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. © 2000 American Institute of Physics.