A. V. Kartashev

Adiabatic calorimetric study of the intense magnetocaloric effect and the heat capacity of (La0.4Eu0.6)0.7Pb0.3MnO3

The temperature dependences of the intense magnetocaloric effect ΔTAD(T, H) and the heat capacity Cp(T) of the (La0.4Eu0.6)0.7Pb0.3MnO3 manganite are directly measured using adiabatic calorimetry. The experimental dependences ΔTAD(T) are in satisfactory agreement with those calculated from the data on the behavior of the magnetization. The factors responsible for the absence of an anomaly in the experimental temperature dependence of the heat capacity Cp(T) in the range of the magnetic phase transition are discussed.

Caloric characteristics of PbTiO3 in the temperature range of the ferroelectric phase transition

The heat capacity and thermal expansion of the PbTiO3 ceramic sample have been measured in the temperature range 80–970 K. The electrocaloric and barocaloric efficiencies of lead titanate in the ferroelectric phase transition range have been investigated by analyzing the experimental data in terms of the thermodynamic theory of phase transitions, the electrical equation of state P(T, E), the Pippard equation, and the S(T, p) diagram.

Caloric and Multicaloric Effects in Oxygen Ferroics and Multiferroics

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.

Modulated magnetic structure in quasi-one-dimensional clinopyroxene NaFeGe2O6

The magnetic structure of the NaFeGe2O6 monoclinic compound has been experimentally investigated using the elastic scattering of neutrons. At a temperature of 1.6 K, an incommensurate magnetic structure has been observed in the form of an antiferromagnetic helix formed by a pairs of the spins of the Fe3+ ions with helical modulation in the ac plane of the crystal lattice. The wave vector of the magnetic structure has been determined and its temperature behavior has been studied. The analysis of the temperature dependences of the specific heat and susceptibility, as well as the isotherms of the field dependence of the magnetization, has revealed the existence of not only the order-disorder magnetic phase transition at the point TN = 13 K, but also an additional magnetic phase transition at the point Tc = 11.5 K, which is assumingly an orientation phase transition.

Investigation of the reconstructive phase transition between metastable (α) and stable (β) modifications of the NH4LiSO4 crystal

Crystals of ammonium lithium sulfate NH4LiSO4 in α and β modifications are studied, and conditions of their nucleation and growth are determined. The α modification of NH4LiSO4 and α → β phase transitions are investigated using polarized light microscopy, x-ray diffraction, and differential scanning calorimetry in the temperature range 80–530 K. It is found that, depending on the conditions of growth and storage, there exist two temperature ranges (Tα → β ≈ 340–350 and ≈440–450 K) in which the crystals can undergo an α → β reconstructive phase transition. The enthalpy of this transformation depends on the symmetry of the final phase. In the former case (340–350 K), the reconstructive phase transition leads to rapid destruction of the sample. In the latter case (440–450 K), the crystal structure undergoes a slow transformation (recrystallization) without noticeable distortions. The results obtained indicate that no structural phase transition occurs in the α modification of NH4LiSO4 at 250 K.

Investigation into phase diagrams of the fluorine-oxygen system: Ferroelastic-antiferroelectric (NH4)2WO2F4-(NH4)2MoO2F4

Thermal, physical, structural, optical, and dielectric investigations have been performed for oxyfluoride solid solutions (NH4)2W1 — xMoxO2F4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1). The character of the influence of the chemical and hydrostatic pressures on the stability of the parent (space group Cmcm) and distorted ferroelastic and antiferroelectric phases has been determined by analyzing the temperature-pressure, unit cell volume-composition, and temperature-composition phase diagrams. The specific features of the nature and mechanism of the phase transitions have been discussed using the available data on the structural, entropy, and dielectric parameters.

State of spin glass in SmFeTi2O7

The SmFeTi2O7 compound has been synthesized using the solid-phase reaction method. In order to determine the magnetic state, X-ray structural, Mössbauer, calorimetric, and magnetic measurements have been performed. The state of spin glass with the freezing point Tf = 7 K has been found for SmFeTi2O7.

Properties of clinopyroxene LiFeGe2O6

The polycrystalline compound LiFeGe2O6 has been synthesized by the solid-phase reaction. The X-ray diffraction, Mössbauer, calorimetric, and magnetic investigations have been carried out. The Mössbauer spectrum at 300 K represents a single quadrupole doublet. The isomer shift with respect to the metal iron α-Fe is 0.40 mm/s, which is characteristic of the Fe3+ high-spin ion in the octahedral coordination. The quadrupole splitting of 0.42 mm/s indicates a distortion of the oxygen octahedron around the iron cation. The results of the measurement of the temperature dependence of the heat capacity in the range 2–300 K have shown the presence of the only anomaly with a maximum at Tm ∼ 20.5 K, which indicates the occurrence of a magnetic phase transition in this point. The data of the measurement of the temperature dependence of the magnetization have confirmed that the magnetic order with the dominant antiferromagnetic interaction of magnetically active ions exists in LiFeGe2O6 at a temperature below 20.5 K. The investigation of the temperature dependence of the heat capacity in the magnetic field H up to 9 T has demonstrated that the external factor insignificantly changes the order-disorder transition point (at H = 9 T, there occurs a shift of ∼0.5 K toward the low-temperature range).

Magnetic state of the GdFeTi2O7 compound

The X-ray diffraction, Mössbauer, calorimetric, and magnetic characteristics of zirconolite GdFeTi2O7 have been measured to determine the ground magnetic state. A kink dependent on the magnetic prehistory of the sample has been revealed in the temperature dependence of the magnetic moment at T = 3 K. Mössbauer spectroscopy has confirmed the nonequivalence of the iron ion positions in GdFeTi2O7. The experimental data obtained allow the conclusion on the formation of a spin glass state with the freezing temperature Tf = 3 K in the GdFeTi2O7 compound.

Magnetic structure of Cu2MnBO5 ludwigite: thermodynamic, magnetic properties and neutron diffraction study

We report on the thermodynamic, magnetic properties and the magnetic structure of ludwigite-type Cu2MnBO5. The specific heat, low-field magnetization and paramagnetic susceptibility were studied on a single crystal and combined with powder neutron diffraction data. The temperature dependence of the specific heat and the neutron diffraction pattern reveal a single magnetic phase transition at T  =  92 K, which corresponds to the magnetic ordering into a ferrimagnetic phase. The cation distribution and the values and directions of magnetic moments of ions in different crystallographic sites are established. The magnetic moments of Cu2+ and Mn3+ ions occupying different magnetic sites in the ferrimagnetic phase are pairwise antiparallel and their directions do not coincide with the directions of the principal crystallographic axes. The small value of the magnetic moment of copper ions occupying site 2a is indicative of partial disordering of the magnetic moments on this site. The magnetization measurements show a strong temperature hysteresis of magnetization, which evidences for field-dependent transitions below the phase transition temperature.