Crystal structure and ferroelectric properties of $A{\mathrm{Bi}}_2{\mathrm{Ta}}_2{\mathrm{O}}_9(A=\mathrm{Ca},$ 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 °C, and that of ${\mathrm{SrBi}}_2{\mathrm{Ta}}_2{\mathrm{O}}_9$ was over 300 °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 °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.