Structure ofABi2Nb2O9(A= Sr, Ba): Refinement of Powder Neutron Diffraction Data

The room temperature structures of the two Aurivillius-type oxides ABi{sub 2}Nb{sub 2}O{sub 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{sub 1}am, Z = 4, a = 5.5189(3) {angstrom}, b = 5.5154(3) {angstrom}, C = 25.1124(9) {angstrom} for SrBi{sub 2}Nb{sub 2}O{sub 9} and a = 5.567(1) {angstrom}, b = 5.567(1) {angstrom}, C = 25.634(1) {angstrom} for BaBi{sub 2}O{sub 9}. Both compounds exhibit a polar and potentially ferroelectric structure. Adoption of perovskite-type layers in the smaller Sr ion containing material results on a tilting of the NbO{sub 6} octahedra, which in turn makes the formation of a fifth strong bond between Bi in the [Bi{sub 2}O{sub 2}]{sup 2+} layer and an O from the [SrNb{sub 2}O{sub 7}]{sup 2{minus}} layer possible. This bond is absent in BaBi{sub 2}Nb{sub 2}O{sub 9}. The formation of the fifth bond in the Sr compound contributes to a larger value of P{sub s} than found in the analogous Ba compound.