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

Ferroelectric materials of the SrBi2(Ta1-xNbx)(2)O-9 solid-solution system were synthesized, and their structural and ferroelectric properties were investigated. Atomic displacements of the ions in the (Ta,Nb)O-6 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 degrees C (SrBi2Nb2O9). Three short (Ta,Nb)-O bonds in the (Ta,Nb)O-6 octahedron, whose lengths are less than 2 Angstrom, 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. (C) 2000 American Institute of Physics. [S0003-6951(00)01543-6].