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 &quot;average&quot; 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.</p

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 &quot;average&quot; 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.</p