Solid Solution in the systems BiMO3-ATiO3 (M=Fe, Cr; A=Ba, Sr) Synthesis, structure and magnetic properties

Abstract

Abstract Compounds exhibiting more than one of the properties ferromagnetism, ferroelectricity and/or ferroelasticity, i.e. so called multiferroics, are promising materials for device applications. Co-existence of electric polarization and magnetization allows for a coupling between the properties, the so-called magnetoelectric effect, which mediate induction of magnetization through an applied electric field and vice versa. The research effort towards magnetoelectric materials has experienced a resurge during the last years. However, it is clear that in order to realize new technological applications it is necessary with further materials development. To understand the relationship between structure and properties, the principles of multiferroism and to predict new multiferroic materials, solid solutions of multiferroic materials have been studied. For this thesis, polycrystalline samples of the binary solid solutions xBiMO3-(1-x)ATiO3 (M = Fe or Cr; A = Ba or Sr; x = 0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1) were prepared by solid state sintering. Phase purity was analyzed by X-ray powder diffraction and it was determined that BiFeO3 formed binary solid solutions with both BaTiO3 and SrTiO3 for all measured x, while BiCrO3 did not form phase pure solid solutions with BaTiO3 and only partially with SrTiO3 (for x ≤ 0.5). All successfully sintered samples showed cubic symmetry except for 0.8BiFeO3 – 0.2SrTiO3 which showed an onset of a transition towards a rhombohedral structure. Neutron time-of-flight data were collected from polycrystalline samples of 0.5BiFeO3-0.5BaTiO3 and 0.5BiCrO3-0.5SrTiO3 at room temperature and the subsequent structure model refinement revealed that the isotropic thermal vibration factors for the A-site was high, indicating disorder at that position and motivating further reverse Monte Carlo analysis of the total scattering. Magnetic measurements were carried out on the same samples, and 0.5BiCrO3-0.5SrTiO3 showed an antiferromagnetic transition at 10 K, whilst the 0.5BiFeO3-0.5BaTiO3 sample showed a ferromagnetic behavior.