Sillen–Aurivillius Intergrowth Phases as Templates for Naturally Layered Multiferroics

The ferroelectric Sillen–Aurivillius phase Bi 5 PbTi 3 O 14 Cl, a layered structure containing three-layer perovskite-type blocks, has been modified by substituting magnetic transition metal cations M 3+ = Cr 3+, Mn 3+, or Fe 3+ for 1 / 3 of the Ti 4+ cations, accompanied by co-doping of Bi 3+ for Pb 2+ . 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 Bi 6 Ti 2 M O 14 Cl undergo non-centrosymmetric ( P 2 an ) to centrosymmetric ( P 4/ mmm ) ferroelectric phase transitions for Bi 6 Ti 2 CrO 14 Cl at 974.6(2) K, Bi 6 Ti 2 MnO 14 Cl at 913.5(6) K, and Bi 6 Ti 2 FeO 14 Cl at 1044.8(1) K. Ferroelectric properties were measured on Bi 6 Ti 2 FeO 14 Cl 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 M 3+ and Ti 4+ across all three perovskite layers, resulting in highly strained structures (enhancing the ferroelectricity compared to Bi 5 PbTi 3 O 14 Cl) and pronounced spin-glass (cluster glass-type) behavior below T irr (0) = 4.46 K that we have characterized by detailed magnetic susceptibility and heat capacity measurements.