Structural and multiferroic properties in double-layer Aurivillius phase Pb0.4Bi2.1La0.5Nb1.7Mn0.3O9 prepared by molten salt method

A single-phase sample of the Aurivillius compound Pb_0.4Bi_2.1La_0.5Nb_1.7Mn_0.3O₉ was prepared by a molten salt method using K₂SO₄/Na₂SO₄ as the flux. The crystal structure, morphology, ferroelectric, and magnetic properties were investigated. Neutron powder diffraction data confirmed a non-centrosymmetric orthorhombic crystal structure with space group A2₁am and Pb/Bi disorder in the bismuth oxide blocks, Bi/Pb/La disorder on the perovskite A-site, and Nb/Mn disorder on the perovskite B-site. The morphology of the sample showed anisotropic plate-like grains as probed by scanning electron microscopy. The dielectric constant exhibits a transition peak between 600 K and 640 K that depends on frequency, indicating relaxor ferroelectric behavior. Electrical polarization versus applied field loops are unsaturated, with a remnant polarization of 0.43 μC/cm² at 40 Hz under the maximum electrical field applied of 160 kV/cm. The ferroelectricity originates from the displacement of oxygen atoms in the BO₆ octahedra, resulting in a polar structural distortion. Magnetic susceptibility measurements showed the presence of mixed Mn³⁺ and Mn⁴⁺, resulting in short-range ferromagnetic order via double exchange interactions below 33 K. The remnant magnetization (M_r) is 0.01 emu/g at 5 K. This mixed valence of Mn cations is mainly responsible for the high electrical conductivity. Thus, Pb_0.4Bi_2.1La_0.5Nb_1.7Mn_0.3O₉ exhibits coexisting ferroelectric and ferromagnetic properties.