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We report the influence of anisotropy and texture on elasto(ElCE)- and baro(BCE)-caloric effects in single-crystal and ceramic (NH4)2SO4. Inverse extensive and intensive ElCE in ceramics, (ΔSElCE)cer = 87 J/kg·K; ΔTAD = - 11.6 K), as well as in a single crystal along the ferroelectric axis a, (ΔSElCE)a = 115 J/kg·K; (ΔTAD)a = - 16 K, significantly exceed BCE, ΔSBCE = 75 J/kg·K; ΔTAD = - 9.8 K, even at low pressure ~ 0.3 GPa. Caloric parameters of ammonium sulphate are comparable with those for promising solid-state refrigerants.

We performed an analysis of the phase diagrams and piezocaloric properties of fluorine–oxygen ferroics (NH4)2MeOxF6-x (Me: W, Mo, Nb; x: 1, 2) with orthorhombic symmetry. The nature of the influenc...

The thermoelectric properties of the NdCoO3 and SmCoO3 rare-earth cobalt oxides with a perovskite structure have been investigated in a wide temperature range. It is shown that, in the low-temperature region, the thermal conductivity of the compounds has a sharp maximum and the electrical conductivity of the samples increases with temperature, whereas the Seebeck coefficient behaves nonmonotonically with increasing temperature. The SmCoO3 oxide is characterized by the positive thermopower over the entire investigated range with a sharp growth in the low-temperature region, attaining the maximum value (S ≈ 1000 μV/K) near room temperature, and a further decrease. It has been established that, in the NdCoO3 oxide, the Seebeck coefficient changes its sign, which was rarely observed in the La-based compounds and is atypical of the undoped rare-earth cobalt oxides. The thermopower maximum obtained at a temperature of 450 K is 400 μV/K. The regions of the fastest growth of the thermoelectric power factor correspond to the anomalies caused by the spin transition of Со3+ ions and the dielectric–metal transition.

In SrxBa1−xNb2O6 crystals (x = 0.33, 0.5, 0.61, and 0.75), temperature dependences of spontaneous polarization, strain, elastic modules, and second harmonic generation (SHG) signal are compared. It...

In this study, the conventional and inverse barocaloric effects (BCE) in ferroelectric NH4HSO4 are reported. Maximum extensive and intensive BCE near order–disorder phase transition can be achieved at low pressure p ≤ 0.1 GPa. Large thermal expansion of the crystal lattice plays a very important role in the developing conventional BCE and conversation between BCE of different sign in the narrow temperature range.

Results are presented from studying the X-ray diffraction, heat capacities, dielectric permittivities, and coefficients of the volumetric thermal expansion of composite materials obtained by embedding of Rb2ZnCl4 salt into the porous matrices of borosilicate glass with average pore diameters of 46 and 320 nm in the temperature range of 120–350 K. The temperatures of transitions to the incommensurate and ferroelectric phases are determined, along with the freezing temperature of the mobility of domain boundaries in Rb2ZnCl4 particles. A substantial increase in their Curie temperature is observed.

Heat capacity, thermal dilatation, permittivity and sensitivity to external pressure of ( NH 4 ) 3 SiF 7 were studied. Due to the absence of cubic phases P m 3 ¯ m and P a 3 ¯ , a strong decrease in the total entropy change ∑ Δ S i = 19 J/mol K associated with four successive transformations P 4 / m b m ↔ P b a m ↔ P 2 1 / c 11 ↔ P 1 ¯ ↔ P 12 1 / c 1 was found in silicate in comparison with other double fluoride salts ( NH 4 ) 3 MeF 7 (Me: Ge, Ti, Sn) Using analysis of the excess heat capacity in the framework of the thermodynamic theory, the entropies associated with each individual phase transition were determined. In accordance with the entropic parameters, the complete ordering of the structural elements occurs in the monoclinic phase P 2 1 / c 11 . Further change in symmetry is associated with small entropy changes which prove insignificant displacement of structural units. A T − p phase diagram was constructed and good agreement was found between measured and calculated baric coefficients.

The effect of cationic and anionic orderings on the crystal structure and magnetic properties of substituted rare-earth cobaltites Gd0.1Sr0.9CoO3 – δ was studied using X-ray diffraction, measurement of XANES spectra, and magnetic and thermodynamic characteristics. The effects of ordering cause a decrease in symmetry to tetragonal and a distortion of the coordination octahedra of CoO6. Anomalous magnetic and thermodynamic quantities are observed at 260 and 360 K, respectively, for disordered and ordered samples. The XANES spectra measured at the CoK edge did not reveal a noticeable shift in the absorption edge compared with the spectrum of original GdCoO3. This suggests that the charge compensation process is associated not only with a change in the electronic state of cobalt ions, but also with the emergence of holes in the 2p states of oxygen.

The paper presents the results of detailed studies of the thermal expansion of (1-x)PbNi1/3Nb2/3O3-xPbTiO3 solid solutions with x = 0-0.8. The anomalous and lattice contributions to deformation and...

The crystals of (ND4)3VO2F4 with a high degree of deuteration (~87%) have been grown and a significant (~1.5%) increase in the unit cell volume has been detected. The decrease in the chemical pressure leads to a change in the sequence of phase transitions due to wedging-out of one of the rhombic phases observed in (NH4)3VO2F4. The thermal physical studies have been carried out and the entropies, deformations, and pressure coefficients related to structural transformations have been determined. The T–p phase diagram has been built and the pressure and temperature boundaries of crystal phase stability have been determined. The dielectric studies demonstrate a nonferroelectric nature of the phase transitions in (ND4)3VO2F4. The experimental and model entropies are compared. Based on the decrease in the entropy as a result of deuteration, a hypothesis on significant but not limited anharmonism of vibrations of ammonia tetrahedral corresponding to the disordering is proposed.

Near the Curie temperature TC = 30.3 K, the temperature dependences of the magnetization and heat capacity of a single-crystal ferromagnet PbMnBO4 in the magnetic fields of 1, 3, 10 and 30 kOe are studied. In the strong magnetic fields, both the magnetic contribution to the specific heat and the nonlinearity of the field dependences of the magnetization are maintained up to the temperatures exceeding TC more than twice. It is assumed that in PbMnBO4 the difference between TC, the paramagnetic Curie temperature θ = 49 K and the broad temperature region above TC where the magnetic contribution to the specific heat is significant is due to the quasi-one-dimensional character of the magnetic structure of this ferromagnet. Using both the estimation of TC from the Ginzburg-Landau field theory and the θ value, the total exchange interaction parameters 2J ≈ 40.4 K (intrachain) and z′J′ ≈ 8.8 K (interchain) are determined, with z′ = 4 being the number of neighboring chains. The estimation shows that the Ginzburg-Landau field theory describing the quasi-one-dimensional behavior of PbMnBO4 is well applicable in the temperature range from to T = S2J ≈ 80 K. Above this temperature, the mean field approximation with the exchange parameter λθ based on the paramagnetic Curie temperature θ describes well the experimental temperature dependences of the magnetization in the strong magnetic field and the specific heat is determined by the lattice contribution.

Crystalline samples of the (La1 – yEuy)0.7Pb0.3MnO3 (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.

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).

A detailed study of the effect of temperature and pressure on heat capacity, entropy and hysteresis phenomena near the ferroelectric phase transition in ammonium sulfate (AS) was performed. An analysis of experimental results within the framework of the phenomenological theory showed that taking into account the temperature-dependent part of the anomalous entropy leads to a significant increase in the barocaloric effect (BCE). The maximum values of extensive and intensive BCE near the tricritical point are outstanding: &Delta, S B C E m a x &asymp, 85 J/kgK, &Delta, T A D m a x &asymp, 12 K and can be achieved at low pressure &sim, 0.5 GPa.

The influence of an electric field on the phase transition parameters and the electrocaloric response in ferrielectric (NH4)2SO4 was studied using a universal multifunctional adiabatic calorimeter. The very low sensitivity of the phase transition temperature to the electric field, dT0/dE ≈ 5·10-3 K/(kV/cm), was shown to be the reason for the low maximum value of the intensive electrocaloric effect, ΔTAD, at E = 11 kV/cm: direct and indirect measurements gave the values of 0.01 K and 0.05 K, respectively. However, the extensive electrocaloric response revealed by an analysis of the entropy–temperature–electric field phase diagram was relatively large, with a value of ΔSECE ≈ − 19 J/kg K, due to the significant contribution of a jump in the excess entropy to the total value, δS0/ΔS0 ~ 0.37. An additional irreversible increase in temperature under the electric field was associated with the Joule effect and indicated a significant electrical conductivity of ammonium sulphate.

Dielectric relaxation and phase transition behaviors in (1–x)Pb(Zn1/3Nb2/3)O3-xBaTiO3 (PZN-BT) binary solid solutions have been systematically studied in this paper. All the compositions display a pure pseudo-cubic perovskite structure. As the BT contents increase from 10 mol% to 70 mol%, the phase transition peak becomes broader and broader, accompanying with decreases of ɛm (the maximum dielectric permittivity) and Tm (the temperature corresponds to the ɛm). Nevertheless, an abnormal increase of ɛm and Tm occurs when the BT contents exceeds 70 mol%, forming a “U” shaped curve of the compositional dependence of ɛm and Tm. Moreover, it is indicated from the new glass model fitting results that the characteristic parameter p, which represents the degree of relaxation, also shows a similar “U” shaped variation curve. Similarly, as shown in polarization-electric field loops, both remnant polarizations (Pr) and coercive fields (Ec) display the “U” shaped curve as the BT content changes from 10 mol% to 80 mol%. Finally, according to the similar variation of these key parameters mentioned above, a polarization mismatched model, which describes the destruction and reestablishment of the long range order arrangement in solid solutions composed of two kinds of ferroelectrics, is proposed to illustrate the underlying mechanism. In this PZN-BT system or other similar ABO3-type perovskite relaxors, the competition between the A-O and B-O coupling plays an important role to form the “U” shaped evolution of these key parameters.

The effect of restricted geometry on specific heat capacity and thermal expansion of the triglycine sulfate (TGS)–borosilicate glass composites have been studied first. A decrease in the entropy and temperature of the P21 ↔ P21/m phase transition in the TGS component with decreasing the glass matrix pore diameter at the invariable specific heat and thermal expansion coefficient has been observed. The estimates are indicative of the minor effect of internal pressure on the TGS pressure coefficient dTC/dp in the composites.

A study of heat capacity, thermal dilatation, susceptibility to hydrostatic pressure, permittivity and polarization loops was carried out on NH4HSO4–porous glass nanocomposites (AHS + PG) as well as empty glass matrices. The formation of dendrite clusters of AHS with a size, dcryst, exceeding the pore size was found. An insignificant anisotropy of thermal expansion of AHS + PG showing statistically uniform distribution of AHS with random orientations of nanocrystallites over the matrix was observed. The effect of internal and external pressures on thermal properties and permittivity was studied. At the phase transition P-1 ↔ Pc, a strongly nonlinear decrease in the entropy ΔS2 and volume strain (ΔV/V)T2 was observed with decreasing dcryst. The linear change in temperatures of both phase transitions P-1 ↔ Pc ↔ P21/c under hydrostatic pressure is accompanied by the expansion of the temperature range of existence of the ferroelectric phase Pc, while this interval narrows as dcryst decreases.

Fe substitution effect on the magnetic behavior of Mn2−xFexBO4 (x = 0.3, 0.5, 0.7) warwickites has been investigated combining Mossbauer spectroscopy, dc magnetization, ac magnetic susceptibility, and heat capacity measurements. The Fe3+ ions distribution over two crystallographic nonequivalent sites is studied. The Fe introduction breaks a long-range antiferromagnetic order and leads to onset of spin-glass ground state. The antiferromagnetic short-range-order spin correlations persist up to temperatures well above TSG reflecting in increasing deviations from the Curie-Weiss law, the reduced effective magnetic moment and “missing” entropy. The results are interpreted in the terms of the progressive increase of the frustration effect and the formation of spin-correlated regions.

Precise direct measurements of the intensive electrocaloric effect (ECE) in commercial multilayer capacitor based on doped BaTiO3 were performed using an adiabatic calorimeter. High reversibility of ECE studied under equilibrium thermal conditions was observed. The nonequilibrium thermal conditions caused by fixing the temperature of one of the ends of the linear EC element lead to the predominance of ECE when the electric field is turned off. The heat flow through the EC element appearing under the influence of a periodic electric field and depending on its frequency makes it possible to create a cooling cycle without thermal keys.

The XANES spectra (X-ray absorption near-edge spectra) at the K edge of Co and the L3 edge of Gd in polycrystalline Gd0.4Sr0.6CoO3 – δ rare-earth oxides with an ordered and disordered distribution of Gd3+ and Sr2+ cations over the A sites in the crystal lattice are measured. The results of XANES measurements do not reveal a noticeable shift in the absorption edge with increasing Sr concentration as compared to the GdCoO3 parent composition. The measured temperature dependences of the heat capacity of polycrystalline ordered and disordered samples and a single-crystal ordered Gd0.4Sr0.6CoO2.85 sample exhibit two Schottky anomalies. These anomalies are thought to be related to the high-spin state of the Co3+ ions in the pyramidal environment caused by oxygen deficiency and to the magnetic state of oxygen p electrons induced by the doping-assisted generation of a hole in the 2p state. The absence of a noticeable shift in the absorption edge and the presence of two Schottky anomalies support the fact that the charge state of cobalt remains unchanged in the compounds under study.

Needle-shape single crystals of Co5/3Nb1/3BO4 warwickite were grown using the flux technique. X-ray diffraction measurements have revealed an orthorhombic structure (Sp. Gr. Pbnm) where the octahedral M1 site is occupied by a mixture of Co2+/Nb5+ ions and the M2 site is exclusively filled by Co2+ ions. Using dc magnetization measurements it was established that the new material undergoes two magnetic transitions: an antiferromagnetic transition at TN1 = 27 K and a ferrimagnetic one at TN2 = 14 K, below which a hysteresis cycle opens. Both magnetic transitions are marked by anomalies in the specific heat. High magnetic anisotropy with c-axis as a hard magnetization direction was detected., This work has been financed by the Russian Foundation for Basic Research (project no. 20-02-00559). We acknowledge financial support from the Spanish MINECO DWARFS project MAT2017-83468-R and Gobierno de Aragón (Group, E12-20R).

Heat capacity, thermal dilatation, susceptibility to hydrostatic pressure and dielectric properties associated with succession of three phase transitions below room temperature in double fluoride salt (NH 4 ) 3 GeF 7 were studied. A possible transformation into the parent Pm -3 m cubic phase was not observed up to the decomposition of compound. Nonferroelectric nature of structural distortions was confirmed. The DTA under pressure studies revealed a high temperature stability of two phases: P 4 /mbm and Pbam . The entropies of the phase transitions agree well with the model of structural distortions. Analysis of the thermal properties associated with the individual phase transitions in the framework of thermodynamic equations has shown a high reliability of the data obtained.

We report the conventional and inverse barocaloric effect (BCE) in ferroelastics (NH4)3NbOF6 and (NH4)3TiOF5. We found a rich set of the structural phase transitions and triple points on the T − p phase diagrams. Extensive and intensive BCE near some transformations and around triple points in both crystals are outstanding (|∆ SBCEmax | = 30–100 J/kg·K; |∆ TADmax | = 4–22 K) and can be achieved at low pressure 0.01–0.60 GPa. PACS 62.50.-p 65.40.-b 81.30.-t

Heat capacity, thermal dilatation, sensitivity to pressure and permittivity of NH4HSO4 embedded into glass matrices with a pore size of 320 nm and 46 nm were studied. Large difference in the thermal expansion of both glass and ferroelectric leads to a “clamped” state of NH4HSO4 in nanocomposites and to the phase transition temperatures change. The restricted geometry does not effect on the order of successive transformations in NH4HSO4 but is accompanied by a significant reduction in entropy of phase transitions. The behavior of DTA-signal and permittivity show the expansion of the temperature range of the ferroelectric phase under hydrostatic pressure.

The direct and indirect measurements of intensive electrocaloric effect in a triglycine sulfate ferroelectric crystal are performed under equilibrium and nonequilibrium thermodynamic conditions implemented in the adiabatic calorimeter. The effect of the electric field parameters (frequency, profile, and strength) on the value of the effect and degree of its reversibility are studied. The difference between the temperature variation values in a switched-on and switched-off dc field under quasi-isothermal conditions is established. The low-frequency periodic electric field induces the temperature gradient along the electrocaloric element and heat flux from its free end to the thermostated base. A significant excess of the field switching-off rate over the switching-on rate leads to a noticeable intensification of the cooling effect.

Thermal and dielectric studies of (ND4)2MoO2F4 crystals undergoing successive phase transitions at T1 = 272 K and T2 = 181 K showed that deuteration is accompanied by an increase in the chemical pressure in the crystal lattice (Δp ≈ 0.02 GPa), which shifts the Cmcm ↔ Pnma transformation for the first order to the tricritical point. The direct participation of ammonium groups in the mechanism of structural distortions is demonstrated by a decrease in the entropy of the high-temperature phase transition (ΔS1 = R ln 6.0). An external hydrostatic pressure leads to an expansion of the temperature interval of the intermediate antiferroelectric Pnma phase. The triple point on the T–p phase diagram, where the Cmcm, Pnma, amd Pnma* phases coexist, can be realized at a negative pressure of ptrp ≈ −0.8 GPa.

The thermal, optical, and dielectric properties of fluoride Rb2TaF7 were investigated. It was observed that the variation in chemical pressure in fluorides A 2 +TaF7 caused by the cation substitution of rubidium for ammonium does not affect the ferroelastic nature of structural distortions, but leads to stabilization of the high- and low-temperature phases and enhancement of birefringence. The entropy of the phase transition P4/nmm ↔ Cmma is typical of the shift transformations, which is consistent with a model of the initial and distorted phase structures. The anisotropy of chemical pressure causes the change of signs of the anomalous strain and baric coefficient dT/dp of Rb2TaF7 as compared with the values for its ammonium analog.

A study of heat capacity, thermal dilatation, permittivity, dielectric loops and susceptibility to hydrostatic pressure was carried out on quasi-ceramic samples of NH 4 HSO 4 obtained from an aqueous solution as well as the melt. The main parameters of the successive P 2 1 / c ( T 1 ) ↔ Pc ( T 2 ) ↔ P 1 phase transitions did not depend on the method of preparation of the samples, and were close to those determined in previous studies of single crystal and powder, except for the sign and magnitude of the baric coefficient for T 2 . Direct measurements of the pressure effect on the permittivity and thermal properties showed d T 2 /d p = −123 K·GPa −1 , which is consistent in terms of magnitude and sign with the baric coefficient evaluated using dilatometric and calorimetric data in the framework of the Clausius-Clapeyron equation. Thus, the temperature region of the ferroelectric Pc phase existence is extended under pressure. A strong decrease in the entropy jump at the Pc ↔ P 1 transformation with an increase in pressure, and the linear dependence of T 2 on pressure, indicate that an increase in pressure shifts this phase transition towards the tricritical point on the T – p phase diagram. A significant barocaloric effect was found in the region of the Pc ↔ P 1 phase transition.

International audience; 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: ΔTmaxAD = 29 mK (E = 2.0 kV/cm) for nBT and ΔTmaxAD = 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.

X-ray, calorimetric and dilatometric studies of RbCdZrF7 revealed the existence of the second order phase transition Cmcm ↔ P21/m at T0 = 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.

X-ray, calorimetric and dilatometric studies of K3ZrF7 revealed the existence of the phase transition Fm-3m ↔ R-3m at T0 = 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)2KZrF7 - (NH4)3ZrF7 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.

Temperature dependences of specific heat C p(T) and coefficient of thermal expansion ;(T) for Na0.95Li0.05NbO3 sodium-lithium niobate ceramic samples are investigated in the temperature range of 100–800 K. The C p(T) and α(T) anomalies at T 3 = 310 ± 3 K, T 2 = 630 ± 8 K, and T 1 = 710 ± 10 K are observed, which correspond to the sequence of phase transitions N ↔ Q ↔ S(R) ↔ T2(S). The effect of heat treatment of the samples on the sequence of structural distortions was established. It is demonstrated that annealing of the samples at 603 K leads to splitting of the anomaly corresponding to the phase transition Q → R/S in two anomalies. After sample heating to 800 K, the only anomaly is observed in both the C p(T) and ;(T) dependence. Possible mechanisms of the observed phenomena are discussed.

We present the results of both direct measurements and modeling of the intensive electrocaloric effect ΔTAD in ferroelectric triglycine sulfate under nonequilibrium thermal conditions and periodically varying electric field. In the narrow region around the phase transition temperature TC, a visible difference was observed between the electrocaloric responses at applying ΔTON and removal ΔTOFF of the constant electric field: |ΔTON| |ΔTOFF| at T TC, respectively. The variation of the frequency and profile of the periodic electric field at T

Using the data about the temperature - pressure phase diagram and the phase transition entropy at ambient and high pressure, intensive and extensive barocaloric effects were analyzed in the region ...

Magnetic and thermodynamic properties of perovskite Gd0.2Sr0.8CoO3−δ, with ordered and disordered states of Gd and Sr in the A-sites of the crystal lattice have been studied revealing remarkable differences in the physical properties of the ordered and disordered states. The ordered samples have larger oxygen nonstoichiometry, heat capacity, and thermal expansion anomalies, and abnormal temperature dependence of the magnetization around 350 K.

Текст статьи не публикуется в открытом доступе в соответствии с политикой журнала. Calorimetric, dilatometric and differential thermal analysis studies were performed on (NH4)3SnF7 for a wide range of temperatures and pressures. Large entropy (δS0 = 22 J/mol•K) and elastic deformation (δ(ΔV/V) 0 = 0.89%) jumps have proven that the Pa-3 ↔ Pm-3m phase transition is a strong first order structural transformation. A total entropy change of ΔS0 = 32.5 J/mol•K agrees well with the model associated with the complete ordering of structural units in the Pa-3 phase, which are disordered in the Pm-3m phase, and is equal to the sum of entropy changes in the related material, (NH4)3TiF7, undergoing transformation between the two cubic phases through the intermediate phases. Hydrostatic pressure decreases the stability of the high temperature Pm-3m phase in (NH4)3SnF7, contrary to (NH4)3TiF7, characterised by a negative baric coefficient. The effect of experimental conditions on the chemical stability of (NH4)3SnF7 was observed.

Powder X-ray diffraction data for the crystal structure, phase composition, and molar specific heat for La1‒xGdxCoO3 cobaltites in the temperature range of 300–1000 K have been analyzed. The behavior of the volume thermal expansion coefficient in cobaltites with isovalent doping in the temperature range of 100–1000 K is studied. It is found that the β(T) curve exhibits two peaks at some doping levels. The rate of the change in the occupation number for the high-spin state of cobalt ions is calculated for the compounds under study taking into account the spin–orbit interaction. With the Birch–Murnaghan equation of state, it is demonstrated that the low-temperature peak in the thermal expansion shifts with the growth of the pressure toward higher temperatures and at pressure P ∼ 7 GPa coincides with the second peak. The similarity in the behavior of the thermal expansion coefficient in the La1–xGdxCoO3 compounds with the isovalent substitution and the undoped LnCoO3 compound (Ln is a lanthanide) is considered. For the whole series of rare earth cobalt oxides, the nature of two specific features in the temperature dependence of the specific heat and thermal expansion is revealed and their relation to the occupation number for the high-spin state of cobalt ions and to the insulator–metal transition is established.

Experimental study of temperature-pressure phase diagrams revealed a slight difference in the sensitivity to hydrostatic pressure of triglycine sulfate (TGS) as a bulk (single crystal, ceramics), and nanoparticles embedded in porous glass (TGS + PG). An analysis of the pressure sensitivity, dT0/p and dT0/dσ, and entropies of the P21 ↔ P21/m phase transition showed the closeness of the barocaloric parameters in various materials under study. Due to the strong anisotropy of thermal expansion, the piezocaloric effect under pressure along the axis c exceeds the barocaloric effect in single-crystal TGS. The reasons for the strong reduction in the electrocaloric effect in the ferroelectric component of nanocomposites compared with a single-crystal are discussed.

Single-phase samples of the layered perovskite-like cobalt oxides Ln0.2Sr0.8CoO3-δ (Ln = Sm, Gd, Dy) with the same oxygen nonstoichiometry index δ = 0.37 ± 0.01 were synthesized. All samples are characterized by a tetragonal unit cell with the space group I4/mmm. The structural, magnetic, electric transport and dilatation properties of the obtained samples are investigated. The studied samples are characterized by two anomalies in magnetic properties, a high-temperature maximum near Тm = 350 К with magnetic field hysteresis below Tm, and a diffuse peak in the intermediate temperature range, which shifts with ionic radius decrease of the rare-earth element to higher temperatures. The high-temperature maxima of the magnetic susceptibility correlate with anomalies in thermal expansion, heat capacity and the features in the temperature dependences of the electrical resistivity, pointing to a strong relationship between the structural, magnetic and electronic degrees of freedom. The given comparative analysis of the experimental data of various substituting rare-earth elements with the same oxygen nonstoichiometry has been carried out for the first time.

Heat capacity, thermal expansion, and sensitivity to the hydrostatic pressure of (NH4)2KZrF7 elpasolite are studied in a wide temperature range. The changes in deformation and entropy during successive phase transitions are determined: Δ(ΔV/V) = 3·10−4; ΔS = 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.

Mixed-valence hydrated fluoridotitanate with idealized formula FeTiF6·6H2O is the main product of fluoride processing of natural mineral ilmenite with hydrofluoric acid. Its single crystals were grown and optical and calorimetric experiments were carried out in the temperature range 200–350 K. Charge re-distribution, Fe2+Ti4+ to Fe3+Ti3+, in the compound studied was evident in XPS spectra. A first order ferroelastic phase transition occurs at temperatures T0↓ = 271.5 K, T0↑ = 274 ÷ 275.5 K with the symmetry change P 3 ‾ ↔ P 1 ‾ . The structural transformation is accompanied by the appearance of a very weak optical anisotropy in the slice (001), a small anomaly of the heat capacity (ΔS = 1.5 J/mol K = 0.2R), and positive baric coefficients dT0/dp ≈ 30 ± 10 K/GPa.

The α-MnS-based Fe X Mn 1− X S (0 x x = 0) to 200 K ( x = 0.29) observed in these materials is accompanied by a decrease in the NaCl-type cubic lattice parameters. It is established that at the symmetry transformation in the compositions with x = 0.25 and 0.29 the structural transition occurs, which is followed by the magnetic transition. These features make the Fe X Mn 1− X S solid solutions interesting for both fundamental study of the interrelation between the magnetic, electrical, and structural properties in MnO-type strongly correlated electron systems and application.

The main problems of the current state-of-the-art research into the caloric effects observed in oxygen ferroics, multiferroics, and composites, as well as the influence of different factors (anisotropy, dimensional parameters, direct and indirect interferroic interactions) on these effects, have been considered. Possible ways to increase the caloric efficiency of materials have been analyzed.

The complete set of material parameters including elastic, dielectric and piezoelectric constants as well as thermal expansion were measured by different methods for the promising piezoelectric single crystal CTGS at extreme temperatures (4.2–1170 K).

Thermal and dielectric studies of (ND

Heat capacity, thermal dilatation, structure, Mossbauer spectra and dielectric permittivity of fluoride KFe 2 F 6 with tetragonal tungsten bronze crystal structure were studied. The as-made sample undergoes two structural phase transitions P 4/ mbm ( T 1 ≈ 340 K) → Pbam ( T 2 ≈ 250 K) → G 2 and magnetic phase transformation at T m ≈ 133 K. Heating up to 600–700 K and subsequent cooling in helium atmosphere leads to a change of phase transition temperatures and diffused anomalies in thermal expansion. The results obtained and influence of thermal prehistory of the sample on its physical properties are discussed in the context of previous studies on related fluorides KFe 2 F 6 , which sometimes suggest conflicting structural details.

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.