New Publications

https://doi.org/10.1134/S0031918X23601439

Powders of maghemite γ-Fe₂O₃ with an average diameter of 8 nm, γ-Fe₂O₃/SiO₂ composites with an agglomerate diameter of about 50 nm and a size of interspersed γ-Fe₂O₃ particles of 6 nm, and γ‑Fe₂O₃/SiO₂/aptamer(FAS9) composites were synthesized by chemical deposition. Mössbauer spectra were measured, the static and dynamic magnetic properties of the powders were studied, and the coercive force was determined, which decreases from 14 Oe for γ-Fe₂O₃ powders to 3 Oe for the γ-Fe₂O₃/SiO₂ composite. It is shown that the particle blocking temperature is close to room temperature. The increment of temperature of the powders was measured in the ferromagnetic resonance mode; the temperature of the Fe₂O₃/SiO₂ composite (ΔT ≈ 16°C) turned out to be higher than that of the pure γ-Fe₂O₃ powder (ΔT ≈ 10°C). It has been experimentally shown that temperature increment ΔT is proportional to the square of the microwave field amplitude. It has been shown that the composition γ-Fe₂O₃/SiO₂/aptamer FAS9 is able to effectively bind to tumor cells, and FMR hyperthermia leads to a decrease in the viability of tumor cells.

https://doi.org/10.1134/S0031918X2360135X

Auger electron spectroscopy was used to determine the phase composition of Cr₂GeC MAX phase thin films. A distinctive feature of the formation of carbon-containing MAX phases is the shape of carbon Auger peaks, which is characteristic of metal carbides spectra. Features of the Auger spectra in the presence of secondary phases of chromium germanides are found. Their presence can manifest itself in an increase in the energy of the germanium peaks, which is caused by a chemical shift during the formation of the Cr–Ge bond. Moreover, we have detected the accumulation of electronic charge, which can be explained by the features of the surface morphology.

https://doi.org/10.1134/S0031918X23601385

In this paper, we solve the inverse problem of magneto-optical ellipsometry for thin ferromagnetic films with optical uniaxial anisotropy. We work within the framework of the approach we developed earlier analyzing magnetoellipsometric data without using fourth-order M-matrices. We work with ellipsometric relations, in which we take into account the magneto-optical contribution as perturbations, and ellipsometric measurements are carried out on a setup with a simple dipole scheme based on the transverse magneto-optical Kerr effect. We add the magneto-optical response to the expressions known in the literature for the reflection coefficients of anisotropic thin films, which are related to the parameters measured by magneto-optical ellipsometry. As a result, by analyzing the obtained expressions for the reflection coefficients, we obtain information on the total permittivity tensor of a thin film.

https://doi.org/10.31857/S0205961423020021

Создана диэлектрическая модель, основанная на рефракционной диэлектрической модели смеси талых и мерзлых лесных органических почв корневой зоны для частоты 435 МГц. Модель разработана на основе диэлектрических измерений четырех почв, в которых содержание органического вещества варьировалось от 15 до 31%. Диэлектрические измерения были проведены в диапазоне массовой влажности от 0 до 0.6 г/г и диапазоне температур от –30 до 25°С. Коэффициент детерминации (R²) между рассчитанными с использованием модели и измеренными значениями действительной (ε') и мнимой (ε") частями комплексной диэлектрической проницаемости составил 0.97. Нормированное среднеквадратическое отклонение составило 16 и 21% для действительной и мнимой частей комплексной диэлектрической проницаемости соответственно. Разработанная диэлектрическая модель может быть применена в алгоритмах дистанционного зондирования при восстановлении значения влажности лесных почв корневой зоны из данных радарного и радиометрического зондирования.

https://doi.org/10.1103/PhysRevB.109.014421

In the substituted ${\mathrm{Nd}}_{1-x}{\mathrm{Tb}}_x{\mathrm{Fe}}_3{\left({\mathrm{BO}}_3\right)}_4$ $(x=0.1\text{and}x=0.2)$, possessing almost easy-axis magnetic structure at low temperatures, an unusual two-step transition in fields along the trigonal $c$ axis was observed by magnetization and single-crystal neutron diffraction studies. At the first step, only part of the Tb Ising-type moments flip to the $c$ axis, which is accompanied by a significant deviation of the antiferromagnetic Fe spins from the $c$ axis. At the second step, the remaining Tb moments flip and the Fe moments flop into the basal plane. The ob- served evolution is qualitatively explained by a model assuming small deviations of Tb moments from the trigonal axis due to local environment distortions, which leads to nonequivalence of the Tb ions with respect to effective Tb-Fe exchange and external field. Thus, an intrinsic “hidden” instability of the magnetic system in the magnetic field occurs.

DOI https://doi.org/10.1039/D4DT00038B

The threshold structural transformation of the DUT-4 metal–organic framework (MOF) from an ordered to distorted phase during exposure to ambient conditions has been revealed. The in situ X-ray diffraction analysis, in situ Raman and FTIR spectroscopy, scanning electron microscopy and synchronous thermal analysis have been used for investigation. The reversible effect of exposure time and humidity on such a phase transition has been confirmed. We also demonstrated that the observed phase transition correlated well with changes in the optical and electronic properties of DUT-4, paving the way to a new family of MOF-based phase change materials for optoelectronic applications.

https://doi.org/10.1016/j.jmmm.2024.171781

Contributions of different magnetic subsystems formed in the systems of synthetic ferrihydrite nanoparticles (characterized previously) with an average size of < d> ≈ 2.7 nm coated with polysaccharide arabinogalactan in different degrees have been separated by measuring the dependences of their magnetization M on magnetic field H of up to 250 kOe on vibrating sample and pulsed magnetometers. The use of a wide measuring magnetic field range has been dictated by the ambiguity in identifying a linear M(H) portion for such antiferromagnetic nanoparticle systems within the conventional field range of 60–90 kOe. The thorough analysis of the magnetization curves in the temperature range of 100–250 K has allowed the verification of the contributions of (i) uncompensated magnetic moments µ_un in the superparamagnetic subsystem, (ii) the subsystem of surface spins with the paramagnetic behavior, and (iii) the antiferromagnetic susceptibility of the antiferromagnetically ordered ferrihydrite particle core. As a result, a model of the magnetic state of ferrihydrite nanoparticles has been proposed and the numbers of spins corresponding to magnetic subsystems (i)–(iii) have been estimated. An average magnetic moment μ_un of ∼ 145 μ_B (μ_B is the Bohr magneton) per particle corresponds approximately to 30 decompensated spins of iron atoms in a particle (about 3 % of all iron atoms), which, according to the Néel’s hypothesis μ_un ∼ <d>^3/2, are localized both on the surface and in the bulk of an antiferromagnetically ordered particle. The fraction of free (paramagnetic) spins is minimal in the sample without arabinogalactan coating of the nanoparticle surface (7 %) and is attained 20 % of all iron atoms in the sample with the highest degree of spatial separation of particles. According to this estimation, paramagnetic spins are located mainly on the edges and protruding areas of particles. Most magnetic moments of iron atoms are ordered antiferromagnetically and the corresponding magnetic susceptibility of this subsystem behaves as in an antiferromagnet with the randomly distributed crystallographic axes, i.e., increases with temperature.

https://iopscience.iop.org/article/10.1088/1361-6463/ad211b

Double fluoride salts (NH₄)₃M⁴⁺F₇ (M⁴⁺: Sn, Ti, Ge, Si) demonstrate a high efficiency of using chemical pressure as a tool for control and tuning structural ordering/disordering, sensitivity to hydrostatic pressure, successions of the phase transitions, etc and, as a result, for purposeful variation within a wide range of parameters of barocaloric effect (BCE). The conventional and inverse BCEs near the triple points were found on the T − p phase diagrams, combination of which can be used to construct original cooling cycle in narrow temperature and pressure ranges. Reconstructive transformation between two cubic phases, , realized in (NH₄)₃SnF₇ at atmospheric pressure and in (NH₄)₃TiF₇ at  0.4 GPa are characterized by rather low thermal hysteresis,  = 1 K, and a great entropy change,  = 110–152 J (kg · K)⁻¹, depending on the size of the central atom. At above 300–350 K, a contribution to BCE associated with the regular thermal expansion of the crystal lattice becomes comparable to entropy and temperature changes under pressure in the region of the phase transitions. An analysis of the absolute, relative and integral barocaloric characteristics of (NH₄)₃M⁴⁺F₇ compounds showed their high competitiveness with respect to other barocaloric materials considered as promising solid-state refrigerants.

https://doi.org/10.48550/arXiv.2307.07208

We analyse the stationary current of Bose particles across the Bose-Hubbard chain connected to a battery, focusing on the effect of inter-particle interactions. It is shown that the current magnitude drastically decreases as the strength of inter-particle interactions exceeds the critical value which marks the transition to quantum chaos in the Bose-Hubbard Hamiltonian. We found that this transition is well reflected in the non-equilibrium many-body density matrix of the system. Namely, the level-spacing distribution for eigenvalues of the density matrix changes from Poisson to Wigner-Dyson distributions. With the further increase of the interaction strength, the Wigner-Dyson spectrum statistics changes back to the Poisson statistics which now marks fermionization of the bosonic particles. With respect to the stationary current, this leads to the counter-intuitive dependence of the current magnitude on the particle number.

https://doi.org/10.1063/5.0178247

Metal nanoparticle (NP) complexes lying on a single-layer graphene surface are studied with a developed original hybrid quantum–classical theory using the Finite Element Method (FEM) that is computationally cheap. Our theory is based on the motivated assumption that the carrier charge density in the doped graphene does not vary significantly during the plasmon oscillations. Charge transfer plasmon (CTP) frequencies, eigenvectors, quality factors, energy loss in the NPs and in graphene, and the absorption power are aspects that are theoretically studied and numerically calculated. It is shown the CTP frequencies reside in the terahertz range and can be represented as a product of two factors: the Fermi level of graphene and the geometry of the NP complex. The energy losses in the NPs are predicted to be inversely dependent on the radius R of the nanoparticle, while the loss in graphene is proportional to R and the interparticle distance. The CTP quality factors are predicted to be in the range ∼10−100⁠. The absorption power under CTP excitation is proportional to the scalar product of the CTP dipole moment and the external electromagnetic field. The developed theory makes it possible to simulate different properties of CTPs 3–4 orders of magnitude faster compared to the original FEM or the finite-difference time domain method, providing possibilities for predicting the plasmonic properties of very large systems for different applications.

https://doi.org/10.1016/j.jre.2022.11.004

We report on the novel heterometallic quaternary selenide EuCeCuSe₃, the fabrication of which has been a challenge until this work. The structure of the reported selenide was elucidated from the powder X-ray diffraction data, which revealed the formation of EuCeCuSe₃ with excellent yield (96.7%) accompanied with a minor fraction of CeSe₂ (3.3%), and was best solved in orthorhombic space group Pnma with the BaLaCuS₃ structural type. Thus, the crystal structure of the title compound completes the row of the heterometallic quaternary selenides EuRECuSe₃ (RE = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y), of which the cerium-based derivative exclusively belongs to the BaLaCuS₃ structural type. The distortion of the CuSe₄ polyhedron was compared for the whole series of EuRECuSe₃ compounds using the τ₄-descriptor for four coordinated ions, which revealed the highest degree of distortion for the Ce³⁺-containing selenide, followed by the La³⁺-based derivative. Furthermore, the crystallographic and geometrical parameters of the reported selenide were discussed in comparison to the Ce³⁺-based sulfides SrCeCuS₃ and EuCeCuS₃. Ab initio calculations of the crystal structure, a phonon spectrum and elastic constants for the crystal of EuСeCuSe₃ were also performed. The types and wavenumbers of fundamental modes were determined and the involvement of ions participating in the phonon modes was assessed. The experimental IR spectrum of the reported selenide was interpreted and found to be in agreement with the calculated spectrum. The experimental direct band gap of EuCeCuSe₃ was measured to be 1.36 eV that is consistent with the concept of its origin due to interband transitions between orbitals emerging mainly from 4f (valence band) and 5d (conduction band) levels of the Eu²⁺ cation. The dependence of the Young's modulus on the direction demonstrates the anisotropy of the elastic properties, while the Vickers hardness for EuCeCuSe₃ was calculated to be 5.2 GPa. Finally, the title compound is paramagnetic above 4 K.

https://doi.org/10.1134/S1063776123120075

The magnetic-field dependence of the superparamagnetic-blocking temperature T_B of systems of antiferromagnetically ordered ferrihydrite nanoparticles has been investigated and analyzed. We studied two powder systems of nanoparticles: particles of “biogenic” ferrihydrite (with an average size of 2.7 nm), released as a result of vital functions of bacteria and coated with a thin organic shell, and particles of biogenic ferrihydrite subjected to low-temperature annealing, which cause an increase in the average particle size (to 3.8 nm) and burning out of the organic shell. The character of the temperature dependences of magnetization, measured after cooling in a weak field, as well as the shape of the obtained dependences T_B(H), demonstrate peculiar features, indicating the influence of magnetic interparticle interactions. A detailed analysis of the dependences T_B(H) within the random magnetic anisotropy model made it possible to estimate quantitatively the intensity of magnetic particle–particle interactions and determine the magnetic anisotropy constants of individual ferrihydrite particles.

https://doi.org/10.1134/S1990793123060064

The results of a study of water absorption processes by samples of polymer composite materials (PCMs) based on fiberglass, subjected to low-speed impact with controlled impact energy and alternating temperature cycling are presented. Using magnetic resonance imaging (MRI), the distribution of absorbed water in the fiberglass structure is visualized and the dynamics of its accumulation in various areas of the sample are studied. It is found that mechanical impact leads to a nonuniform distribution of the absorbed water in the samples and a significant accumulation of free water in the areas of destruction and adjacent layers in the event of a violation of the integrity of the outer layer of the material. It is shown that cyclic alternating temperature effects do not lead to a noticeable change in the water absorption processes and are comparable in effect to mechanical nondestructive impacts. The results obtained using MRI are in close agreement with the data of traditional weight measurements, which shows the effectiveness of the method in diagnosing defects and mechanical damage to PCMs exposed to the humid environment.

https://doi.org/10.3390/ijms25020708 

Gd@C₈₂O_xH_y endohedral complexes for advanced biomedical applications (computer tomography, cancer treatment, etc.) were synthesized using high-frequency arc plasma discharge through a mixture of graphite and Gd₂O₃ oxide. The Gd@C₈₂ endohedral complex was isolated by high-efficiency liquid chromatography and consequently oxidized with the formation of a family of Gd endohedral fullerenols with gross formula Gd@C₈₂O₈(OH)₂₀. Fourier-transformed infrared (FTIR) spectroscopy was used to study the structure and spectroscopic properties of the complexes in combination with the DFTB3 electronic structure calculations and infrared spectra simulations. It was shown that the main IR spectral features are formed by a fullerenole C₈₂ cage that allows one to consider the force constants at the DFTB3 level of theory without consideration of gadolinium endohedral ions inside the carbon cage. Based on the comparison of experimental FTIR and theoretical DFTB3 IR spectra, it was found that oxidation of the C₈₂ cage causes the formation of Gd@C₈₂O₂₈H₂₀, with a breakdown of the integrity of the parent C₈₂ cage with the formation of pores between neighboring carbonyl and carboxyl groups. The Gd@C₈₂O₆(OOH)₂(OH)₁₈ endohedral complex with epoxy, carbonyl and carboxyl groups was considered the most reliable fullerenole structural model.

https://doi.org/10.1007/s11433-023-2282-8

High-Q optical cavity is an indispensable component of many photonic devices, such as lasers, sensors, harmonic generation and photon emission, and chiral dichroism. Conventional way of realizing ultrahigh Q-factor relies on Fabry-Pérot resonators, photonic crystal nanocavities fabricated by a CMOS-compatible process, or whispering gallery modes.

https://doi.org/10.1016/j.jphotochem.2023.115454

We’ve studied the B800 part of Rhodoblastus acidophilus light-harvesting complex (LH2) by several quantum chemical techniques based on the density functional theory (DFT) and determined the specific method and a minimal reliable model suitable for further studies of the LH2. In addition to bacteriochlorophyll a molecules, the minimal model includes two α and one β chain amino acids. Within the model, we are able to reproduce the contribution of the B800 ring of nine bacteriochlorophyll a molecules to the near infrared Q_y absorption band. We also discuss the use of hybrid DFT calculations for precise energy and optical estimations and DFT-based tight binding (DFTB) method for the large-scale calculations. Crucial importance of Hartree-Fock exchange interaction for the correct description of B800 peak position was shown.

https://doi.org/10.1016/j.diamond.2024.110844

In this study, particles of synthetic undoped diamond (D_N) obtained via the high pressure – high temperature method were coated with a nickel shell using metallic nickel plasma in a two-jet plasma generator with gas vortex and magnetic flux stabilization. Through the use of scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy, we observed the formation of a nickel diamond composite with a core-shell structure, where D_N serves as the core and Ni nanoclusters form the shell (D_N@Ni). The results of voltammetric analysis indicated that D_N@Ni, when deposited on a graphite electrode, exhibited significant electrocatalytic activity in the oxidation of methanol and paracetamol in an alkaline electrolyte.

https://doi.org/10.1002/anie.202318401

Zero area compressibility (ZAC) is an extremely rare mechanical response that exhibits an invariant two-dimensional size under hydrostatic pressure. All known ZAC materials are constructed from units in two dimensions as a whole. Here, we propose another strategy to obtain the ZAC by microscopically orthogonal-braiding one-dimensional zero compressibility strips. Accordingly, ZAC is identified in a copper-based compound with a planar [CuO₄] unit, Cu₂GeO₄, that possesses an area compressibility as low as 1.58(26) TPa⁻¹ over a wide pressure range from ≈0 GPa to 21.22 GPa. Based on our structural analysis, the subtle counterbalance between the shrinkage of [CuO₄] and the expansion effect from the increase in the [CuO₄]-[CuO₄] dihedral angle attributes to the ZAC response. High-pressure Raman spectroscopy, in combination with first-principles calculations, shows that the electron transfer from in-plane bonding d_x²_-y² to out-of-plane nonbonding d_z² orbitals within copper atoms causes the counterintuitive extension of the [CuO₄]-[CuO₄] dihedral angle under pressure. Our study provides an understanding on the pressure-induced structural evolution of copper-based oxides at an electronic level and facilitates a new avenue for the exploration of high-dimensional anomalous mechanical materials.

https://doi.org/10.1134/S0031918X23601804

The results of experimental and theoretical studies of the coercivity and the dipole coupling field of the hysteresis loop on the thickness of the nonmagnetic interlayer in magnetic films, which are obtained via chemical deposition, are presented. Using model calculations based on the Landau–Ginzburg equations, the exchange interactions between magnetic layers with the participation of atoms from the nonmagnetic interlayer are studied. The resulting expression for the dipole coupling field describes well the exponential changes in the dipole coupling field as a function of the interlayer thickness in structures with both soft magnetic layers and layers with significantly different values of the coercivity.

https://doi.org/10.1134/S1062873823704518

Planar and vertical hybrid structures, which combine ferromagnetic and semiconductor layers are essential for implementation and study of spin transport phenomena in semiconductors, which is crucial for the advancement and development of spintronics. We have developed approaches for the synthesis of Fe_{3 + x}Si_{1 – x} epitaxial thin films and demonstrated the spin accumulation effect in multiterminal devices based on Fe_{3 + x}Si_{1 – x}/Si. Fe_{3 + x}Si_{1 – x}/Ge/Fe₃Si and Fe_{3 + x}Si_{1 – x}/Ge/Mn₅Ge₃ multilayer hybrid structures were synthesized on a Si(111) substrate, study of their structural, magnetic and transport properties were performed. The effect of synthesis conditions on the growth of epitaxial structures and on their magnetic and transport properties was discussed. The results obtained may prove valuable in the development and fabrication of spintronic devices.

https://iopscience.iop.org/article/10.1088/1402-4896/ad16b2

The microstructure and temperature of martensitic transformation of Ni₄₄Fe₁₉Ga₂₇Co₁₀ single crystals after aging at temperatures from 623 K to 1173 K were studied by electron microscopy and differential scanning calorimetry. The temperature ranges of the second phase precipitation, their lattice structure and volume fraction, and also the modification of the nanodomain structure of the L2₁+B2 high-temperature phase were determined in dependence on aging temperature. The influence of microstructure parameters on the martensitic transformation temperatures, transformation intervals and thermal hysteresis has been discussed.

https://doi.org/10.1007/s10948-024-06690-0

The interlayer interactions in multilayer systems with a non-magnetic semimetallic interlayer are great of interest. The magnetic and structural properties of the Dy_xCo_1-x/Bi/Py systems (17 < x < 26 at.%) have been studied. The temperature dependences of magnetization in the range 4.2–300 K were measured for the first time. The influence of the bismuth interlayer thickness on the exchange interaction between the DyCo and Py layers was found as well as the critical value of its thickness. The obtained atypical value of the period of exchange bias oscillations was explained by the formation of bismuth compounds with dysprosium–pnictogenides at bismuth thicknesses below the critical value. The interface was investigated by spectral ellipsometry in the range 2–5 eV. The information on the structure of the surface obtained by atomic force microscopy was used to create a multilayer model for fitting experimental ellipsometric data. Analysis of the optical properties showed that pnictogenide Dy₃Bi₂ is formed at the interface, which affects the general magnetic state of the samples studied.

https://doi.org/10.1002/jrs.6644

We summarize the current knowledge on crystal structures, synthesis, applications, and Raman spectroscopy of Wadsley phases of vanadium oxide, including VO₂ (B), V₆O₁₃, V₄O₉, V₃O₇, and V₂O₅. While these oxides have garnered significant attention for potential energy storage applications and have been studied for decades, there remains inconsistency in data regarding their characteristic Raman spectra. To address this, we synthesized a series of Wadsley phases by physical vapor deposition of amorphous vanadium oxide films and subsequent annealing in a controlled environment. X-ray diffraction studies confirmed the formation of VO₂ (B), V₆O₁₃, V₄O₉, and V₃O₇. We meticulously measured the room-temperature Raman spectra of these phases, offering robust reference data for the easy identification of vanadium oxides in unknown samples. Finally, we studied low-temperature phase transitions in VO₂ (B) and V₆O₁₃.

https://doi.org/10.1016/j.solidstatesciences.2024.107440

A study of heat capacity, thermal dilatation and sensitivity to hydrostatic and uniaxial pressure was carried out on single-crystal and ceramic samples of NH₄HSeO₄. The main parameters of low-temperature successive phase transitions B2 (T₁) ↔ incommensurate IC (T₂) ↔ ferroelectric P1 (T₃) ↔ non-ferroelectric did not depend on the type of samples. The behavior of the volumetric strain and the results of direct measurements of T₃(p) contributed to the resolution of the longstanding problem associated with the ambiguity of the sign of the corresponding volumetric baric coefficient. The role of thermal expansion anisotropy in the formation of the piezocaloric effect (PCE) near the ferroelectric phase transition at T₃ has been studied. Due to the strong difference in the linear baric coefficients, the main contribution to the barocaloric effect (BCE) comes from the inverse intensive and extensive PCE associated with the a-axis. Compared to a single crystal, ceramics demonstrate lower BCE values, which, however, exist in a wider temperature range, which leads to close values of integral caloric parameters. The strong decrease in both BCE and PCE at low-temperature transformations in NH₄HSeO₄ compared to the ferroelectrics NH₄HSO₄ and NH₄NH₄SO₄ is associated with a small change in entropy during three low-temperature phase transitions, ΣΔS_i = 2.52 J/mol∙K, which is a consequence of a high degree of structural ordering in selenate as a result of a high-temperature transformation at T₀ between the superionic and B2 phases, accompanied by a giant change in entropy, ΔS₀≈Rln21.

https://doi.org/10.1134/S0021364023603457

Nanoparticles of Co_{1 – x}Mg_xFe₂O₄ with x equal to 0, 0.2, 0.4, 0.6, 0.8 and 1.0 have been synthesized. For all values of x, they are nanocrystals with a cobalt ferrite structure and an average linear size (56 ± 3) nm. Based on the analysis of the Mossbauer effect spectra, the Co²⁺ ions were shown to occupy only octahedral positions at all values of x. The experimentally obtained dependence of the nanoparticles magnetization on x corresponds to the dependence calculated using the Mossbauer effect data, except for the sample with x = 1.0. The effective magnetic anisotropy constant estimated for 0 K from the analysis of the coercive force temperature dependences decreases from 5.27 × 10⁶ at x = 0 to 1.29 × 10⁶ erg/cm3 at x = 0.8 and drops sharply to 4 × 10⁴ erg/cm³ at x = 1.0.

https://doi.org/10.1039/D3DT03437B

The magnetic and electronic structures of Fe₄O₅ have been investigated at ambient and high pressures via a combination of representation analysis, density functional theory (DFT+U) calculations, and Mössbauer spectroscopy. A few spin configurations corresponding to the different irreducible representations have been considered. The total-energy calculations reveal that the magnetic ground state of Fe₄O₅ corresponds to an orthogonal spin order. Depending on the magnetic propagation vector k, two spin-ordered phases with minimal energy differences are realized. The lowest energy magnetic phase is related to k = (0, 0, 0) and is characterized by ferromagnetic ordering of iron magnetic moments at prismatic sites along the b-axis and antiferromagnetic ordering of iron moments at octahedral sites along the c-axis. For the k = (1/2, 0, 0) phase, the moments in the prisms are antiferromagnetically ordered along the b-axis and the moments in the octahedra are still antiferromagnetically ordered along the c-axis. Under high pressure, Fe₄O₅ exhibits magnetic transitions with the corresponding electronic transitions of the metal–insulator type. At a critical pressure P_C ∼ 60 GPa, the Fe ions at the octahedral sites undergo a high-spin to low-spin state crossover with a decrease in the unit-cell volume of ∼4%, while the Fe ions at the prismatic sites remain in the high-spin state up to 130 GPa. This site-dependent magnetic collapse is experimentally observed in the transformation of Mössbauer spectra measured at room temperature and high pressures.

https://doi.org/10.1007/s10973-023-12792-z

Samarium monosulfide, a strain gauge and barometric material, exists in equilibrium with Sm₃S₄ and Sm₂O₂S in the S-Sm–O system. Therefore, studying phase equilibria in the refractory Sm₂S_3-X-SmS-Sm₂O₂S system is a scientifically interesting task. In this system, 49 samples were synthesized and studied by powder XRD, differential scanning calorimetry, visual thermal analysis, and microstructural analysis. Melting points of Sm₃S₄, SmS, and Sm₂O₂S compounds were determined. Eutectic diagrams of Sm₃S₄-Sm₂O₂S, SmS-Sm₂O₂S, SmS-Sm₃S₄ systems were constructed. Temperatures and compositions of the binary eutectic points were determined. Fusion enthalpies for Sm₃S₄, SmS, and Sm₂O₂S phases were estimated using the Schröder–Le Chatelier equation. The liquidus lines were calculated using second-degree polynomials and Redlich–Kister model. Coordinates of the ternary eutectic point in the Sm₃S₄-SmS-Sm₂O₂S system were calculated using the cutting-plane method and the Scheffé method. The calculated compositions of ternary eutectic points were averaged at one most probable point, in accordance with the data on the samples microstructure. The experimental temperature of the ternary eutectic point coincides with the calculated values within the margin of error. Positions of eutectic valleys and approximate positions of isotherms in the system were established. Thermodynamic parameters of the α-Sm₂S₃ → γ-Sm₂S₃ polymorphic transition and the dependence of the Sm₂S_3-X composition on heat treatment conditions were determined. According to the scanning electron microscopy data, the approximate composition of the crystallized from the melt Sm₂S₃ sample is Sm₂S_2.95. The Sm₁₀S₁₄O phase decomposes at 1470 ± 15 °C in the course of a solid-phase reaction. The phase diagram of the Sm₂S_3-X-Sm₂O₂S system was revisited. Optical band gaps of Sm₁₀S₁₄O and Sm₂O₂S phases were determined. The Sm₁₀S₁₄O compound was optically characterized for the first time; its direct and indirect optical bandgaps were found equal to 2.48 and 2.37 eV, respectively. The determined direct and indirect optical bandgaps of Sm₂O₂S (4.4 eV and 3.95 eV, respectively) agree with the earlier measurements, thus confirming the accuracy of the chosen synthesis procedures.

https://doi.org/10.1016/j.nanoso.2023.101089

The results of a study of the dynamic (alternating current magnetic susceptibility) and static magnetic properties, as well as ⁵⁷Fe Mössbauer spectrometry and ferromagnetic resonance of two-line ferrihydrite nanoparticle systems with varying intensities of magnetic interparticle interactions are reported. The strength of the magnetic interparticle interactions has been tuned by coating (with various degrees of coating) the ferrihydrite particles (2–4 nm in size and an average size ∼2.7 nm) of the initial synthetic sample by arabinogalactan. Also, a biogenic ferrihydrite sample (an average particle size of 2-nm) with a natural organic coating was studied and it has the weakest magnetic interparticle interactions among of all the samples. Relaxation times of the particle’s magnetic moment were determined by the data of static and dynamic magnetic susceptibilities and from analysis of ⁵⁷Fe Mössbauer spectrometry. Based on the temperature dependences of the relaxation times, it has been concluded that the predominantly collective processes of freezing of the particle magnetic moments occur under the action of the magnetic interparticle interactions. It is shown that an important role in these processes is played by a magnetic subsystem of the surface spins of the particles. The effect of the interplay between the surface spin and magnetic moment subsystems on the static magnetic properties (low-temperature magnetic hysteresis loops) and the parameters of the microwave absorption line under the magnetic resonance conditions is discussed.