Ferroelectric properties and crystal structure of the layered intergrowth phase Bi₃Pb₂Nb₂O₁₁Cl

The crystal structure and physical properties of Bi3Pb2Nb2O11Cl, a member of the Sillen-Aurivillius phase intergrowth family, have been probed using powder neutron diffraction, high-resolution transmission electron microscopy (HRTEM), second harmonic generation (SHG) tests, and alternating current impedance methods. It is shown that this compound undergoes a diffuse ferroelectric type phase transition at T-c similar to 683 K. Although the powder neutron diffraction data show that the crystal structure has pseudo-tetragonal symmetry (parent space group P4/mmm, a(p) = 3.910 38(6) and c(p) = 18.8342(3) Angstrom at 298 K), electron diffraction data clearly show a weak superlattice (a(o) similar to b(o) similar to (2)(1/2)a(p), c(o) = c(p)) compatible with a reduction of symmetry to orthorhombic, analogous to that observed in the related Aurivillius phases, such as SrBi2Ta2O9. Detailed analysis of the SHG data as a function of temperature suggests the existence of ordered polar nanodomains, compatible with the observation of "average" tetragonal symmetry in the neutron diffraction data. These observations, together with the presence of marked frequency dependence of T-c point toward a relaxor type ferroelectric behavior. The nature of the crystal structure, namely, a regular intergrowth of Aurivillius-like and Sillen-like blocks, [M2O2]/[MNb2O7]/[M2O2]/[Cl] [M = Bi, Pb], has been established from both Rietveld refinement of the powder neutron data and directly from HRTEM images. The observation of ferroelectricity in this material suggests the existence of a wider family of new ferroelectrics analogous to the Aurivillius phases.