Sillen–Aurivillius Intergrowth Phases as Templates for Naturally Layered Multiferroics

The ferroelectric Sillen–Aurivillius phase Bi₅PbTi₃O₁₄Cl, a layered structure containing three-layer perovskite-type blocks, has been modified by substituting magnetic transition metal cations <i>M</i>³⁺ = Cr³⁺, Mn³⁺, or Fe³⁺ for ¹/₃ of the Ti⁴⁺ cations, accompanied by co-doping of Bi³⁺ for Pb²⁺. The aim of the modification was to produce naturally layered ferroelectromagnetic compounds. Rietveld refinements against high-temperature synchrotron X-ray powder diffraction data show that the resulting new compounds Bi₆Ti₂<i>M</i>O₁₄Cl undergo non-centrosymmetric (<i>P</i>2<i>an</i>) to centrosymmetric (<i>P</i>4/<i>mmm</i>) ferroelectric phase transitions for Bi₆Ti₂CrO₁₄Cl at 974.6(2) K, Bi₆Ti₂MnO₁₄Cl at 913.5(6) K, and Bi₆Ti₂FeO₁₄Cl at 1044.8(1) K. Ferroelectric properties were measured on Bi₆Ti₂FeO₁₄Cl using piezoresponse force microscopy which showed typical ferroelectric hysteresis behavior in the polarization with varying field strength as well as a piezoelectric strain. Combined Rietveld refinements against X-ray and neutron powder diffraction data indicate a statistical 1:2 distribution of <i>M</i>³⁺ and Ti⁴⁺ across all three perovskite layers, resulting in highly strained structures (enhancing the ferroelectricity compared to Bi₅PbTi₃O₁₄Cl) and pronounced spin-glass (cluster glass-type) behavior below <i>T</i>_irr(0) = 4.46 K that we have characterized by detailed magnetic susceptibility and heat capacity measurements