The kinetic characteristics of thermal frequency phonons in the region of helium temperatures in ceramic samples of the Ce1–xGdxO2–y electrolyte solid solution have been studied. To explain the temperature dependence of the phonon mean free path, we used the previously performed calculations of the energy of vacancy formation in the anion sublattice of a solid solution of zirconium dioxide stabilized by yttrium ZrO2:Y2O3 (YSZ) with a similar crystal structure. It is shown that in the Ce1–xGdxO2–y system under study, the formation of structural defects associated with the presence of vacancies in the anion sublattice with energy Δ = 8.53 K is possible. It has been established that analysis of the temperature dependences of the YSZ heat capacity allows one to trace the degree of disorder (amorphization) of the solid solution depending on its level of stabilization.

The temperature dependences of the heat capacity were measured and the general patterns of the formation of the phonon spectrum of single crystals of solid solutions of yttrium-lutetium aluminum garnets Y3 – xLuxAl5O12 at 0 ≤ x ≤ 3 were investigated in the temperature range from 1.9 to 220 K. According to the data obtained at temperatures below 10 K, the Debye temperatures were calculated. The features of the phonon spectrum in the intermediate temperature range are interpreted as a superposition of optical modes for yttrium and lutetium garnets. It is shown that the low heat capacity values due to the contribution of acoustic phonons for Y2.25Lu0.75Al5O12 correlate with anomalies in the concentration dependences of the phonon transport, the absorption of acoustic waves, and the shape of the aluminum NMR line.

A method for studying solid dielectrics under conditions of nonstationary propagation of subterahertz phonons is presented. The method analyzes the transport characteristics of thermal phonons in the helium (He) temperature range (2–4 K) in diffusion mode and makes it possible to investigate the following: the kinetic characteristics of thermal phonons in single crystals of solid solutions; the contribution to phonon scattering from the concentration, type, and positions of a substitutional impurity, defects comparable with the wavelength (clusters, phase inhomogeneities, dislocations), low-energy excitations of various nature, including those associated with the formation of an equilibrium configuration of vacancies in the anionic sublattice with respect to substitutional impurity cations; equilibrium criteria in the system of nonequilibrium phonons–low-energy excitations; peculiarities of the phonon spectrum of nanostructured materials; the relationship between the diffusion coefficient of subterahertz phonons and the system of grain boundaries (GB), grain size, conditions for the formation of a “gap” in the phonon spectrum of nanostructured ceramics; spectral properties of GB, their relationship with the technological conditions of synthesis, to evaluate the average thickness and acoustic impedance of GB for a sample; the features of thermal phonon transport in amorphous dielectrics (glasses and glasslike materials) near the thermal conductivity “plateau,” the possibility of “gap” formation in the spectrum of phonon states; subterahertz phonon transport in ceramics based on ferroelectrics, electrolyte solid solutions, and cermets.

The specific features of the transport of thermal-frequency phonons at liquid-helium (He) temperatures under the conditions of non-stationary measurements and thermal conductivity in polycrystalline Y 3 Al 5 O 12 (YAG) ceramic samples, synthesized in different technological regimes, have been studied. The relationship between the phonon diffusion coefficient and the structural features of grains (crystallites) and grain boundaries has been revealed by analyzing the non-stationary measurement data and the thermal conductivity.

The transport characteristics of thermal-frequency phonons and specific heat C(T) in single-crystal solid solutions of aluminum–rare-earth garnets have been experimentally investigated in the helium-temperature range in the presence of low-energy paramagnetic excitations. It is shown that, under conditions of time-dependent propagation of a thermal pulse, equilibrium may occur in the “nonequilibrium phonons–two-level systems” system at certain values of sample length and thermostat temperature.

The interaction of weakly nonequilibrium phonons with the low-energy paramagnetic excitations of the rare-earth ions of the yttrium series in rare-earth garnet solid solutions is studied at liquid-helium temperatures. The interaction of nonequilibrium phonons with the low-energy excitations of Ho 3+ and Tb 3+ , which are caused by local electric fields in a crystal lattice, is experimentally investigated. In the row of Kraemrs ions (where the nature of low-energy excitations is caused by the splitting of the ground level of a paramagnetic ion due to the local magnetic fields of neighboring ions), interaction in the nonequilibrium phonon–low-energy excitation system is only detected in the Er-containing solid solutions and is absent in the structures containing Gd 3+ , Dy 3+ , and Yb 3+ rare-earth ions. In the two-level system model, the efficiency of interaction and the transport characteristics of thermal phonons are shown to depend on the type of rare-earth ion, the energy and spectral features of two-level systems, the moments of electrons in the 4 f shell, and spin–lattice relaxation.

Исследованы особенности низкотемпературной теплоемкости, транспортные характеристики фононов в области гелиевых температур и наноструктурные неоднородности в монокристаллах твердых растворов эрбиевых моноалюминатов YAlO3 : Er, синтезированных методом направленной кристаллизации.

We report the caloric and magnetocaloric properties of the DyxY3−xAl5O12 garnet single crystals (x = 0, 0.15, 0.5, 1, 1.5, 2.25, 3) studied within a large temperature range at magnetic fields up to 70 kOe. Heat capacity at zero field revealed that the Schottky anomalies ascribed to magnetic splitting the ground Kramers doublet of Dy3+ ions induced by neighbor ions. The low-temperature heat capacity was fitted by a sum of the Debye and Schottky contributions. The antiferromagnetic transition above 1.9 K was observed only for the pure dysprosium aluminum garnet. The shifts of the Schottky anomalies in magnetic fields agreed with strong anisotropy of the g-factor of Dy3+ ions in the c-sites. The Dy3+ ions in the non-stoichiometric positions caused additional weak anomalies in the heat capacity. The heat capacity for the garnet with x = 1 demonstrated strong clustering for this particular composition. The results obtained showed that the measurements of the heat capacity in magnetic field can be used to get information on substitutional disorder and non-stoichiometry in solid solutions. The magnetic entropy was evaluated for the dysprosium-comprising garnets. The pronounced magnitude of the magnetic entropy owing to the Schottky anomalies shows the applicability of the mixed dysprosium yttrium garnets to adiabatic refrigerators.

The low-temperature specific heat, the transport characteristics of phonons at the helium temperatures, and the nanostructural heterogeneities in the erbium monoaluminate YAlO3:Er solid solution single crystals synthesized by directional solidification are studied.

In this paper, we present the results of studies of the heat capacity of single-crystal garnets Er3 ‒ xTmxAl5O12 (x = 0, 1, 2, 3) in magnetic fields of up to 9 T and in the temperature range 1.9–220 K. The temperature dependences of the heat capacity are approximated using the sum of the contributions of Schottky anomalies associated with magnetic Er3+ and Tm3+ ions and the Debye and Einstein lattice contributions. Entropy and magnetic entropy are calculated using the heat capacity data. With an increase in the magnetic field, entropy is shown to decrease. This indicates the possibility of using the studied garnets in the adiabatic demagnetization method.

The influence of the Schottky low-energy excitations related to the presence of rare-earth metals on the thermodynamic and kinetic characteristics of a number of rare-earth pentaphosphate single crystals and the related glasses is analyzed. The temperature dependences of the heat capacity are found to be related to the nanosized structure of the amorphous state of the material.

AbstractIn this paper, we compared the results of a heat capacity study of an erbium-doped gallium gadolinium garnet crystal with data for an undoped garnet. The measurements were carried out in the temperature range from 1.9 to 220 K and in magnetic fields from 0 to 9 T. The temperature dependences of the specific heat were interpreted with allowance for the Schottky contributions due to the Gd^3+ and Er^3+ ions and the contributions of the thermal vibrations of the crystal lattice. The values of entropy and magnetic entropy are calculated.

The temperature dependences of the specific heat and the transport characteristics of thermal-frequency phonons in single crystals of the solid solutions of rare-earth aluminum garnets are studied at liquid-helium temperatures in the presence of Schottky-type low-energy excitations. The kinetic characteristics of phonons as functions of the solid solution composition are measured. The relation between the kinetic and thermophysical characteristics of the material in the solid solutions of rare-earth aluminum garnets is analyzed under conditions of a nonstationary process and the spatial inhomogeneity caused by the coordinate dependence of the state of low-energy excitations. The thermalization conditions in the nonequilibrium phonon–low-energy excitation system are estimated.

In this paper, we compared the results of a heat capacity study of an erbium-doped gallium gadolinium garnet crystal with data for an undoped garnet. The measurements were carried out in the temperature range from 1.9 to 220 K and in magnetic fields from 0 to 9 T. The temperature dependences of the specific heat were interpreted with allowance for the Schottky contributions due to the Gd 3+ and Er 3+ ions and the contributions of the thermal vibrations of the crystal lattice. The values of entropy and magnetic entropy are calculated.

Проведены исследования температурной зависимости магнитной ac восприимчивости монокристаллического смешанного граната Er2HoAl5O12 в диапазоне от 1.8 до 300 K в нулевом постоянном поле и при приложении смещающих полей до 9 T. В отсутствие постоянного магнитного поля восприимчивость следовала закону Кюри-Вейсса. Приложение постоянного поля индуцировало магнитный фазовый переход, температура которого возрастала с ростом напряженности магнитного поля. Характерный для фазового перехода максимум динамической восприимчивости не демонстрировал заметной зависимости от частоты переменного поля. Работа выполнена при поддержке РФФИ (грант N 16-07-00181). Измерения проводились на оборудовании Ресурсного центра "Центр диагностики функциональных материалов для медицины, фармакологии и наноэлектроники", Научный парк СПбГУ. DOI: 10.21883/FTT.2017.04.44273.345