Structural distortions in the layered perovskites CsANb₂O₇ (A=Nd, Bi)

Structural distortions in the layered perovskites CsBiNb2O7 and CsNdNb2O7, belonging to the Dion-Jacobson series, have been analyzed in detail using powder neutron diffraction. Both phases adopt the polar orthorhombic space group P2(1)am, with neighboring perovskite-like blocks in 'eclipsed' conformation analogous to the tetragonal archetype CsLaNb2O7. The metric relationship between these distorted derivatives and the parent structure is alpha(O)similar tob(O)similar toroot2a(T), c(O)similar to c(T), with the orthorhombic distortion being due to off-center displacements of the A-site cation, together with cooperative BO6 octahedral tilts. This mechanism of distortion is analogous to that in the Aurivillius phase ferroelectric SrBi2Ta2O9, though the title compounds do not display ferroelectric behavior. (C) 2003 Elsevier Science (USA). All rights reserved.