Critical behavior of the heat capacity of the manganite La<Subscript>0.87</Subscript>K<Subscript>0.13</Subscript>MnO<Subscript>3</Subscript>

The heat capacity of the manganite La_0.87K_0.13MnO₃ has been measured in the temperature range 80–350 K. The nature of the ferromagnetic phase transition and the critical properties of heat capacity near the Curie temperature have been studied. The regularities of variations in the universal critical parameters near the phase transition point have been established. The calculated critical exponent and amplitudes of the heat capacity with allowance for corrections on the scaling (α = −0.13 and A ⁺/A ⁻ = 1.178) correspond to the critical behavior of the 3D Heizenberg model.     

Critical behavior of the heat capacity of Ag-doped manganites

The critical behavior of the heat capacity of silver-doped manganites has been studied. The laws of variation in the universal critical parameters near the phase transition point have been established. It has been shown that the universality class of the critical behavior of the heat capacity for the La_1−x Ag _y MnO₃ (y≤x) system does not depend on the silver concentration or the synthesis conditions.     

Effect of γ irradiation on the heat capacity of Rb2ZnBr4 in the vicinity of phase transitions

Heat capacity of Rb₂ZnBr₄ as a function of γ irradiation dose has been measured within the 85–300 K range by the adiabatic calorimeter technique. It is shown that, as the irradiation dose increases, the heat capacity peak in the vicinity of the incommensurate-commensurate first-order phase transition (PT) decreases, and the transition temperature T _c increases. The heat capacity peak in the region of the second-order PT at T ₃=112 K does not depend on γ irradiation, both in magnitude and in position, just as the heat capacity throughout the remainder of the temperature range studied. Fiz. Tverd. Tela (St. Petersburg) 40, 1106–1108 (June 1998)     

Quasi-reversible superprotonic phase transition in K9H7(SO4)8 · H2O crystals

The heat capacity and the bulk conductivity have been measured in the K₉H₇(SO₄)₈ · H₂O crystal. The hysteresis phenomena have been studied upon superprotonic phase transition. It is revealed that the phase transition is reversible upon thermocycling only for the monohydrate form of the crystal. The exponential temperature dependence of the heat capacity in the supercooled disordered phase exhibits a thermally activated behavior and is determined by the defects responsible for the high conductivity of the crystal.     

Heat capacity and the p-T phase diagram of Pb2MgTeO6 elpasolite

The heat capacity of Pb₂MgTeO₆ is measured in the temperature range 80–300 K. It is found that the heat capacity exhibits an anomaly associated with the phase transition at T ₀=186.9 K. The thermodynamic parameters of the structural transformation are determined. The effect of hydrostatic pressure up to 0.5 GPa on the phase transition temperature is examined.     

Specific heat of the elpasolite Pb2MgWO6

The specific heat of Pb₂MgWO₆ has been measured in the temperature interval 83–370 K. An anomaly in the specific heat associated with the phase transition at T ₀=312.8K has been discovered. The thermodynamic parameters of the structural phase transition Fm3m-Pmcn have been determined. Fiz. Tverd. Tela (St. Petersburg) 41, 1686–1688 (September 1999)     

Diffuse phase transition of Fe doped lead ytterbium tantalate ceramics

The effect of different concentration of Fe on the phase transition behavior of Lead ytterbium tantalate is investigated by dielectric and differential scanning calrimetry measurements. The samples are prepared through solid state reaction method and it has been found that the sintering temperature significantly lowered when the proportion of Pb(Fe_1/2Ta_1/2)O₃ increased. It has been observed that the doping in small amounts (0≤x≤0.2) of Fe could meliorate the dielectric and ferroelectric properties. The diffuseness in the mode of phase transition increases and the phase transition temperature decreases as a function of Fe content. It is revealed that the dielectric data and heat capacity data follow a similar trend in the variation of the mode of phase transition and phase transition temperatures. The phase transition temperature values obtained from the heat capacity measurement well agreed with the values obtained from dielectric measurement.     

Theoretical calculations of the high-pressure phases of ZnF2 and CdF2

First-principles calculations based on density functional theory were used to study the high-pressure phases of both ZnF₂ and CdF₂. We found that the sequence of the pressure-induced phase transitions is: Rutile (P4₂/mnm) ↦ CaCl₂ (Pnnm) ↦ PdF₂ (Pa-3) and CaF₂ (Fm3m) ↦ PbCl₂ (Pnma) ↦ Ni₂In (P6₃/mmc) for ZnF₂ and CdF₂ respectively. In ZnF₂ the behavior of the ground-state total energy, of the Gibbs free energy and of the lattice constant vs. pressure shown that the phase transition at 4 GPa from the rutile-type phase to the CaCl₂-type phase is a second-order phase transition. The mechanism of the structural change was also revealed by the transition from the PbCl₂-type phase to the Ni₂In-type phase in CdF₂. Moreover, the high-pressure behavior of divalent metal fluorides was compared and discussed.     

Critical properties of the models of small magnetic particles of the antiferromagnet MnF2

The static critical behavior of the models of small magnetic particles of the real two-sublattice antiferromagnet MnF₂ is investigated by the Monte Carlo method taking into account the interaction of the second nearest neighbors. Systems with open boundaries are considered to estimate the influence of the sizes of particles on the pattern of their critical behavior. The behavior of thermodynamic functions in the phase transition region is investigated. The data on the maxima of the temperature dependences of heat capacity and magnetic susceptibility are shown to be insufficient to unambiguously determine the effective temperature of the phase transition in the models of small magnetic particles. This requires an additional investigation of the spatial orientation of the sublattice (sublattices) magnetization vector for the models under study.     

High-temperature heat capacity of Gd2CuO4

The heat capacity of Gd₂CuO₄ has been studied by differential scanning calorimetry in the temperature range 362–958 K. It has been found that the temperature dependence of the molar heat capacity has an extremum near 590 K; the extremum is related to a phase transition from the high-temperature tetragonal phase to the low-temperature orthorhombic phase.     

Anomalous critical behavior of NaV2O5

The temperature dependences and critical behavior of the dielectric constant were studied in NaV₂O₅ along the c axis in a frequency range of 1 MHz-1GHz and a temperature range of 4.2–300 K. An analysis of the data obtained, along with literature data on the heat capacity, magnetic losses, and the ultrasound velocity, indicates that the various physical quantities demonstrate similar temperature dependences in the vicinity of the transition, which corroborates the conclusion on the universality of the critical behavior in NaV₂O₅. Deviations from the predictions of the standard theory of second-order phase transitions were found, such as the asymmetry of the critical behavior above and below the transition and the presence of an anomalous base line.     

High‐resolution Brillouin scattering and heat capacity studies of bis(guanidinium)zirconium bis(nitrilotriacetate)hydrate, [C(NH2)3]2Zr[N(CH2COO)3]2·H2O

High‐resolution micro‐Brillouin scattering and heat capacitiy studies of polar bis(guanidinium)zirconium bis(nitrilotriacetate)hydrate were performed in the temperature range 230–305 K. Strong anomalies in the Brillouin shift and the attenuation were observed near 290 K upon cooling for the sound waves corresponding to the c₂₂ elastic constant. For this elastic constant, the coupling between the order parameter and the elastic strains is linear quadratic. In case of the c₄₄ and c₆₆, the coupling is biquadratic, giving rise to an increase of these parameters below 290 K. The analysis of the acoustic anomalies and heat capacity results near 290 K revealed that they correspond to a second‐order, cell‐multiplying, displacive phase transition. The second phase transition leads to weak acoustic anomalies at approximately 284 K. Copyright © 2009 John Wiley & Sons, Ltd.     

Relating phase transition heat capacity to thermal conductivity and effusivity in Cu2Se

Accurate measurement of thermal conductivity is essential to determine the thermoelectric figure‐of‐merit, zT. Near the phase transition of Cu₂Se at 410 K, the transport properties change rapidly with temperature, and there is a concurrent peak in measured heat capacity from differential scanning calorimetry (DSC). Interpreting the origin as a broad increase in heat capacity or as a transient resulted in a three‐fold difference in the reported zT in two recent publications. To resolve this discrepancy, thermal effusivity was deduced from thermal conductivity and diffusivity measurements via the transient plane source (TPS) method and compared with that calculated from thermal diffusivity and the two interpretations of the DSC data for heat capacity. The comparison shows that the DSC measurement gave the heat capacity relevant for calculation of the thermal conductivity of Cu₂Se. The thermal conductivity calculated this way follows the electronic contribution to thermal conductivity closely, and hence the main cause of the zT peak is concluded to be the enhanced Seebeck coefficient. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

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.     

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 T _m ～ 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).

     

Calorimetric investigation of (TEA)2MnCl4 crystals

Temperature dependencies of the specific heat in a temperature range of 14–260?K are presented for the crystals (TEA)₂MnCl₄. Two anomalies typical for the first order phase transition at T ₁?=?224?K and T ₂?=?231?K with entropy jumps of 15?J?mol^?1?K^?1 and 12?J?mol^?1?K^?1 were observed. The temperature dependence of the lattice heat was approximated by a linear combination of Debye and Einstein functions. Basing on the results an effective wave number of phonons with an essential contribution to the lattice oscillation energy was determined. Since phase transition sequence observed in (TEA)₂MnCl₄ and (TEA)₂CuCl₄ is similar and similar is the chemical composition one can suppose that the low temperature phase transition observed in (TEA)₂MnCl₄ crystals is an isostructural transition.     

A simple theory of condensation

A simple assumption of the emergence in gas of small atomic clusters consisting of c particles each leads to a phase separation (first-order transition). It reveals itself by the emergence of a “forbidden” density range starting at a certain temperature. Defining this latter value as the critical temperature predicts the existence of an interval with the anomalous heat capacity behavior c _p ∝ ΔT ^−1/c . The value c = 13 suggested in the literature yields the heat capacity exponent α = 0.077.     

Thermodynamic parameters of diffuse phase transitions in Ag2Te

A differential thermal analysis in vacuum and studies of the coefficients of electrical conductivity and thermal conductivity, and of the thermal emf are made in Ag₂Te in the neighborhood of the structural phase transition. It is shown that these data can be used to calculate the switching factor L(T), determine the region in which the phases coexist within the transition, and calculate the thermodynamic parameters. Prior to and after the main phase transition, additional displacement transitions are observed. It is found that the phase transition takes place roughly according to the scheme α _{385 K}→α′_{405 K}→β′_{420 K}→β _{440 K}. The specific heat C _p, changes in entropy ΔS and transition enthalpy ΔH, as well as the minimum phase fluctuation volume V, and the heat Q of the phase transition are determined. It is shown that excesses of Te and Ag have almost no effect on the transition temperatures T ₀, but have a substantial effect on the thermodynamic parameters. Fiz. Tverd. Tela (St. Petersburg) 40, 1693–1697 (September 1998)     

Influence of the lanthanum deficit on electrical resistivity and heat capacity of silver-doped lanthanum manganites La<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ag<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>MnO<Subscript>3</Subscript>

The electrical resistivity and heat capacity of the silver-doped lanthanum manganites La_0.80Ag_0.15MnO₃ and La_0.85Ag_0.15MnO₃ have been investigated. Despite the nonstoichiometry of the composition, the La_0.80Ag_0.15MnO₃ manganite exhibits a bulk homogeneity and better physical properties from the applied point of view as compared to the La_0.85Ag_0.15MnO₃ manganite, viz., the former compound has a higher spontaneous magnetoresistance and a larger jump of the heat capacity with a small width of the phase transition, and the anomalies of the heat capacity and electrical resistivity in the vicinity of the Curie point of this compound agree with the fluctuation nature of the second-order phase transition. The behavior of the properties of lanthanum-deficient manganites under investigation in the region of the phase transition is consistent with the classical theory of indirect exchange interaction. The behavior of the temperature dependence of the electrical resistivity has been analyzed in terms of two models. One of these models is based on the tunneling of charge carriers between ferrons or polarons, and the other model is based on the polaron hopping conduction. Both approaches lead to consistent results, and their combination has made it possible to estimate the tunneling distance of charge carriers. The origin of the influence of technological parameters characterizing the synthesis of La_{1 − x} Ag _y MnO₃ ceramic materials on their physical properties has been elucidated.     

Landau behaviour of the heat capacity of AgNa(NO2)2 close to the nearly tricritical ferroelectric phase transition

The heat capacity of a single crystal of the uniaxial ferroelectric AgNa(NO₂)₂ was measured close to the nearly tricritical phase transition. In the ferroelectric phase a strong temperature dependence of the anomaleous specific heat is found which obeys a temperature law (θ _f —T)^?0.494. A small latent heat was detected nearT _c. The thermal behaviour of AgNa(NO₂)₂ is described by a Landau type theory, from which the non linear coefficientsζ andζ of the free energy expansion are derived. The thermal data fit well to the known dielectric behaviour.