V. S. Bondarev

Phase transitions and caloric effects in ferroelectric solid solutions of ammonium and rubidium hydrosulfates

Structural, calorimetric, dielectric, and electrocaloric investigations of Rbx(NH4)1 − xHSO4 ferroelectric solid solutions have been performed. It has been found that rubidium atoms inhomogeneously occupy nonequivalent crystallographic positions in the structure P21/c. The influence of the rubidium concentration on the sequence of phase transitions in the NH4HSO4 compound has been established. It has been revealed that the consequences of the Landau theory can be applied to the description of the temperature dependences of the anomalous heat capacity and the electrocaloric effect over a wide range of temperatures. Comparative evaluations of the electrocaloric and barocaloric effects in hydrosulfate and triglycine sulfate crystals have been carried out.

Heat capacity of the PbFe1/2Ta1/2O3 perovskite-like compound

The heat capacity Cp(T) of the complex perovskite PbFe1/2Ta1/2O3 has been studied using adiabatic calorimetry in the temperature range 150–370 K. Three diffuse anomalies in Cp(T) are found to exist at temperatures Td≈350 K, Tm≈250–300 K, and Tc≈205 K. The anomalous and the lattice contribution to the heat capacity are separated, and the change in the entropy is determined. The results of the study are discussed jointly with the data on the structure and physical properties of the compound.

Heat Capacity and Thermal Expansion Studies of Relaxors

Heat capacity and thermal expansion of three perovskite-like solid solutions with nonisovalent PbMg1/3Nb2/3O3, PbFe1/2Ta1/2O3 and isovalent Ba0.92Ca0.08Ti0.76Zr0.24O3 substitution were measured using the methods of calorimetry and optical-mechanical dilatometry in a wide temperature range. The blurred anomalies in the Cp(T) and α(T) dependences are observed at Burns temperature, at the temperature of the dielectric permittivity maximum and at the temperature of the phase transition into ferroelectric state. The results are discussed together with the data on the structure and dielectric properties in the framework of spherical random bond-random field model.

Calorimetric and dielectric studies of the (NH4)2MoO2F4 oxyfluoride

The temperature dependences of the heat capacity, dielectric properties, and response to an external pressure and an electric field for the (NH4)2MoO2F4 oxyfluoride (space group Cmcm, Z = 4) have been studied. A comparative analysis of the data on the entropy of phase transitions, p-T phase diagrams, permittivity, and anomalous heat capacity in combination with the results of previous studies of the related compounds (NH4)2WO2F4 and (ND4)2WO2F4 has made it possible to establish that both [MO2F4]2− anions and ammonium groups play a substantial role in the mechanism and nature of the structural transformations.

Heat capacity and thermal expansion study of relaxor-ferroelectric Ba0.92Ca0.08Ti0.76Zr0.24O3

Heat capacity and thermal expansion of the Ba0.92Ca0.08Ti0.76Zr0.24O3 compound is measured using the methods of adiabatic calorimetry and optic-mechanical dilatometry in the temperature range 100–370 K. Three blurred anomalies on the Cp(T) and α(T) dependencies are observed in the temperature range 200–360 K. The results of the studies are discussed together with data on the structure and dielectric properties in the framework of spherical random bond–random field model.

Synthesis and study of structural, thermodynamic, and magnetic properties of NaxLi1–xFeGe2O6 (x = 0.1–0.9) compounds

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.

Electrocaloric effect in triglycine sulfate under equilibrium and nonequilibrium thermodynamic conditions

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.

Heat capacity study of double perovskite-like compounds BaTi1−xZrxO3

The temperature dependence of the heat capacity of two compositions in the solid solution system BaTi1−xZrxO3 (x = 0.25, 0.35) was measured using adiabatic calorimetry. In the T-x phase diagram, these compounds occupy positions near the crossover from conventional ferroelectric behavior to the relaxor state. Both compounds reveal diffuse heat capacity anomalies: two anomalies in the temperature ranges 250–350 and 150–200 K at x = 0.35 and one anomaly within the range ∼150–320 K at x = 0.25. The results obtained are discussed together with structural and dielectric measurements.

Effect of a restricted geometry on thermal and dielectric properties of NH4HSO4 ferroelectric

ABSTRACT 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.

Studies of the heat capacity and thermal expansion of the Na0.95K0.05NbO3 solid solution

The heat capacity and thermal expansion of ceramic samples of the Na0.95K0.05NbO3 solid solution have been investigated over a wide temperature range of 100–750 K. The observed anomalies in the heat capacity and thermal expansion at T4 = 297 K, T3 = 535 K, T2 = 665 K, and T1 ≈ 710 K correspond to the sequences of phase transitions N → Q → G → S → T1. It has been shown that, as a result of the phase transitions, the unit cell volume at T4 and T2 decreases, and at T3 and T1, increases with increasing temperature. The directions of the shift of the phase transition temperatures induced by hydrostatic pressure have been determined. It has been established that all structural transformations are accompanied by relatively small variations in the entropy. Different mechanisms of the structural distortions have been discussed.