Synthesis and characterization of (Bi, AE)MnO₃ (AE = Ca, Sr) system

The effects of electron lone pairs of trivalent bismuth ions on the crystal structure and magnetic properties were examined in Bi_1-xAE_xMnO₃ (AE = Ca, Sr). Polycrystalline perovskite manganites Bi_1-xAE_xMnO₃ were synthesized by using a conventional ceramic technique and a high-pressure technique (4 GPa). Perovskites with triclinic, monoclinic, and tetragonal distortions were found in Bi_1-xSr_xMnO₃ from X-ray powder diffraction. In particular superlattice reflections were found in BiMnO₃ by electron diffraction which revealed two kinds of unit cell, i.e. polymorphism. The bismuth lone pair character was reflected in these structures. Both Bi_1-xCa_xMnO₃ and La_1-xCa_xMnO₃ showed the same crystal symmetry and weak ferromagnetism in the Ca-rich region. However, the magnetic moment of Bi_1-xCa_xMnO₃ was larger than that of La_1-xCa_xMnO₃ with the same x value. Their ferromagnetism was suppressed by charge ordering with further substitution of trivalent cations, although the charge ordering temperature of the La-system is higher than that of the Bi system. The difference is probably associated with the Bi lone pair.