Comprehensive studies of the thermophysical properties (thermal conductivity, thermal diffusivity, heat capacity, thermal expansion coefficient) of solid solutions of the Bi0.9M0.1FeO3 (M = La, Pr, Nd, Sm) multiferroics have been carried out within the temperature range of 300–800 K. Anomalies of the thermophysical properties caused by phase transitions are observed within the Néel temperature domain (640–650 K). Formulas are proposed for the calculation of the studied thermophysical properties within the domains of the structural phase transitions, and a clear correlation is established for the dependences of the thermophysical properties within the domain of the structural phase transitions.

The effects of the modification of bismuth ferriteBismuth ferrite by various rare earth elementsRare Earth Elements (REE) (REE) are considered: the results of the investigation of the influence of crystal-physical parameters of stoichiometrically introduced dopants on the form of the phase diagrams of Bi1−xREExFeO3 systems, the grain structureGrain structure of ceramics, the dielectricDielectric spectra and thermal propertiesThermal properties of samples over a wide range of temperatures are presented.

The heat capacity and the permittivity of multiferroics Bi 1 − x Gd x FeO 3 ( x = 0, 0.05, 0.10, 0.15, 0.20) have been studied in the temperature range 130–800 K. It has been found that insignificant substitution of gadolinium for bismuth markedly shifts the temperature of antiferromagnetic phase transition and increases the heat capacity over a wide temperature range. It has been shown that the temperature dependence of the excess heat capacity is due to the manifestation of three-level states. Additional anomalies characteristic of the phase transitions have been revealed in the temperature dependences of the heat capacity for the compositions with x = 0.1 and 0.15 at T ≈ 680 K and T ≈ 430 K, respectively. The results of studies of the heat capacity have been discussed simultaneously with the data of structural studies.

The heat capacity of Bi1 − x Re x FeO3 (Re = La, Eu, Ho; x = 0, x = 0.05) multiferroics has been studied in the temperature range of 120–800 K. The substitution of a small amount of rare-earth elements for bismuth leads to a significant increase in the heat capacity in the broad temperature range studied. It is established that the temperature dependence of the excess heat capacity is related to the Schottky effect for three-level states certain that appear as a result of structure distortions in the rare-earth-doped compositions.

We have studied the phase transformations, microstructure, and dielectric, piezoelectric, and thermophysical properties of Ag1 − y NbO3 − y/2 (0 ≤ y ≤ 0.20) ceramics. Within its homogeneity range (y ≤ 0.10), silver niobate undergoes a complex sequence of phase transformations, accompanied by anomalies in its physical properties. The observed dispersion effects are interpreted in terms of electrical conductivity above the Curie temperature and in terms of the motion of domain walls and interfaces below the Curie temperature.

We have studied the phase composition, microstructure, and dielectric and thermophysical properties of Na 1 − y NbO 3 − y /2 (0 ≤ y ≤ 0.20) ceramics and identified a complicated sequence of phase transformations within the homogeneity range of sodium niobate ( y ≤ 0.10), accompanied by anomalous variations in its physical properties. At low y values, discontinuous secondary recrystallization occurs. We conclude that dielectric effects above the Curie temperature are related to the electrical conductivity of the material and that those at low temperatures are governed by the motion of domain walls and interfaces. The temperature-dependent structural, dielectric, and thermophysical properties of the materials studied are shown to correlate.

The mechanisms of CdSnAs 2 heat and charge transfer in solid and liquid states were investigated. It was demonstrated that heat transfer in solid state is accomplished by phonons, electrons, diffusion, and the recombination of electron-hole pairs. A sharp increase in thermal and electrical conductivity up to the values characteristic of metals is observed upon melting. In contrast to metals, the electrical conductivity of CdSnAs 2 melt increases with temperature.

The temperature dependences of the thermal conductivity (300–800 K), specific heat, and thermal expansion coefficient of PCR-1 and PCR-37 piezoceramics have been experimentally determined. Anomalies are found near the Curie temperature, which are caused by changes in the phase composition variations and structure rearrangement.

The structural, dielectric, and thermal properties of the Na0.875Li0.125NbO3 solid solution doped with strontium and other elements have been studied in wide temperature and frequency ranges. The material has been shown to undergo a sequence of phase transitions accompanied by anomalies in its structural, dielectric, and thermal properties. The observed low-frequency dispersion of its dielectric permittivity is attributed to the effect of electrical conductivity.

We present the results of our experimental studies of the thermal conductivity for SiC and SiC + 1.2% BeO ceramic samples as a function of the hydrostatic pressure up to 400 MPa in the temperature range 273–523 K. We show that pressure leads to a nonlinear increase in thermal conductivity and to an additional phonon scattering by lattice defects.

The permittivity and thermal expansion of polarized and unpolarized samples of the ferroelectric ceramic PKR-1 have been investigated in the temperature range 300–700 K. Anomalous behavior of the thermal expansion coefficient is found in the range of ferroelectric phase transitions. The results obtained are analyzed taking into account the specific features of the structure of polar ceramics.

The thermal properties of piezoelectric ceramics (PKR-8, PKR-7M) based on lead zirconate titanate solid solutions Pb(Ti, Zr)O 3 were studied over the temperature range 300–800 K. The thermal conductivity and thermal expansion coefficients were found to exhibit an anomalous behavior in the region of the ferroelectric phase transition.

The temperature dependence of the thermal conductivity, electrical conductivity, thermoelectromo-tive force, and thermal expansion coefficient for sulfides of lanthanum, gadolinium, praseodymium and dysprosium of the composition Ln3-хVxS4 is investigated in the temperature range from 300 to 1200 K. It is shown that the transfer phenomena and thermoelectrical properties of the investigated compositions depend on the concentration of current carriers, cation vacancies, and mobility. Gadolinium sulfide is found to have the highest thermoelectrical efficiency. The scattering from ions of rare-earth elements has a noticeable effect on the magnitude and temperature dependence of the lattice thermal conductivity and electrical resistance. INTRODUCTION Sulfides of rare-earth elements (REE) with boundary compositions Ln2S3 and Ln3S4 form with each other a continuous series of solid solutions within a single phase of the type of Th5P4, whose crystal-chemical formula may be written as Ln3-xVxS4 , where 0 ≤ x ≤ 0.333 (Vr is the concentration of cation vacancies in the REE sublattice) (Golubkov at el 1973). They are high-energy-gap semiconductors with the energy-gap width of over 2.1 eV. The concentration of vacancies on transition from Ln2S3 (x = 0.333) to Ln3S4 (x = 0) decreases from (1.5-2) x 10 cm to zero. Simultaneously, the concentration of current carriers increases as n = n0(l 3x) cm , where n0 = (4.5-6) x 10 cm. At a temperature of 300 K, the electrical resistance of such sulfides varies approximately from 10 to 10 Ohm cm, i.e., by 13 orders of magnitude (Wood at el 1985; Gadjiev 1988). These factors introduce a number of specific features into their electrical, thermal, and elastic properties. The urgency of the problem of the investigation of thermal and electric properties of REE sulfides is due, among other things, to the possibility of their practical use as hightemperature branches during the thermoelectric conversion of energy. Owing to high values of the melting temperature (Tmelt > 2000 K), thermal stability, and the possibility of obtaining, in a series of solid solutions Ln3-xVxS4, compositions with optimal thermoelectric parameters, they may offer a substantial advantage as efficient high-temperature materials for thermoelectric energy conveners over the presently used materials. In the literature, data are available on the temperature dependence of the electrical conductivity, thermal conductivity, and thermoelectromotive force of boundary compositions of La3S4, Ce3S4, Pr3S4, and Nd3S4 (Zhuze at el 1971). In accordance with the data of different authors, these parameters considerably differ from one another. This may be attributed to a number of reasons, such as the pollution with oxychalcogenides in the process of synthesis, the presence of eutectic inclusions of the composition LnS characterized by different values of thermal and electrical parameters, and the possibility of propagation of the boundary of the homogeneity region beyond the composition Ln3S4. No data on the thermoelectric properties of G3dS4 and Dy3S4 are available in the literature. EXPERIMENTAL DATA AND THEIR DISCUSSION This paper contains experimental data on the temperature dependence of the electrical conductivity (σ), thermoelectromotive force (α), thermal conductivity (λ), and thermal expansion coefficient (β) of sulfides La3S4 (LaS1.33), Pr3S4, Gd3S4, Dy3S4, and LaS2.70S4 (LaS1.48), PrS1.48, GdS1.48, and DyS1.48. We also investigated the temperature dependences of the specific heat capacity (Cp) and of the velocity of propagation of longitudinal (Vl) and transverse (VS) ultrasonic waves in order to estimate the Debye temperature (θ) and the mechanisms of heat and electric transfer. Samples of similar compositions were prepared by both hot pressing (Vavilov State Optical Institute, St. Petersburg) and induction melting (Institute of Inorganic Chemistry, Siberian Division, Russian Academy of Sciences, Novosibirsk). Whether the samples had the required compositions or not was checked by the direct gravimetric method enabling one to determine the content of sulfide sulfur within ±0.03 wt % (Kamarzin at el 1981), as well as by the gas-chromatographic method used for reliable determination of compositions differing by 0.001 sulfur atom per REE atom (Chuchalina at el 1978). With respect to density, the samples prepared by hot pressing were close to cast ones. The homogeneity of specimens was assessed by measuring the thermoelectromotive force (by the thermal probe method) and electrical conductivity over the sample length at room temperature. The thermal conductivity was measured by an absolute compensation method (Magomedov at el 1990), and the electrical conductivity and thermoelectromotive force were measured by the four-probe compensation method. Note that the thermoelectromotive force (α) was determined in the course of measuring both the heat conductivity and electrical conductivity. A capacitive dilatometer was used to measure the thermal expansion coefficient (Kamilov at el 1992). Figure 1 gives the experimental data on the temperature dependence of thermoelectromotive force and electrical conductivity of La3S4, Pr3S4, Gd3S4, and Dy3S4. Our data on a and a are in satisfactory agreement with those given in. Fig. 1. The temperature dependence of the electrical conductivity and thermoelectromotive force: (7) Gd3S4, (2) La3S4, (3)Pr3S4,(.f)Dy3S4. Fig. 2. The temperature dependence of the thermal conductivity and thermal expansion coefficient: (1) La3S4, (2) Gd3S4, (3)Pr3S4,(4)Dy3S4.inclusions) that produce additional electrical and thermal resistance. The thermoelectromotive force of all of the nvestigated compositions increases linearly with temperature and corresponds to the electron conduction. The electrical conductivity decreases with rising temperature, as in the case of degenerated semiconductors and metals, whereas the electrical resistance ρ(T) increases linearly with temperature. The results of measurements of the concentration of current carriers n by the Hall effect substantiate this assumption. The experimentally determined values of n (see table) coincide in order of magnitude with the calculated ones (provided that there is one conduction electron per formula unity of Ln3S4, i.e., (4.5-6) x 10 cm), although they are somewhat smaller than the predicted data. The values of σ, α, λ, and their temperature dependences for different compounds are different, though they have much in common, which is associated with the specific features of a compound. Figure 2 gives the temperature dependence of the thermal conductivity and thermal expansion coefficient of the investigated compositions. Here, the value of λ varies insignificantly with temperature. The total thermal conductivity depends on the contribution of the lattice and electronic components, and λ = λ1+λe The electronic component λe, calculated by the Wiede-mann-Franz law for the case of degeneracy, λe = LσT, amounts to almost half the lattice thermal conductivity (L is the Lorentz number). We were interested in the dependence of the value of λ1 on the composition and temperature. As is known, the value of λi within the limits of one structure depends on the mean atomic weight (A), characteristic Debye temperature (θ), and anharmonic Griineisen coefficient (γ). At high temperatures (T >θ), the value and temperature dependence of λ1 in real crystals may be estimated by the LeibfriedSchlemann formula λ1 = BAθa/γ T. (1) Physical properties of REE sulfides at 300 K Compositions σ,ohm cm n×10, cm α, μVK -1 λ, W mK β×10, K Cp, J molK Vm, m s θ, K