Ferroelectric Properties and Crystal Structure of the Layered Intergrowth Phase Bi₃Pb₂Nb₂O₁₁Cl

The crystal structure and physical properties of Bi 3 Pb 2 Nb 2 O 11 Cl, 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 ∼ 683 K. Although the powder neutron diffraction data show that the crystal structure has pseudo-tetragonal symmetry (parent space group P 4/ mmm, a p = 3.910 38(6) and c p = 18.8342(3) Å at 298 K), electron diffraction data clearly show a weak superlattice ( a o ∼ b o ∼ (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 SrBi 2 Ta 2 O 9 . 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, [M 2 O 2 ]/[MNb 2 O 7 ]/[M 2 O 2 ]/[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.