Репозиторий Университета

© 2020 Elsevier Ltd Neutron diffraction and magnetization measurements of the Bi0.85Ca0.15Fe1-xMnxO3+δ (x = 0.4, 0.5) compounds have been performed at room and low temperatures to disclose the effect of the mixed-valence Mn substitution on the magnetic structure and properties of the Ca-doped bismuth ferrites near the polar/nonpolar phase boundary. It has been confirmed that the Mn substitution results in the filling of anion vacancies produced by the aliovalent replacement of Bi3+ by Ca2+. The Bi0.85Ca0.15Fe0.6Mn0.4O3+δ compound has the acentric structure specific to the pure BiFeO3 (space group R3c) and displays a G-type antiferromagnetic order at room temperature (m300K = 1.35(2) μB). The magnetic moments localized on the Fe/Mn ions are directed along the polar axis. The spin-reorientation transition from the c to a axis takes place with decreasing temperature. An increase in the Mn concentration gives rise to the polar → nonpolar (R3c → Pnma) structural phase transformation. The nonpolar (x = 0.5) compound has a G-type antiferromagnet structure (TN = 210 K) with spins aligned along the orthorhombic b axis. The low-temperature magnetic moments (m5K = 2.67(2) μB and m5K = 1.80(3) μB for the samples with x = 0.4 and x = 0.5, respectively) are considerably smaller than those predicted for complete spin ordering of the interacting ions of Fe3+, Mn3+ and Mn4+ (>4 μB). While the neutron diffraction measurements reveal no contribution associated with a long-range ferromagnetic order at T = 5 K, a significant increase in the magnetization of the samples, suggesting the formation of a glassy phase, is observed with decreasing temperature.

© 2020 Elsevier B.V. The room- and low-temperature neutron diffraction measurements of the Bi0.9Ca0.1Fe0.6Mn0.4O3+δ compound have been carried out to disclose the influence of Mn substitution on the multiferroic properties of the low-doped Bi1−xCaxFeO3−x/2 perovskites combining ferroelectric and weak ferromagnetic behavior. It has been proven that the material under study retains a polar R3c structure specific to the parent Bi0.9Ca0.1FeO2.95. The Mn doping results in the elimination of oxygen vacancies giving rise to the increase in spontaneous electric polarization. The chemical modification stabilizes the collinear antiferromagnetic structure at room temperature. The reorientation of the antiferromagnetic vector from the c to a axis takes place with decreasing temperature. Reflecting the competitive character of the superexchange interactions between Fe3+, Mn3+ and Mn4+, the coexistence of ferromagnetic glassy and antiferromagnetic long-range-ordered phases is observed at low temperatures.

© 2019 Elsevier B.V. Herein, we report on the crystal structure, magnetic and local ferroelectric properties of the Bi1−xCaxFe1−xTixO3 and Bi1−xCaxFe1−x/2Nbx/2O3 perovskites prepared by a solid state reaction method. It has been found that the Ca2+/Nb5+-containing series is characterized by a narrower concentration range (x ≤ 0.2) over which the acentric R3c structure specific to the pure BiFeO3 can be stabilized. The compositional variation in the critical concentration defining the polar/nonpolar (R3c/Pnma) phase boundary can be understood as related to the chemical modification-induced changes in the lattice spacing diminishing the stability of the a−a−a− tilting in favor of the a−b+a− one. Both the Ca2+/Ti4+ and Ca2+/Nb5+ substitutions ensure the suppression of a cycloidal antiferromagnetic order, thus leading to the formation of a weak ferromagnetic polar state. While this effect is proven to be associated with a composition-driven reduction in polar displacements, lattice defects are supposed to contribute to the instability of the cycloidal spin arrangement.

© 2019 Elsevier B.V. The structural parameters of the Bi0.85AE0.15Fe0.85Ti0.15O3 (AE = Ca and Ba) multiferroics have been determined using variable temperature neutron powder diffraction. The compounds adopt the polar rhombohedral R3c structure near room temperature and undergo phase transitions into either the nonpolar orthorhombic Pnma (AE = Ca) or cubic Pm3-m (AE = Ba) structures on heating. In the ferroelectric phase, a temperature-driven lattice expansion is accompanied by both a diminishing of the off-center ionic displacements (thus resulting in a decrease in the spontaneous electric polarization) and a reduction in the magnitude of the antiphase oxygen octahedra tilting. Being largely different for the materials under study, the latter parameter is supposed to specify the dissimilarity in their magnetic properties.