Structural distortion and ferroelectric properties of $SrBi2(Ta1−xNbx)2O9$

Ferroelectric materials of the $SrBi2(Ta1−xNbx)2O9$ solid-solution system were synthesized, and their structural and ferroelectric properties were investigated. Atomic displacements of the ions in the $(Ta,Nb)O6$ octahedron significantly increase as x increases, which leads to more structural distortion of the perovskite-type unit. The $Bi2O2$ layer, in contrast, is less distorted in $SrBi2Nb2O9$ than in $SrBi2Ta2O9.$ The contribution of the perovskite-type unit to total ferroelectric polarization is greater in the $SrBi2Nb2O9$ sample, while that of the $Bi2O2$ layer is less; consequently, the total calculated polarization slightly increases. The ferroelectric Curie temperature also increases from 300 $(SrBi2Ta2O9)$ to $440 °C$$(SrBi2Nb2O9).$ Three short (Ta,Nb)–O bonds in the $(Ta,Nb)O6$ octahedron, whose lengths are less than 2 Å, have a covalent character, and the substitution of Nb for Ta makes the bonds more covalent. The strong covalent interaction of the (Ta,Nb)–O bonds increases the structural distortion, resulting in the higher ferroelectric Curie temperature and the larger contribution of the perovskite-type unit to the total spontaneous ferroelectric polarization.