The influence of Ca. and Ba modification on the ferroelectric properties of
SrBi2Ta2O9 (SBT) was investigated using dense ceramics, and their crystal
structures were analyzed by the Rietveld method through high-resolution neu
tron powder diffraction. The 30% substitution of Sr by a larger Ba lowered
the Curie temperature (T-C) and decreased remanent polarization (2P(r)). In
contrast, an equal amount of smaller Ca modification resulted in a marked
increase in T-C and in a much higher coercive field, but did not improve 2P
(r) at an applied field lower than 200 kV/cm. Structure refinement revealed
that spontaneous polarization (P-s) is determined by the average shift of
perovskite blocks along the a axis, and that the Ca-modified SBT had almost
the same P-s as SBT. The 30% Ca incorporation did not improve the intrinsi
c polarization, however enhanced TaO6 octahedral rotation in the a-b plane,
which would lead to the higher T-C.