Crystal structure and ferroelectric properties of mixed bismuth layer-structured Bi7Ti4+x/2Nb1−xWx/2O21 ceramics

The effects of Ti and W substitutions for Nb on the ferroelectric properties and the crystal structure of Bi7Ti4+x/2Nb1−xWx/2O21 ceramics were investigated in this study. From the XRPD patterns, no impurity phase was observed in the composition range from 0 to 1. The solid solutions have orthorhombic symmetry, with I2cm space group. The curve representing the relationship between composition x and remanent polarization (Pr) passes through a maximum at approximately x = 0.75; therefore, the maximum Pr value of 12.4 μC/cm2 was obtained for this composition. The variation in the structural distortion of oxygen octahedra in the pseudo-perovskite block showed the same tendency as that of the Pr values. On the other hand, the coercive field (Ec) of the samples at the composition range from 0 to 1 remained the approximately constant value of 90 kV/cm. Moreover, the Curie temperature (Tc) of the samples was decreased from 840 to 715 °C by the increase of the structural distortion of the pseudo-perovskite block, which was caused by the Ti and W substitutions for Nb.