Репозиторий Университета

A correlation between the crystal structure and magnetic properties of system (1-x)BiFeO3 – (x)BaTiO3 with compounds across the morphotropic phase boundary was studied using X-ray and neutron diffraction, magnetometry, and Mössbauer spectroscopy measurements. Increase in the dopants content leads to the structural transition from the rhombohedral phase to the cubic phase via a formation of the two-phase region (0.2 < x < 0.33), wherein the magnetic structure changes from the modulated G-type antiferromagnetic to the collinear antiferromagnetic via a stabilization of the non-collinear antiferromagnetic phase with non-zero remanent magnetization. The value of magnetic moment calculated per iron ion based on the Mössbauer and neutron diffraction data decreases from m ≈ 4.4 μB for the compound with x = 0.25 to m = 3.2 μB for the compound with x = 0.35 testifying a dominance of 3 + oxidation state of the iron ions. Increase in the amount of the cubic phase leads to a reduction in the remanent magnetization from 0.02 emu/g for the compounds with the dominant rhombohedral phase (x < 0.27) down to about 0.001 emu/g for the compounds with dominant cubic structure (x ≥ 0.27). Rapid decrease in the remanent magnetization observed in the compounds across the phase coexistence region points at no direct correlation between the type of structural distortion and non-zero remanent magnetization, while the oxygen octahedra tilting is the key factor determining the presence of non-zero remanent magnetization.

© 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 formation of novel ternary cerium germanide with the chemical composition Ce33.3Ir22.2Ge44.5 (at.%) was established. As derived from powder X-ray diffraction data, the compound crystallizes with two polymorphic modifications. The high-temperature modification, established for as-cast alloy, possesses a tetragonal ThSi2-type structure and can be named as Ce(IrxGe1-x)2, x = 0.32. The low-temperature modification forms in the course of heat treatment at 1073 K, crystallizes with an orthorhombic structure of its own type and can be labeled as Ce3Ir2-xGe4+x, x = 0.02. Both structures belong to the AlB2 structural family and are closely related to each other. In both modifications, statistical mixtures of Ir and Ge atoms occupy same crystallographic sites. As found by means of magnetization measurements, the two structural variants of Ce33.3Ir22.2Ge44.5 (at.%) exhibit very similar magnetic properties. Both polymorphs are Curie-Weiss paramagnets with strong crystalline electric field features observed at low temperatures.

© 2019 International Union of Crystallography. This study of 3-(5-phenyl-1,3,4-oxadiazol-2-yl)-2H-chromen-2-one, C17H10N2O3, 1, and 3-[5-(pyridin-4-yl)-1,3,4-oxadiazol-2-yl]-2H-chromen-2-one, C16H9ON3O3, 2, was performed on the assumption of the potential anticancer activity of the compounds. Three polymorphic structures for 1 and two polymorphic structures for 2 have been studied thoroughly. The strongest intermolecular interaction is stacking of the 'head-to-head' type in all the studied crystals. The polymorphic structures of 1 differ with respect to the intermolecular interactions between stacked columns. Two of the polymorphs have a columnar or double columnar type of crystal organization, while the third polymorphic structure can be classified as columnar-layered. The difference between the two structures of 2 is less pronounced. Both crystals can be considered as having very similar arrangements of neighbouring columns. The formation of polymorphic modifications is caused by a subtle balance of very weak intermolecular interactions and packing differences can be identified only using an analysis based on a study of the pairwise interaction energies.

© 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.

© 2019 Elsevier B.V. Crystal structure of the Bi1-xBaxFe1-xTixO3 ceramics across the phase boundary region between the polar rhombohedral and pseudo-cubic phases has been investigated by means of X-ray and neutron diffraction, scanning electron microscopy and differential scanning calorimetry techniques. The structural measurements have revealed a decrease in the rhombohedral distortions upon increase in the dopant content followed by the stabilization of the pseudo-cubic phase at x ≥ 0.20. While temperature increase gives rise to the similar pattern of structural changes for the lightly-doped samples, an unpredictable temperature-driven strengthening of the rhombohedral distortions has been found for the compounds belonging to the phase boundary region. The study specifies the peculiarities of the temperature-driven evolution of the crystal structure depending on the structural state of the compounds at room temperature.