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-x)AE(x)MnO(3) (AE = Ca, Sr). Polycrystalline perovskite manganites Bi(1-x)AE(x)MnO(3) 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 Bi1-xSrxMnO3 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 Bi1-xCaxMnO3 and La1-xCaxMnO3 showed the same crystal symmetry and weak ferromagnetism in the Ca-rich region. However, the magnetic moment of Bi1-xCaxMnO3 was larger than that of La1-xCaxMnO3 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. (C) 1998 Elsevier Science B.V. All rights reserved.