Crystal structure and ferroelectric properties ofABi2Ta2O9(A=Ca,Sr, and Ba)

Crystal structures and ferroelectric properties of a series of Bi-layered compounds, ${\mathrm{CaBi}}_{2}{\mathrm{Ta}}_{2}{\mathrm{O}}_{9},$ ${\mathrm{SrBi}}_{2}{\mathrm{Ta}}_{2}{\mathrm{O}}_{9},$ and ${\mathrm{BaBi}}_{2}{\mathrm{Ta}}_{2}{\mathrm{O}}_{9},$ were investigated. The structures of ${\mathrm{CaBi}}_{2}{\mathrm{Ta}}_{2}{\mathrm{O}}_{9}$ and ${\mathrm{SrBi}}_{2}{\mathrm{Ta}}_{2}{\mathrm{O}}_{9}$ are orthorhombic, while that of ${\mathrm{BaBi}}_{2}{\mathrm{Ta}}_{2}{\mathrm{O}}_{9}$ is pseudotetragonal on the macroscopic scale but consists of microdomains with orthorhombic distortion. The ferroelectric Curie temperature of ${\mathrm{CaBi}}_{2}{\mathrm{Ta}}_{2}{\mathrm{O}}_{9}$ was over 600 \ifmmode^\circ\else\textdegree\fi{}C, and that of ${\mathrm{SrBi}}_{2}{\mathrm{Ta}}_{2}{\mathrm{O}}_{9}$ was over 300 \ifmmode^\circ\else\textdegree\fi{}C. ${\mathrm{BaBi}}_{2}{\mathrm{Ta}}_{2}{\mathrm{O}}_{9},$ in contrast, showed relaxor-type ferroelectric behavior; that is, in the plot of temperature dependence of dielectric constant, a broad peak appeared around 60 \ifmmode^\circ\else\textdegree\fi{}C. As the size of the A-site cation decreases from ${\mathrm{Ba}}^{2+}$ to ${\mathrm{Ca}}^{2+},$ the lattice mismatch between ${\mathrm{TaO}}_{2}$ and $A\mathrm{O}$ planes in the perovskite-type unit of $A{\mathrm{Ta}}_{2}{\mathrm{O}}_{7}$ increases and the structural distortion becomes more pronounced. This distortion leads to the higher Curie temperature and the larger spontaneous ferroelectric polarization.