm{)Bi}}_{\mathrm{2}}{\mathrm{Ta}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{9.0}}$ was obtained. The calculated polarization of ${\mathrm{Sr}}_{\mathrm{0.8}}{\mathrm{Bi}}_{\mathrm{2.2}}{\mathrm{Ta}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{9}}$ is larger than that of the stoichiometric sample, which is consistent with observations of remanent polarization in thin-film capacitors. The Bi substitution and the cation vacancies at the Sr site enhance structural distortion in the ${\mathrm{TaO}}_{\mathrm{6}}$ octahedra and lead to the larger ferroelectric spontaneous polarization and the higher Curie temperature.