THE CRYSTAL STRUCTURE OF PbBi2Nb2O9

Crystal of PbBi2Nb2O9 belongs to orthorhombic system with space group D2h23 - Fmmm, z = 4 and cell parameters a = 5.4806(7) ?, b = 5.4791(7)?, c = 25.416(3) ?, V = 763.2(2) ?3, Dobs = 7.91g/cm3, Dx = 8.26g/cm3, μ(Mo,Kα) = 678.38cm-1, F(000) = 1608. The crystal structure was determined by the direct method (Multan-78) and Fourier-technique. The coordinations of atoms, isotropic and anisotropic thermal parameters were refined to R = 0.107 for 396 independent reflections by block-diagonal least-squares refinement technique. The Nb-O octahedra are deformed from Oh to C4v and the atoms Nb deviate 0.225 ? from the center along the directions of the 4-fold axes. The deformation of the Nb-O octahedra and the position deviation of metal ions Nb may be the structual reasons resulting in the ferroelectric properties of PbBi2Nb2O9.