Baric Coefficient Research Articles

Phase transitions, baro- and piezocaloric effects in single crystal and ceramics of ferroelectric NH4HSeO4

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, ΣΔSi = 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.

Baric properties of CdSe-core / ZnS/CdS/ZnS-multilayer shell quantum dots

The model of spherical quantum dot of the core/multilayer shell type, which is subjected to external comprehensive pressure, is constructed taking into account the mismatch between the lattice parameters of the contacting layers and the pressure due to the curvature of the surface. Within the method of the deformation potential, the influence of external pressure on the band structure of CdSe-core/ZnS/CdS/ZnS-shell quantum dot was investigated. It is shown that by changing the thickness of the shell layers and the amount of pressure, it is possible to change the width of the optical band gap of quantum dot in a wide range. It was found for the first time that in the case of small CdSe QD cores (less than 2.5 nm) there is a critical value of comprehensive pressure, above which the value of baric coefficient decreases abruptly. It is shown that an increase in the thickness of CdS layer in the three-layer ZnS/CdS/ZnS shell leads to a decrease in the critical pressure, and in the two-layer ZnS/CdS shell leads to an increase in the critical pressure. It is established that in the case of a three-layer shell for small radii of cores (less than 2.5 nm) at pressure greater than 16.5 kbar there is an anomalous dependence of the energy of electron in the ground state and, accordingly, the width of the optical band gap on the core radius: increasing the radius of core leads to an increase in the width of the optical band gap of quantum dot. • The model of strain CdSe-core/ZnS/CdS/ZnS-shell quantum dot is constructed. • Within the method of the deformation potential, the influence of external pressure on the band structure of CdSe-core/ZnS/CdS/ZnS-shell quantum dot was investigated. • In the case of small cores of CdSe QD there is a critical value of pressure, above which the value of the baric coefficient decreases abruptly. • Increasing the thickness of the CdS layer in a three-layer ZnS/CdS/ZnS shell leads to a decrease in critical pressure.

The baric coefficient of CdSe quantum dot with a three-layer ZnS/CdS/ZnS shell

The model of CdSe quantum dot with a multilayer shell that undergoes mechanical deformation is constructed. This theory takes into account the influence of external hydrostatic pressure, the Laplace surface pressure and the mismatch of lattice parameters of contacting materials of the nanoheterosystem. Within the framework of the developed model, the baric coefficient of CdSe/ZnS/CdS/ZnS quantum dot was calculated for different values of the CdSe core radius and at different thicknesses of the ZnS/CdS/ZnS shell layers. It is established that for small cores (with a diameter of less than 4.8 nm) of quantum dots with a three-layer shell, two values of the baric coefficient are possible depending on the magnitude of external pressure. The analysis of use of the offered model for research of interaction of quantum dots with elements of biological systems through an elastic field of deformation is carried out.

Синтез, микроструктура и влияние высокого давления на транспортные свойства GdBaCo2O5,5.

In polycrystalline samples of double layered cobaltite GdBaCo2O5.5 the structure and resistivity at the first order “insulator-metal” (I-M) phase transition were studied at normal and high pressures. The strong dependence of the shape of the temperature hysteresis loop on the rate of temperature change indicates an infra-slow thermal relaxation of conductivity. Baric studies have shown an increase in the transition temperature ТIM at increasing pressure P with baric coefficient dТIM/dP ≈ 10 K/GPa. The spin blockade model is used to explain the observed effects.

Slow Relaxation of Magnetic Susceptibility in Polycrystalline GdBaCo2O5.53 above the Curie Point and the Effect of High Hydrostatic Pressures

A slowly decaying anomaly of magnetic susceptibility (characteristic time τ ~ 103 s) is revealed in studying the ferromagnetic–paramagnetic phase transition in polycrystalline cobaltite GdBaCo2O5.53 that occurs due to the remaining short-range magnetic order at temperatures above the Curie point (TC = 275 K). High hydrostatic pressures (up to 1.48 GPa) are found to have a weak effect on TC with baric coefficient dTC/dP = 2.1 K/GPa.

Effect of Isovalent Cation Substitution on the Thermal, Caloric, and Magnetocaloric Properties of the (La1 – yEuy)0.7Pb0.3MnO3 Manganites

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.

Влияние изовалентного замещения катионов на термические, калорические и магнетокалорические свойства манганитов (La-=SUB=-1-y-=/SUB=-Eu-=SUB=-y-=/SUB=-)-=SUB=-0.7-=/SUB=-Pb-=SUB=-0.3-=/SUB=-MnO-=SUB=-3-=/SUB=-

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.

Influence of Isotropic Pressure on the Current–Voltage Characteristics of Surface-Barrier Diodes Sb–p-Si〈Mn〉–Au

The influence of hydrostatic pressure on the current–voltage characteristics of surface-barrier diode structures of the Sb–p-Si〈Mn〉–Au type are investigated. The potential-barrier height and the baric coefficient of its variation are found to be eϕδ = 0.75 eV and δ =–1.54 × 10–11 eV/Pa, respectively.

Влияние изотропного давления на вольт-амперную характеристику поверхностно-барьерных диодов Sb-p-Si<Mn>-Au

AbstractThe influence of hydrostatic pressure on the current–voltage characteristics of surface-barrier diode structures of the Sb– p -Si〈Mn〉–Au type are investigated. The potential-barrier height and the baric coefficient of its variation are found to be e ϕ_δ = 0.75 eV and δ =–1.54 × 10^–11 eV/Pa, respectively.

T–p phase diagrams and the barocaloric effect in materials with successive phase transitions

An analysis of the extensive and intensive barocaloric effect (BCE) at successive structural phase transitions in some complex fluorides and oxyfluorides was performed. The high sensitivity of these compounds to a change in the chemical pressure allows one to vary the succession and parameters of the transformations (temperature, entropy, baric coefficient) over a wide range and obtain optimal values of the BCE. A comparison of different types of schematic T–p phase diagrams with the complicated dependences observed experimentally shows that in some ranges of temperature and pressure the BCE in compounds undergoing successive transformations can be increased due to a summation of caloric effects associated with distinct phase transitions. The maximum values of the extensive and intensive BCE in complex fluorides and oxyfluorides can be realized at rather low pressure (0.1–0.3 GPa). In a narrow temperature range around the triple points conversion from conventional BCE to inverse BCE is observed, which is followed by a gigantic change of both and .

Thermal, optical, and dielectric properties of fluoride Rb2TaF7

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.

Thermal, dielectric and barocaloric properties of NH4HSO4 crystallized from an aqueous solution and the melt

A study of heat capacity, thermal dilatation, permittivity, dielectric loops and susceptibility to hydrostatic pressure was carried out on quasi-ceramic samples of NH4HSO4 obtained from an aqueous solution as well as the melt. The main parameters of the successive P21/c (T1) ↔ Pc (T2) ↔ P1 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 T2. Direct measurements of the pressure effect on the permittivity and thermal properties showed dT2/dp = −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 ↔ P1 transformation with an increase in pressure, and the linear dependence of T2 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 ↔ P1 phase transition.

Thermal properties and phase transition in the fluoride, (NH4)3SnF7

Calorimetric, dilatometric and differential thermal analysis studies were performed on (NH4)3SnF7 for a wide range of temperatures and pressures. Large entropy (δS0=22J/molK) 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=32.5J/molK is characteristic for the order–disorder phase transition, 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.

Magnetization and magnetocaloric effect in La0.7Pb0.3MnO3 ceramics and 0.85(La0.7Pb0.3MnO3)–0.15(PbTiO3) composite

Abstract

Heat capacity, entropy, dielectric properties and T–p phase diagram of (NH4)3TiF7

Successive phase transitions G1→G2→G3 in the double salt (NH4)3TiF7 have been studied by detailed calorimetric, DTA under hydrostatic pressure and dielectric measurements. Rather large entropy jumps at phase transition points were found to be followed by large additional contributions associated with the temperature dependence of the excess heat capacity. The permittivity behaviour and the tangent of the dielectric losses proved the nonferroelectric nature of both transformations. Two triple-points were found on the temperature-pressure phase diagram, suggesting the existence of a hypothetical parent G0Pm−3m cubic phase. A direct transformation between high (G0) and low (G3) temperature cubic phases takes place at p>0.41GPa with a baric coefficient dTG0→G3/dp=−40K/GPa.

Pressure-induced change in the order of the phase transition in lead titanate: Structural aspects

The crystal structure of lead titanate PbTiO3 have been studied by energy-dispersive X-ray diffraction under pressures of 0–4 GPa in the temperature range of 300–950 K. At a temperature of T = 747 K, a structural phase transition from a ferroelectric tetragonal phase to a paraelectric cubic phase is observed. The application of pressure leads to a significant decrease in the phase transition temperature; under pressures of P ∼ 2 GPa, the type of the phase transition changes from the first to second order. In the low pressure range, the baric coefficient value is dTc/dP = −20(3) K/GPa; under pressures above 2 GPa, it increases up to −113(5) K/GPa.

Study of the p-Ge-n-GaAs heterojunction under hydrostatic pressure

The current-voltage characteristic of the p-Ge-n-GaAs heterojunction is experimentally studied under hydrostatic pressure as high as 8 GPa and temperature of 300 K. The baric coefficient of the edge of the valence band of Ge is calculated using the experimental results.

Effect of pressure on Raman spectra of SnS2 single crystals

The 2H polytype of a SnS2 layered crystal has been studied using Raman spectroscopy at pressures of up to 5 GPa in a diamond anvil cell. The Raman frequency of the intralayer mode increases linearly with increasing pressure at baric coefficients of 5.2 cm−1/GPa for P 3 GPa. This change in the baric coefficient for Raman scattering and the available data on X-ray measurements of the compressibility of 2H-SnS2up to 10 GPa suggest that the crystal structure undergoes a transformation at about 3 GPa.

Thermography of the phase transformations in a hypoeutectic Al-7% Si-0.5% Mg silumin at high pressures and temperatures

The phase equilibria in a hypoeutectic AK7 aluminum alloy (Al-7% Si-0.5% Mg silumin) are studied by differential barothermal analysis in the pressure range 5–131 MPa at temperatures up to 960 K. The baric dependences of the liquidus temperature during heating and the solidus and liquidus temperatures during cooling are found to have a minimum at 5 MPa. The solidus temperature during heating increases sharply as the pressure increases from 5 to 53 MPa and remains almost unchanged at a pressure p ≥ 53 MPa. The baric coefficients of the solidus and liquidus temperatures in the pressure range 5 ≤ p ≤ 53 MPa achieve ~0.6 K/MPa, which is much higher than the baric coefficient of the melting temperature of pure aluminum. A pT phase diagram is proposed for the AK7 alloy.

Baric properties of InAs quantum dots

In the context of the deformation potential model, baric dependences of the energy structure of InAs quantum dots in a GaAs matrix are calculated. Under the assumption of the absence of interaction between the spherical quantum dots of identical sizes, the energy dependence of the baric coefficient of energy of the radiative transition in the quantum dot is determined. A similar dependence is also found experimentally in the photoluminescence spectra under uniform compression of the InAs/GaAs structures. Qualitative agreement between the theory and experiment as well as possible causes for their quantitative difference are discussed. It is concluded that such factors as the size dispersion, Coulomb interaction of charge carriers, and tunnel interaction of quantum dots contribute to this difference.