O. M. Fesenko’s research while affiliated with National Academy of Sciences of Ukraine and other places

Using the model of four sublattices, the Landau-Ginzburg-Devonshire-Kittel phenomenological approach and the Stephenson-Highland ionic adsorption model for the description of coupled polar and antipolar long-range orders in ferroics, we analytically calculated the phase diagrams and polar properties of B i 1 − x S m x Fe O 3 nanoparticles covered by surface ions with dependence on their size, surface ions density, samarium content x , and temperature. The size effects and ferroionic coupling govern the appearance and stability conditions of the long-range ordered ferroelectric, reentrant ferrielectric, and antiferroelectric phases in B i 1 − x S m x Fe O 3 nanoparticles. Calculated phase diagrams are in qualitative agreement with the x-ray diffraction phase analysis, electron paramagnetic resonance, infrared spectroscopy, and electrophysical measurements of B i 1 − x S m x Fe O 3 nanopowders sintered by the solution combustion method. The combined theoretical-experimental approach allows us to explain the influence of the ferroionic coupling and size effects in B i 1 − x S m x Fe O 3 nanoparticles on their polar properties. Published by the American Physical Society 2024

The x-ray diffraction, Raman, and infrared spectroscopies and magnetic measurements were used to explore the correlated changes of the structure, lattice dynamics, and magnetic properties of the LuFeO3 nanoparticles, which appear in dependence on their sintering temperature. We revealed a gradual substitution of the hexagonal phase by the orthorhombic phase in the nanoparticles, with sintering temperature increasing from 700 to 1100 °C. The origin and stability of the hexagonal phase in the LuFeO3 nanoparticles are of the special interest, because the nanoparticles in the phase can be a room-temperature multiferroic with a weak ferromagnetic and pronounced structural and ferroelectric long-range ordering. The antiferromagnetic and nonpolar orthorhombic phase is more stable in the bulk LuFeO3. To define the ranges of the hexagonal phase stability, we determine the bulk and interface energy densities of different phases from the comparison of the Gibbs model with experimental results. Using effective parameters of the Gibbs model, we predict the influence of size effects and temperature on the structural and polar properties of the LuFeO3 nanoparticles. Analysis of the obtained results shows that the combination of the x-ray diffraction, Raman and infrared spectroscopies, magnetic measurements, and theoretical modeling of structural and polar properties allows us to establish the interplay between the phase composition, lattice dynamics, and multiferroic properties of the LuFeO3 nanoparticles prepared under different conditions.

Using the model of four sublattices, the Landau-Ginzburg-Devonshire-Kittel phenomenological approach and the Stephenson-Highland ionic adsorption model for the description of coupled polar and antipolar long-range orders in ferroics, we calculated analytically the phase diagrams and polar properties of Bi$_{1-x}$Sm$_x$FeO$_3$ nanoparticles covered by surface ions in dependence on their size, surface ions density, samarium content "x" and temperature. The size effects and ferro-ionic coupling govern the appearance and stability conditions of the long-range ordered ferroelectric, reentrant ferrielectric and antiferroelectric phases in the Bi1-xSmxFeO3 nanoparticles. Calculated phase diagrams are in a qualitative agreement with the X-ray diffraction phase analysis, electron paramagnetic resonance, infra-red spectroscopy and electrophysical measurements of the Bi$_{1-x}$Sm$_x$FeO$_3$ nanopowders sintered by the solution combustion method. The combined theoretical-experimental approach allows to establish the influence of the ferro-ionic coupling and size effects in Bi$_{1-x}$Sm$_x$FeO$_3$ nanoparticles on their polar properties.

To describe the polar properties of nanosized HfxZr1−xO2−y, we evolve the “effective” Landau–Ginzburg–Devonshire (LGD) model based on the parametrization of the Landau expansion coefficients for polar and antipolar orderings. We have shown that the effective LGD model can predict the influence of screening conditions and size effects on phase diagrams, polarization reversal, and structural properties of nanosized HfxZr1−xO2−y of various shapes and sizes. To verify the model, we use the available experimental results for HfxZr1−xO2 thin films and oxygen-deficient HfO2−y nanoparticles prepared under different annealing conditions. X-ray diffraction, which was used to determine the phase composition of the HfO2−y nanoparticles, revealed the formation of a ferroelectric orthorhombic phase in them. Micro-Raman spectroscopy was used to explore the correlation of lattice dynamics and structural changes that depend on the oxygen vacancy concentration in the HfO2−y nanoparticles. Since our approach allows us to determine the conditions (shape, sizes, Zr content, and/or oxygen vacancy amount) for which nanosized HfxZr1−xO2−y are ferroelectric or antiferroelectric, we hope that the obtained results are useful for creation of next generation Si-compatible ferroelectric gate oxide nanomaterials.

The dispersion kinetics of niobium-palladium thin double films deposited onto leucosapphire, alumina, and zirconia ceramics and annealed in vacuum at temperatures up to 1200°C at different exposure intervals at each temperature (from 5 to 20 min) is studied. The double films consisted of two layers. The first one consists of metallized layer–niobium nanofilms with 150 nm thickness deposited onto oxide surface. The second–palladium layer with 1.5 μm thickness is deposited onto first one and supposed to serve as a solder when the metallized oxide samples are joined together. According to our study results, kinetic curves of the investigated thin double metallic films decomposition have been built, from which the basic process parameters (temperature and exposition time at this temperature) for brazing or pressure welding of ceramic materials can be determined. Using the data obtained, process regimes for joining by brazing and pressure welding were selected. According to these regimes, joints’ prototypes with seams up to 2 μm thick were made, which strength up to 150 MPa.

Abstract: High-temperature electrochemical synthesis (HTES) in molten salts is highly promising among the up-to-date methods for the production of carbide powders. Ultrafine composite powders of tungsten carbides (WC|C, WC|C|Pt, W2C|WC, and W2C|W) were synthesized using the HTES method in electrolytic baths with different chemical compositions under various synthesis conditions (cathode current density, CO2 pressure in the electrolyzer, temperature, and cathode material). Composite powders (up to 3 wt.% free carbon) with a WC particle size of 20–30 nm were prepared using Na, K|Cl (1 : 1)–Na2W2O7 (6.4 wt.%)–CO2 (1.25 MPa) and Na, K|Cl (1 : 1)–Na2WO4 (12.0 wt.%)– NaPO3 (0.7 wt. %)–CO2 (1.25 MPa) electrolytic baths at a temperature of 750°C. When the CO2 pressure was reduced to 0.75 MPa, composite W2C|WC powders formed at the cathode. The ratio of carbide phases in the composites depended on the initial concentration of tungsten salts in the electrolyte and on the CO2 gas pressure in the electrolyzer. The addition of Li2CO3 (4.5 wt.%) to the electrolytic salt mixture decreased the tungsten carbide particles to 10 nm, changed their morphology, and increased the free carbon content in the composite up to 5 wt.%. The specific surface area of the powder increased by a factor of 4 to 7 (from 20–35 to 140 m2/g). The resulting products were modified with fine platinum particles through the use of platinum cathodes. The HTES method demonstrated its potential for producing tungsten carbide powders with the properties allowing their use as electrocatalysts in the hydrogen evolution reaction. For the WC|C composite powders synthesized in the Na, K|Cl–Na2W2O7–Li2CO3–CO2 system, the hydrogen evolution potential was – 0.02 V relative to the normal hydrogen electrode, the overpotential at a current density of 10 mA/cm2 was –110 mV, the exchange current was 7.0 10–4 A/cm2, and the Tafel slope was –85 mV/dec. Keywords: electrochemical synthesis, molten salts, nanosized powders, tungsten carbides, surface morphology

Micro-Raman spectroscopy and X-ray diffraction have been used to explore the lattice dynamics of Nb-substituted SrBi2(Ta1-xNbx)2O9 (SBTN) crystalline thin films annealed at low temperature, 700 °C. It turned out that SrBi2(Ta1-xNbx)2O9 films consist of fine-grained spherical structures for x = 0.1–0.4, while the formation of rod-like grains occurs for x = 0.5 due to the stress-induced transformation of the thin film perovskite structure. Moreover, it was revealed that during Nb cationic substitution Aurivillius phase formation was enhanced and became dominated in SBTN thin films and fluorite/pyrochlore phase formation was highly suppressed. We assume that these changes are conditioned by the ferrodistortion occurring in ferroic perovskites, namely by the tilting distortion of (Ta,Nb)O6 octahedra for x = 0.2–0.5. The octahedral tilting distortion can change the coordination environment of the A-cite cation, as well as it lowers the SBTN symmetry due to the differences in ionic radius and mass between Ta and Nb in the B-sites that can lead to significant changes of the SBTN crystal structure. The same nonmonotonic trends were observed for the ferroelectric perovskite phase fraction and remanent polarization on Nb content in SBTN films. The substituting Nb atoms with a concentration of 10–20% made it possible to increase the remanent polarization in 3 times and raise the perovskite phase fraction from 66 to 87%. Therefore, obtained results can be used for the production of lead-free thin films with a high remanent polarization under low annealing temperature, being promising for advanced nonvolatile random access ferroelectric memory (NvFeRAM) applications.

Dispersion kinetics research of two-layer hafnium-copper coatings (films) onto oxide materials (leucosapphire, alumina, and zirconia ceramics) during annealing in vacuum at 900–1100 °C at different annealing time in the range of 2–20 min are describe. According to the research results, the kinetic curves of film dispersion are built. Based on these graphs, the parameters of the joining of metallized non-oxide materials by soldering were selected.KeywordsDispersionKineticsTwo-layer hafnium-copper coatingsOxide materialsAnnealingVacuumTemperature