Новые публикации

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.