Thermophysical properties of PLZT ferroelectric ceramics with a nanopolar structure

The heat capacity, thermal expansion coefficient, and deformation of the PLZT-9/65/35 compound are studied in the temperature range 150–800 K. Diffuse anomalies are detected in the temperature dependences of the heat capacity and thermal expansion coefficient over wide temperature ranges of 250–650 and 330–550 K, respectively. The anomalous behavior of the heat capacity in the temperature range 250–650 K is shown to be caused by the appearance of two-level states (Schottky anomaly). The results obtained are discussed along with the data of structural and dielectric studies.     

Thermal physical properties of ferroelectric ceramics PKR-7M near the diffuse phase transition

The thermal physical properties (heat capacity, thermal expansion coefficient, and deformation) of a relaxor piezoelectric ceramics based on the lead zirconate titanate PKR-7M have been studied in the temperature range 200–800 K. Diffuse anomalies have been revealed in the temperature dependences of the heat capacity and thermal expansion over wide temperature ranges 270–650 and 450–600 K, respectively. It has been shown that the anomalous behavior of the heat capacity is due to the manifestation of two-level states (Schottky anomalies). The results of the study have been discussed together with the data of structural studies.     

Specific heat and magnetic susceptibility of MnF2 and Mn0.98Fe0.02F2 near TN

The antiferro- to paramagnetic phase transition of the weakly anisotropic compound MnF₂ has been studied by means of heat capacity, magnetic susceptibility and thermal expansion measurements. The critical-point parameters associated with the specific heat indicate a transition according to the theoretical Ising-model. The temperature derivative of the parallel magnetic susceptibility times temperature (d(χ∥T)/dT) and the c-axis thermal expansion coefficient show a critical behaviour very similar to that of the specific heat. The influence of iron doping on the critical behaviour has been investigated by studies on Mn_0.98Fe_0.02F₂. Specific heat and magnetic susceptibility measurements show an unexpectedly sharp transition although some rounding off is noticed as compared to pure MnF₂.     

Investigation of the thermal expansion and heat capacity of the CaCu3Ti4O12 ceramics

The thermal expansion of the CaCu₃Ti₄O₁₂ ceramics has been measured over a wide temperature range 120?C1200 K. The high quality of the samples under study has been confirmed by good agreement of the results of measurements of the heat capacity in the range 2?C300 K and in the vicinity of the phase transition of magnetic nature at 25 K with the data for the single crystal. No anomalies in the thermal expansion that can be associated with the phase transition at 726?C732 K assumed by other investigators have been found. The influence exerted on the thermal expansion by the heat treatment of the sample in a helium atmosphere and in air has been investigated.     

Anomalous behavior of the electrical resistivity of MnSi near the ferromagnetic phase transition

The results of measuring the electrical resistivity of a MnSi single crystal near the ferromagnetic phase transition at atmospheric and high pressures are reported. In contrast to the previous works, compressed helium is used as a pressure-transferring medium. It is shown that the temperature derivative of the electrical resistivity has the form of sharp maximum on the phase-transition line over the entire its length. Moreover, the observed maxima have a fine structure exhibiting a pronounced shoulder at temperatures slightly higher (by approximately 0.5 K) than the peak temperature, which indicates the existence of nontrivial fluctuations in the paramagnetic phase of MnSi. This feature disappears at a pressure of about 0.35 GPa, which corresponds to the tricritical-point coordinate.     

Thermodynamic and kinetic properties of an icosahedral quasicrystalline phase in the Al-Pd-Tc system

The properties of a quasicrystalline phase in the Al-Pd-Tc system are studied for the first time. X-ray investigations demonstrate that the quasicrystalline phase in the Al₇₀Pd₂₁Tc₉ alloy has a face-centered icosahedral quasi-lattice with parameter a=6.514 ?. Annealing experiments have revealed that this icosahedral phase is thermodynamically stable. The heat capacity of an Al₇₀Pd₂₁Tc₉ sample is measured in the temperature range 3–30 K. The electrical resistivity and magnetic susceptibility are determined in the temperature range 2–300 K. The electrical resistivity is found to be high (600 μΩ cm at room temperature), which is typical of quasicrystals. The temperature coefficient of electrical resistivity is small and positive at temperatures above 50 K and negative at temperatures below 50 K. The magnetic susceptibility has a weakly paramagnetic character. The coefficient of linear contribution to heat capacity (γ=0.24 mJ/(g-atom K²)) and the Debye characteristic temperature (Θ=410 K) are determined. The origin of the specific features in the vibrational spectrum of the quasicrystals is discussed. __________ Translated from Fizika Tverdogo Tela, Vol. 42, No. 12, 2000, pp. 2113–2119. Original Russian Text Copyright ? 2000 by Mikheeva, Panova, Teplov, Khlopkin, Chernoplekov, Shikov.     

Thermal properties of PCR-1 and PCR-37 piezoceramics

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.     

Calorimetric investigation of electronic and lattice excitations of the icosahedral quasicrystal in the range of moderate temperatures

The precise measurements of the specific heat and the linear expansion coefficient of polygrain samples of the ordered icosahedral phase Al₆₃Cu₂₅Fe₁₂ have been performed in the temperature range 1.8–400.0 K. The deviations from the Grüneisen law, according to which the temperature dependences of the lattice specific heat at constant volume and the linear expansion coefficient are identical to each other, have been analyzed. The proofs that the specific heat of the quasicrystals contains latent electronic and lattice contributions of the Schottky type have been obtained. The revealed contributions can be thermodynamic consequences of the fractal energy spectra.     

Low-temperature vibrational properties of tin nanoparticles in porous glass

The heat capacity has been studied in the temperature range 2.2–40 K and in magnetic fields up to 2 T in tin, which has been embedded in nanometer-size pores in glass having diameter ∼7 nm, in bulk tin and in glass with empty pores. Comparison of the properties of tin nanoparticles and bulk tin has been performed. An increase in the coefficient of electronic heat capacity has been found in nanostructured tin as compared with the bulk tin, and also a considerable deviation of the low-temperature lattice heat capacity from the Debye law in the temperature region T > 3 K has been found. The fact that the density of thermal vibrations in nanocrystalline tin for low energies is higher than in bulk tin has been established using low-temperature heat capacity data.     

Coexistence of superconductivity and long-range magnetic order in TmRh4B4

Electrical resistance, heat capacity, thermal conductivity and linear thermal expansion coefficient measurements on TmRh₄B₄ indicate that superconductivity and long-range magnetic order coexist in this compound for temperatures less than 0.4 K.     

Electronic structure and magnetic phase transition in MnSi

Temperature variations of the amplitude of zero-point and thermal spin fluctuations in a helicoidal ferromagnetic (MnSi) are characterized using the electronic structure model that follows from ab initio LDA + U + SO calculations. It is found that a drastic reduction in the amplitude of zero-point spin fluctuations at temperature T _S (in the vicinity of the magnetic phase transition) leads to ferromagnetic solution instability (a change in the sign of the intermode interaction parameter). The observed magnetovolume effect and a sharp change in the radius of spin correlations have the same underlying cause. The results of calculation of the volumetric coefficient of thermal expansion agree well with the observed anomaly in the region of the magnetic phase transition.     

Thermal expansion of niobium diselenide in the basal plane at low temperatures

The thermal expansion of a high-quality 2H-NbSe₂ single crystal is precisely measured in the basal plane and the temperature range 5.7–50 K. An anomaly of the thermal expansion in the basal plane is detected, and it is found to be caused by a superconducting transition. The change in thermal expansion coefficient α _ab is used to calculate the pressure derivative of the critical temperature (dT _c /dp _ab ) = (19.0 ± 2.0) × 10^?9 K/Pa, and this derivative agrees well with the reported data. An additional anomaly, which indicates a phase transition, is also detected in the pretransition range. The nature of the detected phase transition is discussed.     

On the Induction of the First-Order Phase Magnetic Transitions by Acoustic Vibrations in MnSi

The main result of the paper contains the conclusion that the magnetic phase transition in MnSi always remains first order at any temperature and magnetic field. In these aims, a model of coupling of an order parameter with other degrees of freedom is used. The coupling of magnetic order parameters with long-wave acoustic phonons, in the presence of the nonsingular parts of the bulk and shear moduli, a first-order transition occurs, participle near the transition the heat capacity and the compressibility remain finite, if the heat capacity becomes infinite in the system disregarding the acoustic phonons. The role of the Frenkel heterophase fluctuations is discussed. The impurity effect shows that, for some phases, the heat capacity of the system remains continuous and finite at the transition point. It is supposed that the transition is progressively smoothed by these fluctuations at the application of the magnetic field.     

Thermodynamic and magnetic properties of LaSn3

Thermal expansion coefficient between 77 and 900K, isothermal compressibility in the 0–80 Kbar pressure range, magnetic susceptibility between 77 and 1300 K and heat capacity at constant pressure in the 20–300 K temperature range were determined for the LaSn₃ compound. From the experimental data, the specific heat at constant volume was calculated and the thermal dependence of the Debye's parameter θ_D was obtained. The electron contribution to the heat capacity was also determined from the high temperature data. The magnetic properties confirm that there is no evidence of the existence of a magnetic moment localized on La atoms, in contrast with a previous report and in agreement with the general assumptions. A little anomaly found in the expansion coefficient, in the isothermal compressibility and in the specific heat is discussed in terms of a lattice order-disorder phenomenon.     

An apparatus for the measurement of thermal expansion of solids at low temperatures

A dilatometer, using the three terminal capacitance technique, suitable for measurement of linear thermal expansion of solids in the temperature range 1.3–300 K is described. The dialtometer is designed such that the mounting system for the specimen does not undergo any significant changes in dimensions when the specimen is heated. The apparatus, therefore, yields in principle absolute values of α, the coefficient of linear thermal expansion. The performance of the apparatus has been checked by measurements on copper in the temperature range of 77–300 K. Some preliminary results on the behaviour of α for Y₁Ba₂Cu₃O_6.9 compound in the vicinity of superconducting transition temperature,T _c are also described. The system can detect relative changes in length Δl/l ₀ of about 10⁻⁸. Attempts are being made to improve the sensitivity.     

Lost heat capacity and entropy in the helical magnet MnSi

The heat capacity of MnSi at B = 0 and B = 4 T was measured in the temperature range 2.5-100 K. To analyze the data, calculations of the phonon spectrum and phonon density of states in MnSi were performed. The calculated phonon frequencies were confirmed by means of inelastic neutron scattering. The analysis of the data suggests the existence of negative contributions to the heat capacity and entropy of MnSi at T > T(c) that may imply a specific ordering in the spin subsystem in the paramagnetic phase of MnSi.     

Tricritical phenomena at the gamma-->alpha transition in Ce0.9-xLaxTh0.1 alloys

The gamma-->alpha isostructural transition in the Ce0.9-xLaxTh0.1 system is measured as a function of La alloying using specific heat, magnetic susceptibility, resistivity, thermal expansivity or striction measurements. A line of discontinuous transitions, as indicated by the change in volume, decreases exponentially from 118 K to close to 0 K with increasing La doping, and the transition changes from being first-order to continuous at a critical concentration, x(c) approximately 0.14. At the tricritical point, the coefficient of the linear T term in the specific heat gamma and the magnetic susceptibility increase rapidly near x(c) and approach large values at x=0.35 signifying that a heavy Fermi-liquid state evolves at large doping. The Wilson ratio reaches a value above 2 for a narrow range of concentrations near x(c), where the specific heat and susceptibility vary most rapidly with the doping concentration.     

Positive magnetoresistivity of Fe0.95Co0.05Ge2 single crystals inside the region of a magnetic phase transition

We report on the detection of two narrow peaks of positive magnetoresistivity in the temperature dependence of the magnetoresistivity of a Fe_0.95Co_0.05Ge₂ single crystal in the vicinity of a “smeared” first-order phase transition. The position of these peaks correlates with the position of singularities in the temperature dependence of the temperature derivative of the electrical resistivity and magnetic susceptibility. We show that these singularities in the transport and magnetic properties are, probably due to the presence of two percolation transitions with temperature in the magnetic subsystem of the crystal. Zh. éksp. Teor. Fiz. 112, 690–697 (August 1997)     

Temperature dependence of the heat capacity and lattice constant of lanthanum and samarium hexaborides

The temperature dependence of isobaric heat capacity and [411] interplanar spacing in lanthanum and samarium hexaborides have been determined experimentally within the 5–300 K region. The variation of the lattice parameters and thermal expansion coefficients α(T) with temperature has been calculated. Fiz. Tverd. Tela (St. Petersburg) 40, 2051–2053 (November 1998)     

Characteristics of the thermodynamic and kinetic properties of NbH0.83 in the phase transition region

Measurements were made of the specific heat (in the temperature range 80–400 K), electrical resistivity (4.2–300 K), thermo-emf (4.2–300 K), thermal conductivity (7–300 K), magnetic susceptibility (4.2–400 K), and lattice parameters (30–300 K) of the alloy NbH_0.83. The λ′→λβ phase transitions were studied. It was established that the structure of the niobium matrix of the hydride remains unchanged as a result of these transitions. It is shown that the β→λ phase transition (one-dimensional hydrogen ordering) is a three-stage one and is accompanied by a substantial change in the shear modulus of the hydride. The λ′ phase existing at T<110 K was determined. The electron thermal conductivity κ_el calculated for the λ′ phase in the range T<23 K is 25% higher than the measured thermal conductivity. In order to explain this fact and also the discontinuity in the concentration dependence of the coefficient of thermal expansion of NbHx for x?0.83–0.84 and the approximately 1.5% compression of the NbH_0.84 volume it is assumed that in the region of x _c in the λ′ phase the topology of the Fermi surface of the NbH_x interstitial alloy changes substantially.