Study of the23Na EFG tensor as determined from first order NMR satellite lines near the ferroelectric phase transition of AgNa(NO2)2

The²³Na NMR first order quadrupolar satellite lines are detected and studied in AgNa(NO₂)₂ single crystals near the paraelectric-ferroelectric phase transition. Although the intensities of the satellites are small the components of the electric field gradient tensor (EFG) at the²³Na sites could be deduced from the angular dependence of the first order quadrupolar line splitting with respect to rotations around the three crystallographic axes by applying the Volkoff method. In the paraelectric phase the principal axes system of the EFG coincides with the crystallographic axes system whereas in the ferroelectric phase there is a strongly temperature dependent small non diagonal element _xz (T). The EFG principal components _xx and _yy are strongly influenced whereas the EFG principal component _zz is nearly not affected by the ferroelectric phase transition. The observed temperature dependences of the EFG components are related to the temperature variation of the normalized spontaneous polarizationS(T) by assuming a coupling term which is quadratic inS(T). Finally the problem of the intensities of the satellite lines is discussed.     

Critical behavior of heat capacity of the SC(NH2)2 ferroelectric in the region of incommensurate phase transition

The anomalous (nonclassical) behavior of heat capacity in the region of the second-order phase transition “initial phase-incommensurate phase” was experimentally observed in the SC(NH₂)₂ ferroelectric. Such a critical behavior of heat capacity above and below the temperature of incommensurate phase transition is shown to be qualitatively consistent with the fluctuation theory of XY-type systems.     

Influence of hydrostatic pressure on the polarization dynamics of order-disorder ferroelectrics: AgNa(NO2)2

The characteristic ferroelectric dispersion of the complex dielectric constant of the ferroelectric AgNa(NO₂)₂ is measured as function of temperature and hydrostatic pressure. For the first time the temperature and pressure dependences of the static order parameter susceptibility and the order parameter relaxation time could be reproducibly determined. The results are interpreted in terms of a kinetic Ising model, whose parameters show a characteristic pressure dependence. The results are discussed on the basis of thermodynamic relations and related to the dynamical behaviour of other order-disorder ferroelectrics.     

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)     

Application of the Landau theory on the2d structural phase transition of CH4 on (0001) graphite

A few degrees before melting CH₄ absorbed on (0001) Graphite undergoes a structural phase transition from a commensurate to an incommensurate2d solid. In the commensurate phase the molecules show a hexagonal structure. Since it is known from specific heat measurements that the corresponding phase transition is of continuous type, Landau's theory of second order phase transition should be applicable. Application of this theory allows predicting that the structure of the CH₄ molecules in the incommensurate phase should show a rectangular distortion, driven by a one or two component symmetry breaking order parameter. The rectangular distortion is in agreement with other more general theoretical investigations.     

A systematic approach to the calculation of tricritical temperature with application to the nematic-smecticA phase transition

Using the Landau expansion equations have been found that allow for systematic calculations of tricritical temperature for a given molecular model. The theory has been applied to nematic-smecticA (NA) phase transition of liquid crystals. It has been shown exactly that the NA tricritical temperature depends only on the couplings between the two lowest order translational order parameters and the orientational degrees of freedom. Numerical results of Mayer and Lubensky for the NA tricritical point have been derived exactly. Also a stability condition of the obtained solution has been discussed.Alexander von Humboldt Foundation Fellow, 1985–1986     

Simple Landau model of the smectic- A-isotropic phase transition

A simple Landau-type free energy function is presented to describe the smectic-A-isotropic phase transition. Varying the coupling between orientational and positional order parameters, a smectic-A-isotropic or a nematic-isotropic phase transition occurs. Within this model the smectic-A-isotropic phase transition is found to be always more strongly first order than the nematic-isotropic phase transition. The theoretical results are found to be in good agreement with all published experimental results. Received 27 June 2000     

Effect of an organic dye on the ferroelectric phase transition in KH2PO4 (KDP)

This paper reports on the first measurement of the dielectric permittivity and heat capacity of a KDP crystal doped by Chicago Sky Blue organic dye within a temperature interval including the ferroelectric phase transition at T _c =122 K. Similar measurements were made on a pure KDP crystal under the same conditions for the sake of comparison. The heat capacities of the pure and doped crystals were shown to differ substantially within an interval 1 K wide in the vicinity of T _c , where an anomaly in the heat capacity of the doped crystal was observed to wash out without producing any change in the temperature position of its maximum. The doping reduces the permittivity in the polar phase markedly. The observed effects are associated with the influence of nonisomorphic defects on the ferroelectric phase transition in a piezoelectric crystal.     

Effect of electron irradiation on the heat capacity of [NH2(CH3)2]2 · CuCl4 in the phase transition region

The specific heat of crystalline [NH₂(CH₃)₂]₂ · CuCl₄ in the ferroelectric phase, both nonirradiated and irradiated by electrons, was measured calorimetrically. The temperature region of existence of the ferroelectric phase was shown to broaden under electron irradiation. The existence of an incommensurate phase above the Curie point T _c1 in the crystal was confirmed. The phase-transition sequence observed in [NH₂(CH₃)₂]₂ · CuCl₄ is shown to be described by a phenomenological model for A ₂ BX ₄-type ferroelectrics with an organic cation.     

Dielectric study of the ferroelectric phase transition of KH2PO4

We report high precision measurements of the dielectric constant of Potassium Dihydrogen Phosphate as a function of temperature and electric field. Landau model parameters are evaluated and the biasing field E_c which drives the transition second order is determined.     

Dielectric relaxation in the weak ferroelectric Li2Ge7O15 near the ferroelectric phase transition

The dielectric permittivity of Ni-doped Li₂Ge₇O₁₅ crystals was studied in the vicinity of the ferroelectric phase transition. Introduction of Ni has been shown to suppress the dielectric anomaly and to reduce substantially the transition temperature. A temperature hysteresis in ɛ (T) has been observed in nominally pure and Ni-doped Li₂Ge₇O₁₅ crystals near the transition point. Measurements performed under cooling from the paraphase reveal dispersion of dielectric permittivity at Debye relaxation frequencies of the order of 10⁴–10⁵ Hz at T _c . It is proposed that the hysteresis phenomena and the low-frequency dispersion are caused by residual defects (of the type of random local fields), which become polarized in the ferroelectric phase and become disordered above T _c . Fiz. Tverd. Tela (St. Petersburg) 40, 2198–2201 (December 1998)     

Landau phenomenological theory of the pressure-induced phase transition of phospholipid bilayers

Acta physica Academiae Scientiarum Hungaricae - A theoretical model is developed to describe the temperature and pressure-induced phase transitions of phospholipid bilayer membranes simultaneously....     

Thermal-wave investigation of the ferroelectric phase transition in colemanite

Thermal waves with penetration depths ranging from 150 m to 3 m and amplitudes smaller than 0.1 mK were used to investigate the ferroelectric phase transition in colemanite. The critical behavior of the pyroelectric current is found to be independent of the penetration depth. It agrees well with the prediction from mean field theory for the temperature derivative of the bulk order parameter. The results are discussed in terms of a possible application of the thermal wave method to the investigation of surface critical phenomena.     

Anomalous light scattering at the ferroelectric phase transition of KH2PO4

The temperature-dependent scattered light intensity accompanying the ferroelectric phase transition of KH₂PO₄ is presented as an isofrequency-temperature scan. The temperature dependence of the data is shown to be better fit to the coupled harmonic oscillator model than to the single overdamped oscillator. This mode of data presentation also displays the anomalous light scattering near the phase transition.     

Absence of the small tricritical deviation from the mean-field approximation for the isotropic-nematic phase transition

The tricritical behaviour of the isotropic-nematic phase transition is studied. The presence of two independent sixth-order terms in the free energy expansion breaks the conventional tricritical behaviour and eliminates the tricritical restriction for the deviation from the mean-field approximation.     

Splitting of the phase transition in ferroelectric solid solutions Sn2P2(SexS1−x)6 at high pressures

The influence of the temperature and hydrostatic pressure on the forbidden band width and dielectric permittivity of the ferroelectric solid solutions Sn₂P₂(Se_xS_1–x)₆ (x=0.04, 0.20, 0.30) is investigated. A change in the species and splitting of ferroelectric phase transition lines are detected for p = 0.140 and 0.026 GPa, respectively, for the solid solutions Sn₂P₂(Se_0.04S_0.96)₆ and Sn₂P₂(Se_0.20S_{0.80 6}, which is due to the existence of a Lifshits critical point separating the transitions into codimensional and noncodimensional phases on the p, T diagram. Shift coefficients with the pressure of the phase transition temperature are found for the solid solutions investigated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 28–32, February, 1988.