The cubic Fe2-xMnxO3 is an intriguing material that has recently been investigated for various applications, including lithium-ion battery anodes, catalysts, energy storage media, humidity sensors, and photocatalysts. Despite its wide range of promising applications, the magnetic properties of Fe2-xMnxO3 remain controversial, with different sources reporting conflicting information regarding the type of magnetic ordering, phase transition temperature, and magnetic moment of this compound. This work presents a study of the magnetic state of three Fe2-xMnxO3:Ga solid solutions with varying Mn:Fe:Ga ratios, along with one gallium-free Fe2-xMnxO3 reference sample. We performed a detailed analysis of the actual chemical composition and crystal structure of the synthesized samples using energy-dispersive X-ray spectroscopy (EDX), powder X-ray diffraction (XRD), and X-ray absorption spectroscopy (XAS) to evaluate compositional differences. The magnetic states of the three Fe2-xMnxO3:Ga samples and the gallium-free Fe2-xMnxO3 were investigated using magnetometry and Mossbauer spectroscopy. The low-temperature magnetic anomalies were found to be more consistent with spin-glass-like freezing than with conventional long-range antiferromagnetic ordering. Although variations in magnetic behavior were observed and found to depend on composition and the cooling rate during synthesis, our results demonstrate that these factors do not account for the drastically different magnetic properties reported for similar bixbyite-type oxides. Instead, the apparent room-temperature ferrimagnetism observed in one sample is most likely extrinsic and can be attributed to a trace spinel-type impurity phase, as supported by magnetizations and ESR measurements. Thus, the origin of these discrepancies lies primarily in the chemical purity of the samples and, to a significant extent, in the synthesis technique employed.

Calorimetric, dilatometric and pressure studies of (NH4)3WO2F5 were performed over a wide temperature range, including the Pm-3m <-> Pa-3 phase transition. Comparison of the obtained results with data for related fluorides (NH4)3SnF7 and (NH4)3TiF7 undergoing the same structural changes showed a significant role of chemical pressure in the formation of thermal and barocaloric properties. A decrease in anomalous entropy in oxyfluoride, S0 = 12.2 J/mol center dot K, is accompanied by a significant increase in sensitivity to hydrostatic pressure, dT0/dp = 93 K/GPa, the preservation of a large change in anomalous deformation delta( V/V)0 = 0.45% and a small temperature hysteresis, delta T0 < 1 K. This combination of thermal characteristics has led to both a significant increase in extensive and intensive barocaloric parameters in the low pressures area, and to their high reversibility in the modes of increasing and decreasing pressure.

The content of metal ions and their valence state for three Ni x Cu y Mn 3-x-y BO 5 compositions were estimated by EXAFS and XANES spectroscopy methods. In all three compositions, the copper y content did not exceed 0.25; in the third composition, some manganese ions entered the compound not only in the trivalent, but also in the divalent state. A detailed study of the magnetic properties showed that two magnetic transitions are observed in all compositions: one in the region of 60-75 K, and the second in the region of 10 K. Based on the exchange interactions calculated within the framework of the empirical model, the magnetic ordering temperatures for two- and three-lattice magnets were determined and it was assumed that the magnetic transition in the region of 60-75 K is associated with the ordering of magnetic moments in positions 2 and 3. Keywords: ludwigites, magnetic phase transition, indirect exchange interactions.

The content of transition‐metal ions and their valence state for three compositions of NixCuyMn3−x−yBO5 are estimated by the methods of X‐ray absorption spectroscopy. In all the three compositions, the copper content does not exceed 0.25. One of the compositions has manganese ions both in the trivalent and bivalent states according to the K‐edge position. The calculations of the total energies of various cation‐ordered structures using the software package Wien2K show that copper ions predominately occupy one site, namely, 2(2d). The measurements of the magnetic properties show two types of magnetic transitions in all the three compositions, with one of them being in the range of 60–75 K and the other one at 10 K. Based on the exchange interactions calculated in the framework of the empirical model, magnetic ordering temperatures are estimated for two‐ and three‐sublattice structures and the magnetic transition in the range of 60–75 K is supposed to be connected with magnetic moment ordering in sites 2 and 3.

Методами спектроскопии EXAFS и XANES были оценены содержание металлических ионов и их валентное состояние для трех составов NixCuyMn3-x-yBO5. Во всех трех составах содержание меди y не превысило 0.25; в третьем составе часть ионов марганца вошли в соединение не только в трехвалентном, но и в двухвалентном состоянии. Подробное исследование магнитных свойств показало, что во всех составах наблюдается два магнитных перехода: один в районе 60-75 K, а второй в районе 10 K. На основании вычисленных в рамках эмпирической модели обменных взаимодействий были определены температуры магнитного упорядочения для двух- и трехподрешеточного магнетика и сделано предположение, что магнитный переход в районе 60-75 K связан с упорядочением магнитных моментов в позициях 2 и 3. Ключевые слова: людвигиты, магнитный фазовый переход, косвенные обменные взаимодействия.

Using the flux technique single crystals of (Ni,Cu)2MnBO5 were grown. The crystal structure of this compound belongs to the space group Pbam(55). There are no monoclinic distortions of the crystal structure. The analysis of the structural data shows that the content of copper ions in the crystal is lower than 10 percent. The magnetic ordering into the ferrimagnetic phase occurs at 75 K. The assessment of the exchange interactions shows that changes in the exchange interactions lead to increasing frustrations in the system which could result in a decrease in the temperature of magnetic ordering.

Visualization of nanorelief measurements by a scanning tunneling microscope is a built-in function of the control system, which takes into account the peculiarities of the device's operation. In order to improve the distinguishability of small formations, measurements are projected onto the underlying plane, which is chosen so that the height difference of the entire set of points is as small as possible. The relative height of a relief point is visualized using the gradient shading method. This paper proposes a method for increasing the accuracy of presenting the measurement results, which is based on the use of the standard functions of the control system's software. The experimental results show that the proposed solution significantly improves the distinguishability of small details of a nanorelief.

The Pb 3 Mn 7 O 15 crystals doped with titanium ions have been synthesized. The study of the structural properties has shown that titanium ions occupy positions in the interplane columns, which weakens the exchange coupling between the planes. Two anomalies in the temperature dependences of magnetization and heat capacity at 62 and 35 K have been found. It has been supposed that, at 62 K, the long-range magnetic order arises and, at 35 K, a spin-reorientation transition, as in the Pb 3 Mn 7 O 15 compound, occurs.

X-ray, calorimetric and dilatometric studies of RbCdZrF7 revealed the existence of the second order phase transition Cmcm <-> P2(1)/m at T-0 = 200 K. The structure of the initial and distorted phases is ordered. The phase transition is associated with displacements of fluorine atoms, which leads to minor rotations of the CdF7 and ZrF7 pentagonal bipyramids. A small change in entropy, 0.1R, is characteristic of displacive-type transformations. An anomalously high susceptibility of the transition temperature to hydrostatic pressure was found.

Crystals of Pb3Mn7O15 doped with titanium ions have been synthesized. The study of the structural properties showed that titanium ions occupy positions in the “columns” between the planes, which leads to a weakening of the exchange interaction between the planes. The temperature dependences of magnetization and heat capacity revealed two anomalies at 62 K and 35 K. At 62 K, a long-range magnetic order presumably appears, and at 35 K a spin-reorientation transition occurs, as in Pb3Mn7O15.

X ray, calorimetric and dilatometric studies of K3ZrF7 revealed the existence of the phase transition Fm-3m <-> R-3m at T-0 = 320 K. The structural model assumes a disorder of a pentagonal bipyramid ZrF7 with the following ratio of equivalent orientation positions in the initial and distorted phases: 12 to 6. A good agreement was found between the experimental and model-calculated changes in strain and entropy during the phase transition. A comparative analysis of entropy and structural parameters of related fluorides K3ZrF7 - (NH4)(2)KZrF7 - (NH4)(3)ZrF7 was performed. The anomalous behavior of thermodynamic properties in the range 140 230 K is not typical for phase transitions and is accompanied by a significant change in the entropy of the crystal lattice.

The role of anisotropy of the thermal expansion in formation of piezocaloric effect (PCE) near ferroelectric phase transitions in NH4HSO4 was studied. Strong difference in linear baric coefficients and as a result in intensive and extensive PCE associated with the different crystallographic axes was found. PCE giving the main contribution to the barocaloric effect were determined at both phase transitions. Rather strong effect of the lattice dilatation on the tuning of PCE was observed. Comparative analysis of PCE at the phase transitions in different materials showed that NH4HSO4 can be considered as a promising solid-state refrigerant. A hypothetical cooling cycle based on alternate using uniaxial pressure along two axes was considered.

The (NH4)3VOF5 crystals have been synthesized and their homogeneity and single-phase structure has been established by the X-ray diffraction, energy dispersive spectroscopy, and X-ray photoelectron spectroscopy studies. The investigations of the temperature dependences of specific heat, entropy, strain, and pressure susceptibility show the occurrence of three phase transitions caused by the structural transformations in the (NH4)3VOF5 crystals. The T–p phase diagram shows the temperature limits of stability of the crystalline phases implemented in (NH4)3VOF5. The optical and dielectric studies disclose the ferroelastic nature of the phase transitions. An analysis of the experimental data together with the data on the isostructural (NH4)3VO2F4 crystal makes it possible to distinguish the physical properties of oxyfluorides containing vanadium of different valences (IV and V).

Heat capacity, thermal expansion, and sensitivity to the hydrostatic pressure of (NH4)(2)KZrF7 elpasolite are studied in a wide temperature range. The changes in deformation and entropy during successive phase transitions are determined: Delta(Delta V/V) = 3.10(-4); Delta S = 8 J/mol K The temperatures and entropies of phase transitions turned out to be slightly sensitive to pressure changes. An analysis of the entropy of phase transformations was performed in the framework of the model of the cubic phase structure Fm-3 m. In the low temperature phase, an anomalous behavior of thermodynamic properties, which is not characteristic of phase transitions, was observed, accompanied by a significant change in the crystal lattice entropy.

Abstract The (NH_4)_3VOF_5 crystals have been synthesized and their homogeneity and single-phase structure has been established by the X-ray diffraction, energy dispersive spectroscopy, and X-ray photoelectron spectroscopy studies. The investigations of the temperature dependences of specific heat, entropy, strain, and pressure susceptibility show the occurrence of three phase transitions caused by the structural transformations in the (NH_4)_3VOF_5 crystals. The T – p phase diagram shows the temperature limits of stability of the crystalline phases implemented in (NH_4)_3VOF_5. The optical and dielectric studies disclose the ferroelastic nature of the phase transitions. An analysis of the experimental data together with the data on the isostructural (NH_4)_3VO_2F_4 crystal makes it possible to distinguish the physical properties of oxyfluorides containing vanadium of different valences (IV and V).

AbstractCrystalline samples of the (La_1 – _ y Eu_ y )_0.7Pb_0.3MnO_3 ( y = 0, 0.2, 0.4, 0.5, 0.6, and 0.8) manganite solid solutions have been grown. Temperature dependences of the specific heat, thermal expansion, and high-intensity magnetocaloric effect have been investigated. A comparative analysis of the effect of isovalent cationic substitution on the thermal and caloric parameters of the ferromagnetic phase transition has been made. It has been shown that the growth of the Eu atom concentration leads to a decrease in the phase transition entropy and an increase in the baric coefficient dT / dp . Field and temperature dependences of the magnetocaloric effect have been established. It is demonstrated that, varying the ratio of cations, one can obtain the solid solutions with the maximum magnetocaloric effect value in fields of up to 6 kOe in a temperature range of 90–340 K. Owing to the similar reduced relative cooling powers, the investigated solid solutions can be used as reference solid-state cooling media in designing the cascade cooling systems.

The specific heat, thermal expansion, permittivity, and electrocaloric effect in bulk of BaTiO3 (BT) samples in the form of nano- (nBT-500 nm) and micro- (mBT-1200 nm) ceramics fabricated using spark plasma sintering and solid-state plasma techniques have been investigated. The size effect has been reflected, to a great extent, in the suppression of the specific heat and thermal expansion anomalies and in the changes in the temperatures and entropies of phase transitions and permittivity, and a decrease in the maximum intensive electrocaloric effect: $$\Delta T_{{{\text{AD}}}}^{{\max }}$$ = 29 mK (E = 2.0 kV/cm) for nBT and $$\Delta T_{{{\text{AD}}}}^{{\max }}$$ = 70 mK (E = 2.5 kV/cm) for mBT. The conductivity growth at temperatures above 360 K leads to the significant irreversible heating of the samples due to the Joule heat release in the applied electric field, which dominates over the electrocaloric effect.

AbstractThe specific heat, thermal expansion, permittivity, and electrocaloric effect in bulk of BaTiO_3 (BT) samples in the form of nano- (nBT-500 nm) and micro- (mBT-1200 nm) ceramics fabricated using spark plasma sintering and solid-state plasma techniques have been investigated. The size effect has been reflected, to a great extent, in the suppression of the specific heat and thermal expansion anomalies and in the changes in the temperatures and entropies of phase transitions and permittivity, and a decrease in the maximum intensive electrocaloric effect: $$\Delta T_{{{\text{AD}}}}^{{\max }}$$ = 29 mK ( E = 2.0 kV/cm) for nBT and $$\Delta T_{{{\text{AD}}}}^{{\max }}$$ = 70 mK ( E = 2.5 kV/cm) for mBT. The conductivity growth at temperatures above 360 K leads to the significant irreversible heating of the samples due to the Joule heat release in the applied electric field, which dominates over the electrocaloric effect.

The heat capacity and thermal expansion of potassium heptafluorotantalate were studied. The room temperature phase P21/c\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$P2_1/c$$\end{document} is stable at least to 4 K. The strong first-order phase transition P21/c-Pnma\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$P2_1/c - Pnma$$\end{document} at T0=486.2K\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T_0=486.2\,\hbox {K}$$\end{document} is accompanied by giant changes in the entropy, ΔS0=22.3J(molK)-1,\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta S_0=22.3\,\hbox {J}\,(\hbox {mol}\,\hbox {K})^{-1},$$\end{document} and volume strain, δV0/V=-3.6%\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\delta V_0/V=-\,3.6\%$$\end{document}. A rather high sensitivity of K2TaF7\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {K}_{2}\hbox {TaF}_{7}$$\end{document} to pressure was found, dT0/dp=-220KGPa-1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text {d}}T_0/{\text {d}}p=-\,220 \,\hbox {K}\,\hbox {G}\,\hbox {Pa}^{-1}$$\end{document}. Significant extensive and intensive barocaloric effects are found at low pressure. The possibility of improving the barocaloric properties is discussed.