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Bi4–xPrxTi3O12 (x = 0.4, 0.8, 1.2, 1.6) solid solutions have been prepared by solid-state reactions, via multistep firing of stoichiometric mixtures of their constituent oxides in air at temperatures from 1003 to 1323 K. Their crystal structure has been determined using X-ray diffraction, and their luminescence spectra have been measured at room temperature. High-temperature heat capacity of polycrystalline substituted bismuth titanate samples has been determined by differential scanning calorimetry. The Cp(T) curves of the solid solutions with x = 0.4 and 0.8 have extrema related to phase transitions. Experimental data have been used to calculate the main thermodynamic functions of the solid solutions.

We consider thermo-optic hysteresis in a silicon structure supporting bound state in the continuum. Taking into account radiative heat transfer as a major cooling mechanism we constructed a non-linear model describing the optical response. It is shown that the thermo-optic hysteresis can be obtained with low intensities of incident light $I_0\approx 1~\rm{W/m^2}$ at the red edge of the visible under the critical coupling condition.

Erbium gallium titanate was prepared by solid-phase synthesis via the sequential calcination of precursor oxides in an air atmosphere at 1273 and 1573 K. The crystal structure of ErGaTi2O7 was characterized by full-profile analysis for the X-ray diffraction pattern of the synthesized powder sample as follows: space group Pcnb, a = 9.77326(15) A, b = 13.5170(2) A, c = 7.33189(11) A, V = 918.58(3) A3, ρ = 6.10 g/cm3. The high-temperature heat capacity of erbium gallium titanate was measured by differential scanning calorimetry within a temperature range of 320–1000 K. Based on these data, the basic thermodynamic functions of ErGaTi2O7 were calculated.

Germanate TbGaGe2O7 has been obtained from the initial Tb2O3, Ga2O3, and GeO2 oxides by the solid-phase synthesis. The germanate structure has been established by X-ray diffraction. Room-temperature luminescence spectra of the compound have been recorded. The effect of temperature on the heat capacity of the oxide compound has been investigated by differential scanning calorimetry. The thermodynamic properties of the compound have been calculated from the experimental Cp = f(T) data.

Single-phase LuGaTi2O7 samples have been prepared by solid-state reaction in a starting mixture of Lu2O3, Ga2O3, and TiO2 via sequential firing in air at temperatures of 1273 and 1573 K. The crystal structure of the lutetium gallium dititanate has been determined by the Rietveld method (profile analysis of X-ray diffraction patterns of polycrystalline powders): sp. gr. Pcnb; a = 9.75033(13) A, b = 13.41425(17) A, c = 7.29215(9) A, V = 957.32(2) A3, d = 6.28 g/cm3. The heat capacity of LuGaTi2O7 has been determined as a function of temperature by differential scanning calorimetry in the range 320–1000 K. The Cp(T) data thus obtained have been used to calculate the principal thermodynamic functions of the oxide compound.

The Pb10 – xBix(GeO4)2 + x(VO4)4 – x (x = 0–3) compounds with an apatite structure have been obtained for the first time from the initial PbO, Bi2O3, GeO2, and V2O5 oxides by the solid-state synthesis in the temperature range of 773–1073 K. The structure of the compounds has been determined by X-ray diffraction analysis. The effect of temperature on specific heat of the synthesized compounds has been investigated by differential scanning calorimetry. The thermodynamic properties of the compounds have been calculated from the experimental Cp = f(T) data.

In the process of studying the system Tl2MoO4–In2(MoO4)3–Hf(MoO4)2, a new thallium indium hafnium molybdate was found. The crystal structure of the molybdate Tl5InHf(MoO4)6 was determined in the centrosymmetric space group R $$ \overline{3} $$ c (a = 10.63893 (5) A, c = 38.1447(3) A; V = 3739.04 (4) A3, Z = 6). The structure is a three-dimensional framework consisting of alternating (Hf,In)O6-octahedra connected by МоО4-tetrahedra. Each octahedron has common vertices with tetrahedra. The atoms arranged in this way form channels extended along with the a and b axes, in which thallium atoms are located. The conductivity behavior of Tl5InHf(MoO4)6 ceramics was studied in the temperature range from 300 to 870 K. The conductivity of the heavy cations of thallium is activated with increasing temperature.

The europium gallium germanate EuGaGe2O7 has been prepared by solid-state reaction in air in the temperature range 1273–1473 K using a stoichiometric mixture of Eu2O3, Ga2O3, and GeO2. Its crystal structure has been determined by X-ray diffraction (sp. gr. P21/c, a = 7.1693(7) A, b = 6.57008(6) A, c = 12.7699(1) A, β = 117.4522(5)°, V = 533.768(8) A3). The heat capacity of polycrystalline samples has been determined by differential scanning calorimetry in the temperature range 350–1053 K and the experimental data have been used to calculate the thermodynamic properties (enthalpy increment, entropy change, and reduced Gibbs energy change) of EuGaGe2O7.

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).

The structural, thermal, static magnetic, and resonance properties of the low-dimensional NaCuFe2(VO4)3 compound obtained by the solid-phase synthesis have been investigated. In the temperature range of 110–300 K, the electron spin resonance in the X band with a g factor of 2.008 has been detected. The magnetic properties of a sample with a high frustration level in the paramagnetic, antiferromagnetic, and disordered states have been examined. A shift of the Neel temperature to the high-temperature region in an external magnetic field has been observed. The origin of the disordered magnetism in NaCuFe2(VO4)3 are discussed. The features of substitution of sodium for lithium on the physical properties of the ACuFe2(VO4)3 (A = Na, Li) system have been established. It is shown that the chemical pressure changes the crystal lattice parameters, spacings between magnetic ions, and crystallite size, which is reflected in the physical properties of the material.

The SmGaGe2O7 oxide material has been obtained from initial Sm2O3, Ga2O3, and GeO2 oxides by solid-phase synthesis with annealing in air in the temperature range of 1273–1473 K. The structure of the investigated germanate (sp. gr. P21/c, a = 7.18610(9) A, b = 6.57935(8) A, and c = 12.7932(2) A) has been established by X-ray diffraction and the high-temperature heat capacity has been determined by differential scanning calorimetry. Using the experimental data on Cp = f(T), the thermodynamic properties of the compound have been calculated.

A magnetic resonance method is applied to the investigation of a number of isostructural diamagnetic compounds ABO3 (A = Sc, In, and Ga) with small additions of Cr3+ ions (S = 3/2) sufficient to observe single-ion and pair spectra. It is shown that the resonance spectra for isolated Cr3+ ions can be described to a good accuracy by the ordinary axial spin Hamiltonian for 3d ions in octahedral oxygen environment. The parameters of the spin Hamiltonian are determined for single Cr3+ ion and Cr3+–Cr3+ pair. Lattice distorsions of the parent ABO3 crystals caused by the Cr3+ impurities is discussed.

We have investigated the spin and oxidation states of Co in Co3BO5 using x-ray magnetic circular dichroism (XMCD) and dc magnetic susceptibility measurements. At low temperatures, XMCD experiments have been performed at the Co K-edge in Co3BO5 and Co2FeBO5 single crystals in the fully ferrimagnetically ordered phase. The Co (K-edge) XMCD signal is found to be related to the Co2+ magnetic sublattices in both compounds, providing strong experimental support for the low-spin (LS) Co3+ scenario. The paramagnetic susceptibility is highly anisotropic. An estimation of the effective magnetic moment in the temperature range 100–250 K correlates well with two Co2+ ions in the high-spin (HS) state and some orbital contribution, while Co3+ remains in the LS state. The crystal structure of the Co3BO5 single crystal has been solved in detail at the T range 296–703 K. The unit cell parameters and volume show anomalies at 500 and 700 K. The octahedral environment of the Co4 site strongly changes with heating. The generalized gradient approximation with Hubbard U correction calculations have revealed that, at low-temperatures, the system is insulating with a band gap of 1.4 eV, and the Co2+ ions are in the HS state, while Co3+ are in the LS state. At high temperatures (T > 700 K), the charge ordering disappears, and the system becomes metallic with all Co ions in 3d7 electronic configuration and HS state., We are grateful to the Russian Foundation for Basic Research (Project No. 20-02-00559) and President Council on Grants (Project No. МК-2339.2020.2) for supporting this paper. This paper was carried out within the state assignment of the Russian Ministry of Science and High Education via program “Quantum”’ (No. AAAA-A18-118020190095-4). We also acknowledge support by Russian Ministry of Education and Science via Contract No. 02.A03.21.0006. We acknowledge financial support from the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, Grant No. MAT2017-83468-R) and from the regional Government of Aragon (E12-20R RASMIA project).

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.

Core-shell Fe

Single crystals of SmAl

We report on the synthesis conductions and results of experimental investigations of the crystal structure and magnetic properties of a new magnetic compound YbFeTi2O7. According to the X-ray diffractometry data, the crystal structure of the investigated compound is described by the rhombic space group Pcnb with unit cell parameters of a = 9.8115(1) A, b = 13.5106(2) A, and c = 7.31302(9) A and atomic disordering in the distribution of iron ions Fe3+ over five structural sites. The magnetic measurements in the lowtemperature region revealed a kink in the temperature dependence of the magnetic moment and its dependence on the sample magnetic prehistory. The experimental results obtained suggest that with a decrease in temperature the sample passes from the paramagnetic state to the spin-glass-like magnetic state characterized by a freezing temperature of T f = 4.5 K at the preferred antiferromagnetic exchange coupling in the sample spin system. The chemical pressure variation upon replacement of rare-earth ion R by Yb in the RFeTi2O7 system does not change the crystal lattice symmetry and magnetic state.

The LiGaTe 2 (LGT) single crystal up to 12 mm in size was grown in 3 stages: synthesis, homogenization and crystal growth. The initial charge with 6–10 at% excess of Li 2 Te was produced by melting elementary Li, Ga and Te components. The homogenization step was carried out with the maximum value of melt overheating not exceeding 50 K. The Bridgman-Stockbarger technique was used to grow the LiGaTe 2 crystals. A furnace of special design, providing the enlargement of LGT linear size, was used. At each stage (synthesis, homogenization and crystal growth) the DTA analysis was carried out: during heating and cooling we recorded peaks corresponding to melting or crystallization of different components in the charge, such as Te 0 (melting point 739 K) and LiGaTe 2 (melting point 945 K) and in the as-grown crystal. The XRD analysis was also carried out at each technological stage. The analysis showed that there are 2 side phases (Te 0 , Ga 2 Te 3 ), and their phase contents decrease to 1.2 at%, while the LiGaTe 2 phase increases up to 93.5 at% during the homogenization process. The synthesized charge composition was analyzed by flame photometry (for Li) and atomic absorption (for Ga and Te). To provide a flat crystallization front and optimal conditions for crystal growth, spatial distribution of thermal field in the furnace was simulated. Transmission spectrum was recorded for the as-grown LGT crystal.

The effect of replacing manganese ions on the structural, dielectric, transport, and magnetic properties of Bi2(Sn0.9Mn0.1)2O7 has been studied and the correlation between them has been determined. The change in the type of thermal processes and the thermopower sign upon polymorphous transitions were detected by differential scanning calorimetry. The paramagnetic Curie temperature and the antiferromagnetic interaction were determined in the martensite and austenite phases. The type of current carriers has been established.

The indirect-coupling model is used to analyze the exchange magnetic structure of Pb3Mn7O15 in the hexagonal setting. The ratios of manganese ions Mn4+/Mn3+ in each nonequivalent position are determined. Pb3(Mn0.95Ge0.05)7O15 and Pb3(Mn0.95Ga0.05)7O15 single crystals are grown by the solution–melt method in order to test the validity of the proposed model. The structural and magnetic properties of the single crystals are studied. The magnetic properties of the grown single crystals are compared with those of nominally pure Pb3Mn7O15.

Complex studies have been performed for the structural, static magnetic, and resonance properties of a new magnet LiCuFe2(VO4)3 prepared by solid-phase synthesis. The temperature dependence of the susceptibility has an anomaly at temperature T max = 9.6 K. At high temperatures, the LiCuFe2(VO4)3 sample is in the paramagnetic state described by the Curie–Weiss law at T > 50 K and mainly determined by iron ions with effective magnetic moment μeff(exp) = 8.6μB per formula unit. At low temperatures, a long-range magnetic order is observed in the magnetic subsystem of the sample; the order is predominantly characterized by the antiferromagnetic exchange interaction and high frustration level. The exchange interaction parameters are estimated in a six-sublattice representation of the LiCuFe2(VO4)3 magnet. It is shown that the LiCuFe2(VO4)3 compound is an antiferromagnet with strong intrachain and frustrating interchain exchange interactions.

The properties of NaxLi1–xFeGe2O6 (x = 0.1–0.9) solid solutions obtained via a solid-phase synthesis have been measured by X-ray diffraction, calorimetry, and magnetic method. The order–disorder transformations in low-dimensional NaxLi1–xFeGe2O6 (x = 0.1–0.9) spin systems with predominately antiferromagnetic exchange interaction have been revealed in the low-temperature susceptibility dependences. The study of thermal and physical properties has confirmed that substituting the sodium ions with the lithium ones induces the first-order structural phase transitions of the displacement type which are characterized by a symmetry change in monoclinic crystals from high-temperature C2/c space group to low-temperature P21/c space group.

The LiGaTe

The correlation of the electrical, magnetic, and structural properties of Bi2(Sn1 − xCrx)2O7 solid solutions has been investigated for two compositions with x = 0.05 and 0.1. The temperatures of structural phase transitions and anomalies of the electrical resistance in the vicinity of these transitions at temperatures in the range 300 K < T < 1000 K have been determined using differential scanning calorimetry. It has been found that the thermoelectric power changes sign upon the polymorphic transition. The paramagnetic Curie temperature and the chromium-ion spin have been found from the temperature dependence of the magnetic susceptibility in the range 4 K < T < 1000 K and from the field dependence of the magnetic moment in magnetic fields of up to 6 T.

Single crystals of silicon double salt (NH

The Bi2Sn2O7 compound existing simultaneously in two polymorphic modifications, namely, orthorhombic and cubic, has been synthesized for the first time by solid-phase synthesis. The dielectric and electrical properties of the compound have been studied in the temperature range 100 K < T < 500 K. Anomalies in the temperature dependences of the electrical resistivity and the permittivity (imaginary and real parts) have been found at both low and high temperatures. These features are explained in terms of the model of martensitic phase transitions.

In this paper we are reported about a peculiarity of the crystal structure and the magnetic state of TmFeTi2O7. The compound TmFeTi2O7 has been synthesizedusing the solid-phase reaction method. Using X-ray diffraction method the disorder in the distribution of the iron ions over five nonequivalent crystal sites was observed, also the populations of the iron atoms positions were determined. We show that below Tf = 6 K the magnetization of TmFeTi2O7 depends on the magnetic history of the sample. There are indications for spin glass state. This results allow us to assume the state of spin glass is realized below freezing temperature Tf = 6 K in TmFeTi2O7.

HoAl3-xGax(BO3)4 (0 ≤ х ≤ 3) single crystals were grown from Bi2Mo3O12Li2MoO4-based fluxes. Structural inversion twinning depending on crystal composition and heat treatment conditions is revealed. It is established that upon substitution of Ga3+ ions for Al3+ the magnetic characteristics of the crystals do not significantly change, while the magnetoelectric polarization decreases with increasing gallium content. The measured value and sign of polarization charges depend on a ratio between the left and right isomers.

The structure, infrared and Raman spectra, heat capacity, and thermal expansion have been investigated. It has been shown that, down to liquid-helium temperatures, the cubic pyrochlore structure of the CsMnMoO3F3 and CsZnMoO3F3 oxyfluorides remains stable. The influence of cation substitution on individual features of the properties of the oxyfluorides has been analyzed.

New single crystals of Li8FeSm22O38 have been grown by spontaneous crystallization from a solution in melt. The structure of these crystals has been determined: it corresponds to the space group Im \(\bar 3\) m. Investigations of the magnetic susceptibility in the ranges of temperatures 2–24 K and fields up to 50 kOe have revealed a magnetic transition near 3 K. The temperature behavior of the magnetic susceptibility of the two-level system has been simulated. The results of the simulation agree with the experimental data. The resonance properties of Li8FeSm22O38 have been studied in the temperature range 100–300 K.

The CrxMn1 − xS single crystals have been synthesized based on manganese monosulfide as a result of cation substitution, and their magnetic properties have been studied. It has been established that the CrxMn1 − xS solid solutions with a face-centered cubic NaCl structure are formed in the concentration region 0 ≤ x < 0.3. The unit cell parameter of the solid solution decreases as the degree of substitution increases due to the variation in the ionic radius of cations. These substances are antiferromagnets. An increase in the degree of cation substitution in the CrxMn1 − xS solid solutions is accompanied by a decrease in the number of 3d electrons in the d shell of manganese monosulfide and causes a decrease in the magnetic transition temperature from 149 K (x = 0) to 96 K (x = 0.29), which differs from previously known results.

The structures of two phases of the (NH4)3Nb(O2)2F4 crystal, namely, the parent cubic phase and the most distorted low-temperature phase, have been determined from data of an X-ray diffraction experiment performed for a powder sample. The profile and structural parameters have been refined according to the procedure implemented in the DDM program. The results obtained have been discussed with invoking the group-theoretical analysis of the complete order parameter condensate, which takes into account the critical and noncritical atomic displacements and allows the interpretation of the obtained experimental data. It has been found that the most probable sequence of structural transformations occurring in the crystal can be schematically represented in the following form: $$\begin{gathered} Fm\bar 3m\xrightarrow[{(\eta 1,\eta 1,\eta )}]{{\Gamma _5^ + (11 - 7)}}C2/m\xrightarrow[{(\eta 1,\eta 1,\eta )(0,0,0,0,0,\xi ,0 - \xi ,0,0,0,0)}]{{\Gamma _5^ + (11 - 7) \otimes \Sigma _2 (4 - 2)}}P2_1 /m \hfill \\ \xrightarrow[{(\eta 1,\eta 1,\eta )(0,0,0,0,0,\xi ,0 - \xi ,0,0,0,0)(0,0,0,0,0,\varepsilon ,0,\varepsilon ,0,0,0,0)}]{{\Gamma _5^ + (11 - 7) \otimes \Sigma _2 (4 - 2) \otimes \Sigma _3 (4 - 3)}}P2_1 /n. \hfill \\ \end{gathered} $$

The structure of the low-temperature triclinic phase of the (NH4)3WO3F3 crystal has been determined and the structure of the cubic phase of this crystal has been refined from data of an X-ray diffraction experiment performed for a powder sample. The profile and structural parameters have been refined according to the procedure implemented in the DDM program. The results obtained have been discussed with invoking the group-theoretical analysis of the complete order parameter condensate, which takes into account the critical and noncritical atomic orderings and allows one to interpret the obtained experimental data. It has been found that the symmetry transformation in the crystal can be schematically represented in the following form: \(Fm\bar 3m(Z = 4) \to P\bar 1(Z = 1) \to P\bar 1(Z = 6)\). This transformation is accompanied by the complete ordering of WO3F3 polyhedra and the displacement of NH4 ions.

The structures of two phases of the (NH4)2KWO3F3 crystal have been determined from X-ray diffraction data obtained for a powder sample. The profile and structural parameters have been refined according to the procedure implemented in the DDM program. The results obtained have been discussed with invoking the group-theoretical analysis of the complete order parameter condensate, which takes into account the critical and noncritical atomic orderings and allows one to interpret the experimental data. It has been found that the symmetry transformation in the crystal can be schematically represented in the form \(Fm\bar 3m\xrightarrow[{(\eta ,0,0,0,0,0)}]{{10 - 10(X_5^ - )}}Pmmn\). This transformation is accompanied by the ordering of WO3F3 polyhedra and the displacement of NH4 and K ions.

According to the results of calorimetric and structural studies, the Fm{ie1202-1}m phase in K2NaMoO3F3 remains stable at least to 100 K. No ferroelectric transformation assumed earlier has been revealed in a series of Rb2KMoO3F3 samples prepared using various technologies. Only a phase transition of nonferroelectric origin has been observed near 195 K, and its thermodynamic characteristics have been determined. An analysis of the stability of the cubic structure of molybdenum fluorine-oxygen elpasolites-cryolites has been performed in the framework of the hypothesis on strengths of interatomic bonds. The barocaloric effect in Rb2KMoO3F3 has been estimated.

The thermal and dielectric properties of the (NH4)2NbOF5 oxyfluoride have been investigated. It has been established that the structural phase transitions Cmc21 → C2 → Ia observed at the temperatures T1 = 258.0 K and T2 = 218.9 K exhibit a nonferroelectric nature. The hydrostatic pressure, which stabilizes the initial phase and destabilizes the low-temperature phase, hardly affects the temperature range of stability of the intermediate phase. The model of sequential ordering of the structural elements due to phase transitions has been analyzed using experimental data on the entropies of the phase transitions.

Crystal structures of (NH4)2KWO3F3 at 298 K and 113 K were solved from X-ray powder diffraction data and refined by the Rietveld technique. The compound is isostructural with elpasolite K2NaAlF6 at room temperature with space group Fm-3m, a=8.95850(5) Å, V=718.961(7) Å3, Z=4, Dx=3.363 g/cm3, and MW=364.02. The structure was refined over 18 parameters to Rwp=12.6%, Rp=10.9%, Rexp=5.03%, and RB=3.27% from 40 independent reflections. (NH4)2KWO3F3 was transformed upon cooling to a ferroelastic monoclinic phase with space group P21/n, a′=6.3072(3) Å, b′=6.3028(3) Å, c′=8.9882(3) Å, β′=90.242(2)°, V=357.30(3) Å3, Z=2, and Dx=3.383 g/cm3. The low-temperature structure at 113 K was refined over 28 parameters to Rwp=20.9%, Rp=21.3%, Rexp=12.5%, and RB=6.93% from 453 independent reflections.