Structure of ABi(2)Nb(2)O(9) (A=Sr, Ba): Refinement of powder neutron diffraction data

The room temperature structures of the two Aurivillius-type oxides ABi(2)Nb(2)O(9) (A = Sr, Ba) have been refined by Rietveld analysis of powder neutron diffraction data. The two oxides are isostructural, with orthorhombic space group A2(1)am, Z = 4, a = 5.5189(3) Angstrom, b = 5.5154(3) Angstrom, c = 25.1124(9) Angstrom for SrBi2Nb2O9 and a = 5.567(1) Angstrom, b = 5.567(1) Angstrom, c = 25.634(1) Angstrom for BaBi2Nb2O9. Both compounds exhibit a polar and potentially ferroelectric structure. Adoption of perovskite-type layers in the smaller Sr ion containing material results in a tilting of the NbO6 octahedra, which in turn makes the formation of a fifth strong bond between Bi in the [Bi2O2](2+) layer and an O from the [SrNb2O7](2-) layer possible. This bond is absent in BaBi2Nb2O9. The formation of the fifth bond in the Sr compound contributes to a larger value of P-s than found in the analogous Ba compound. (C) 1996 Academic Press, Inc.