Siarhei I. Latushka

Solid solutions of the 0.7BiFeO₃–0.3Ba_1-xSr_xTiO₃ system with compositions in the vicinity of the rhombohedral-cubic morphotropic phase boundary (x ≤ 0.3) have been synthesized via solid state reactions. The crystal structure and morphology of the solid solutions have been studied using X-ray diffraction, scanning electron microscopy and Raman spectroscopy, and the chemical composition has been studied using energy dispersive X-ray spectroscopy. The dielectric properties of the compositions have been studied using impedance spectroscopy as a function of the concentration of strontium ions over a wide range of temperatures and frequencies. The structure of the solid solutions with x ≤ 0.3 contains coexisting rhombohedral and cubic phases, strontium ion substitution leading to higher cubic phase volume fraction. The x = 0 composition contains almost equal parts of coexisting phases, whereas the x > 0.25 solid solutions have single-phase cubic structures. The concentration-related changes in the phase structure of the solid solutions lead to a decrease in the lattice parameters of the coexisting phases and a nonmonotonic decrease in the electrical conductivity and the dielectric permeability of the compositions. The experimental data provide insight into specific changes of the structure and phase composition of the solid solutions in the vicinity of the rhombohedral-cubic morphotropic phase boundary and deliver better understanding of the relationship between the observed structural changes and the electrical properties of the compositions.

The triangle-based magnetic subsystem of borates with the mineral norbergite structure M₃BO₆ (M = Fe, Cr, V) makes these compounds unique to investigate rare quantum ground states influenced by strong magnetic frustration. In this work, we investigated the thermal and magnetic properties of Cr₃BO₆ to find that despite very large negative Weiss temperature Θ = −160.7 K, it orders only at T_N = 4.5 K and experiences a spin-flop transition at µ₀H = 5 T. Density functional theory (DFT) calculations of exchange interaction parameters allow for suggesting the model of magnetic subsystem in chromium borate Cr₃BO₆. The results prove the decisive role of magnetic frustration on the formation of long-range order, providing therefore a basis for future study. Both experimental data and first-principles calculations point to the coexistence of chromium spin-singlets with long-range antiferromagnetic order.

Solid solutions 0.65BiFeO₃–0.35Ba_1-xSr_xTiO₃ (0 ≤ x ≤ 1) with the compositions in the vicinity of the morphotropic phase boundary “rhombohedral-cubic” were synthesized by the Solid-state reaction method. The crystal structure and morphology of the ceramics 0.65BiFeO₃–0.35Ba_1-xSr_xTiO₃ were studied based on the data obtained by X-ray diffraction, scanning electron microscopy, Raman spectroscopy, as well as energy-dispersive X-ray spectroscopy methods. It was determined that the chemical substitution of barium ions with strontium ions leads to a decrease in the magnitude of rhombohedral distortions, while the unit cell parameters decrease in the whole substitution concentration range. The solid solutions with x ≥ 0.25 are characterized by a single-phase structural state with a cubic unit cell; the average crystallite size decreases with increase of the dopant ions. The results of the structural studies carried out using Raman spectroscopy indicate the presence of rhombohedral distortions in the structure of all studied compounds, which is caused by the presence of nanosized clusters with rhombohedral symmetry. The obtained results made it possible to determine the sequence of the changes occurred in the phase state and the unit cell parameters in the region of the morphotropic phase boundary “rhombohedral -pseudocubic”; the concentration intervals corresponding to the single-phase and two-phase structural states of the compounds were determined. The region of concentration stability of the polar rhombohedral phase was clarified using the structural data obtained by local and microscopic research methods.

Исследование корреляции между многофазным состоянием системы и ее магнитными свойствами было проведено на керамических образцах Bi1-xEuxFeO3 с 0,12 ≤ x ≤ 0,18. Результаты рентгеновской дифракции свидетельствуют о том, что с увеличением концентрации ионов Eu наблюдается структурный переход от ромбоэдрической (R3c) к неполярной орторомбической (Pnma) фазе через частичное образование антиполярной орторомбической структуры (Pbam). Смешанное фазовое состояние в полученных соединениях наблюдается в интервале 0,12 ≤ x ≤ 0,16. Магнитные измерения в сильном магнитном поле указывают на слабое ферромагнитное взаимодействие. Магнитный отклик в сильном поле объясняется расположением спинов на фазовой границе, а также наличием собственных антиферромагнитных спинов.

Сrystal structure, piezoelectric and magnetic properties of solid solutions BiMn_1-xFe_xO₃ (x ≤ 0.4) prepared by solid-phase reactions from a stoichiometric mixture of simple oxides at high pressures and temperatures have been studied. The structure of the compounds is characterized by the concentration driven phase transition from the monoclinic structure to the orthorhombic structure at x ≈ 0.2; wherein the ordering d_z2 of the orbitals of Mn³⁺ ions is destroyed, and the inhomogeneous magnetic state is stabilized. Solid solutions with 0.2 ≤ x ≤ 0.4 are characterized by a nonzero piezoelectric response, wherein both ferroelectric and magnetic domain structures exist, the ferroelectric switching voltage decreases with an increase of iron ions concentration, while the residual magnetization value decreases. The maximum value of the piezoresponse signal is observed in the compound BiMn_0.7Fe_0.3O₃. The work clarifies the relationship between the chemical composition, the crystal structure, piezoelectric and magnetic properties of solid solutions BiMn_1-xFe_xO₃. The presence of both magnetic and electric dipole ordering indicates the perspectives for the practical usage of such materials.

The crystal structures of Bi_1−xNd_xFeO₃ and Bi_1−xGd_xFeO₃ solid solutions (0 ≤ x ≤ 0.2) with chemical compositions across structural transformations from the polar rhombohedral phase to the orthorhombic phase with an antipolar distortion and then to the nonpolar orthorhombic phase have been investigated using X-ray diffraction and infrared reflective spectrometry. The obtained results clarify details of the structural transitions assuming the changes that occurred in the crystal lattice dynamics of the compounds. Increase in the dopant content causes a notable change in the intensity and position of the reflectance lines at 18.2 μm and 22.6 μm (550 cm⁻¹ and 440 cm⁻¹) ascribed to the transverse optical phonon modes associated with Bi (Nd, Gd)–O and Fe–O bonds. In the concentration region attributed to the dominant rhombohedral phase, the chemical substitution leads to an increase in intensity of the modes A₁ for solid solutions of both systems. Meanwhile, in the case of Gd doping, the mode A₁ shifts towards the red side of the spectrum, but there is an opposite tendency in the case of Nd doping; the intensity of the modes E decrease regardless of both the dopant-ion type and concentration. This behavior is discussed assuming the change in mass for the chain of chemical bonds caused by different dopant ions and the structural transformations occurring in the compounds upon chemical doping.

The compound BiFe_0.7Mn_0.3O₃ consisting at room temperature of coexistent anti-polar orthorhombic and polar rhombohedral phases has a metastable structural state, which has been studied by laboratory X-ray, synchrotron and neutron diffraction, magnetometry, differential thermal analysis, and differential scanning calorimetry. Thermal annealing of the sample at temperatures above the temperature-driven phase transition into the single phase rhombohedral structure (~700 K) causes an increase of the volume fraction of the rhombohedral phase at room temperature from ~10% up to ~30%, which is accompanied by the modification of the magnetic state, leading to strengthening of a ferromagnetic component. A strong external magnetic field (~5 T) applied to the sample notably changes its magnetic properties, as well as provides a reinforcement of the ferromagnetic component, thus leading to an interaction between two magnetic subsystems formed by the antiferromagnetic matrix with non-collinear alignment of magnetic moments and the nanoscale ferromagnetic clusters coexisting within it. The modification of the structural state and magnetic properties of the compounds and a correlation between different structural and magnetic phases are discussed focusing on the effect of thermal annealing and the impact of an external magnetic field.

The crystal structure and magnetic state of the (1 − x)BiFeO₃-(x)BiMnO₃ solid solution has been analyzed by X-ray diffraction using lab-based and synchrotron radiation facilities, magnetization measurements, differential thermal analysis, and differential scanning calorimetry. Dopant concentration increases lead to the room-temperature structural transitions from the polar-active rhombohedral phase to the antipolar orthorhombic phase, and then to the monoclinic phase accompanied by the formation of two-phase regions consisting of the adjacent structural phases in the concentration ranges 0.25 < x₁ < 0.30 and 0.50 ≤ x₂ < 0.65, respectively. The accompanied changes in the magnetic structure refer to the magnetic transitions from the modulated antiferromagnetic structure to the non-colinear antiferromagnetic structure, and then to the orbitally ordered ferromagnetic structure. The compounds with a two-phase structural state at room temperature are characterized by irreversible temperature-driven structural transitions, which favor the stabilization of high-temperature structural phases. The magnetic structure of the compounds also exhibits an irreversible temperature-induced transition, resulting in an increase of the contribution from the magnetic phase associated with the high-temperature structural phase. The relationship between the structural parameters and the magnetic state of the compounds with a metastable structure is studied and discussed depending on the chemical composition and heating prehistory.

The crystal structure of BiMnO_3+δ ceramics has been studied as a function of nominal oxygen excess and temperature using synchrotron and neutron powder diffraction, magnetometry and differential scanning calorimetry. Increase in oxygen excess leads to the structural transformations from the monoclinic structure (C2/c) to another monoclinic (P2₁/c), and then to the orthorhombic (Pnma) structure through the two-phase regions. The sequence of the structural transformations is accompanied by a modification of the orbital ordering followed by its disruption. Modification of the orbital order leads to a rearrangement of the magnetic structure of the compounds from the long-range ferromagnetic to a mixed magnetic state with antiferromagnetic clusters coexistent in a ferromagnetic matrix followed by a frustration of the long-range magnetic order. Temperature increase causes the structural transition to the nonpolar orthorhombic phase regardless of the structural state at room temperature; the orbital order is destroyed in compounds BiMnO_3+δ (δ ≤ 0.14) at temperatures above 470 °C.