Crystal structure and ferroelectric properties of ABi(2)Ta(2)O(9) (A = Ca, Sr, and Ba)

Crystal structures and ferroelectric properties of a series of Bi-layered compounds, CaBi2Ta2O9, SrBi2Ta2O9, and BaBi2Ta2O9, were investigated. The structures of CaBi2Ta2O9 and SrBi2Ta2O9 are orthorhombic, while that of BaBi2Ta2O9 is pseudotetragonal on the macroscopic scale but consists of microdomains with orthorhombic distortion. The ferroelectric Curie temperature of CaBi2Ta2O9 was over 600 degrees C, and that of SrBi2Ta2O9 was over 300 degrees C. BaBi2Ta2O9, in contrast, showed relaxer-type ferroelectric behavior; that is, in the plot of temperature dependence of dielectric constant, a broad peak appeared around 60 degrees C. As the size of the A-site cation decreases from Ba2+ to Ca2+, the lattice mismatch between TaO2 and AO planes in the perovskite-type unit of ATa(2)O(7) increases and the structural distortion becomes more pronounced. This distortion leads to the higher Curie temperature and the larger spontaneous ferroelectric polarization.