Synthesis, structural and property studies of bismuth containing perovskites
Several bismuth-containing transition metal perovskites that are of interest as potential multiferroic materials have been synthesised and studied. These materials have been structurally characterised and their physical properties have been examined at varying temperatures and pressures. The new series of substituted bismuth ferrite perovskites BixCa1-xFeO3, where x = 0.4 - 1.0, has been prepared. A disordered cubic phase (x = 0.4 - 0.67) and the coexistence of rhombohedral and cubic phases (x = 0.8 and 0.9) have been observed. The x = 0.8 sample is located at the phase boundary and shows a transformation from cubic to rhombohedral symmetry at 473 - 573 K. All samples are antiferromagnets at room temperature and have Néel temperature of 623 - 643 K. Ferroelectric order is suppressed in the disordered cubic phase. BixLa1-xMnO3 materials with x = 0.8, 0.9 and 1.0 were synthesised at 3 - 6 GPa. For x = 1.0 and 0.9 samples a highly distorted perovskite structure with monoclinic space group C2/c was adopted and ferromagnetic behaviour was observed with Curie temperatures of 101 and 94 K, respectively. Bi0.8La0.2MnO3 shows an O'-type orthorhombic Pnma structure and canted A-type antiferromagnetic ordering below 80 K. A new phase of BiNiO3 has been discovered at 4 - 5 GPa below 200 K, in which a Pb11 symmetry has been revealed with a = 5.2515(2) Å, b = 5.6012(3) Å, c = 7.6202(4) Å and β = 90.20(1) º at 4.3 GPa and 100 K. This new Phase Id is derived from the ambient Phase I Bi3+ 0.5Bi5+ 0.5Ni2+O3, where the charge disproportionated Bi3+/Bi5+ cations become disordered. The updated P-T phase diagram of BiNiO3 is presented. BiCu3Mn4O12 has been studied by neutron diffraction from 5 to 400 K. The incorporation of Mn3+ into the Cu site has been observed, showing that the true composition is BiCu2.5Mn4.5O12. The ordering of Mn and Cu moments below transition temperature 320 K is found to be ferromagnetic rather than ferrimagnetic as proposed previously.