V. K. Novik

Relationship between the pyroelectric coefficient and the lattice dynamics of the pyroelectrics

Abstract The experimental results for a temperature dependence of the pyroelectric coefficient are represented for twenty different crystals in a wide temperature range. Current theories are used for an analytical fitting of this result. It is shown that in the ferroelectrics the pyroelectric coefficient γσ is defined mainly by optical phonon modes in comparison with the linear pyroelectrics in which γσ follows the T3 law due to acoustic phonon. The possibility of an appearance of the linear temperature dependence of γσ ≈ T suggested by Born is discussed briefly. Our experimental results show that such behavior indeed exists but only in very imperfect crystals and at very low temperature.

The low-temperature behaviour of the pyroelectric coefficient

Abstract The results of both theoretical and experimental investigations of the temperature dependence of the pyroelectric coefficient γδ (T) are given. It is shown that in ideal crystals the γδ (T) may be represented by the sum of Debyes and Einsteins functions, which describe the contributions of acoustic and optical modes, respectively. Experimentally obtained γδ (T) dependences for different kinds of linear pyroelectrics and ferroelectrics are listed. The experimental data for linear pyroelectrics (except the region of the lowest temperatures) are described well by the mentioned sum of Debyes and Einsteins functions. In ferroelectrics the acoustic modes contribution to the γδ (T) is not found out in the frames of the accuracy of measurements. The simple estimations of the coefficient of anharmonicity and of the nonlinear dipole moment show that in ferroelectrics the relative contribution of the optical modes is greater than of the acoustic ones to the extent of the ratio (ω D /ω oj )2, where ω D i...

On the role of a weak hydrogen bond OH…O in the formation of the anomalous dielectric response of crystals and polymers near 40°C

In some crystals, polymers, and gels that contain hydrogen bonds OH…O, NH…O of length 2.8–3 Å or water molecules, gigantic anomaly of dielectric permittivity (ɛ ∼ 103–106) is observed in certain circumstances at frequencies of 1–106 Hz, which is accompanied by peculiarities in conductivity σ and dielectric losses tanδ. In crystals this effect appears after a sudden cooling to −50°C and it is observed at slow heating in the range of 20–40°C. At the return temperature course from 40°C dependences ɛ(T), σ(T), and tanδ(T) have their usual values. Anomalies in objects that differ by their compositions are unified by their temperatures, which are all close to 40°C. Authors have made an attempt to explain the similarity of these phenomena by the features of hydrogen bonds that are present in the objects.

Proustite-a new ferroelectric with no 180° polarization reversal

In this work the peculiarities of ferroelectric phase transition in proustite at T = 28. 7 K, which is of the first order and is accompanied by the change of symmetry from 3m to 1, are examined.

Point defects and singularities of the low-temperature (T < 15 K) behavior of the pyroelectric coefficient and the spontaneous polarization of TGS, LiTaO3 and LiNbO3

Abstract The common phenomenon of the formation of the additional component Psi of the spontaneous polarization resulting from the ordered localization of impurity ions (charged point defects) in the field of the host polar lattice for polar dielectrics (linear pyroelectrics and ferroelectrics) is established. The additional component is not, in the general case, collinear to the matrix lattice, the spontaneous polarization Psm, and correlates with it as |Psi| |Psm|−1 < 10−4. In spite of the small value of |Psi|, its temperature dependence at T ⩽ 7 K is the main contribution to the value of the pyroelectric coefficient. Based on the results of investigations of single crystals TGS, LiTaO3 and LiNbO3 with different contents of point defects, the following mechanisms are responsible for changes of the spontaneous polarization, as the temperature decreases from the paraelectric phase: for pure crystals, the mechanism of phase transition (order-disorder or displacive type) → some optical modes anharmonism; fo...

Dielectric loss as an indication of the kinetics of the ferroelectric phase transition

The dependences tanδ(f, T) for triglycine sulfate and diglycine nitrate single crystals are investigated in the frequency range f = 1−2 × 107 Hz at temperatures from −150 to 50°C for triglycine sulfate and from −150 to −60°C for diglycine nitrate. The results are discussed in the framework of the model according to which two or more types of media with different dielectric properties are formed in the ferroelectric phase. The polar volumes of domains themselves and the volumes of domain walls with different symmetries are treated as individual media. The dependences tanδ(f, T) make it possible to separate the contribution of a particular medium and to trace its evolution with temperature. The results of the analysis of the relaxation properties confirm that the phase transitions lead to the formation of two media (the polar medium and domain walls) in triglycine sulfate and three media (the polar medium and two types of domain walls) in diglycine nitrate. It is demonstrated that the relaxation properties are not steady-state characteristics of the samples but depend on their prehistory.

Bioferroelectricity and related problems: Hydrogen-bonded ferroelectric-like systems

Abstract Short review of bioferroelectricity and related problems is presented. One of the important cases the hydrogen-bonded systems, are analysed in detail. The proposed model describes the soliton-like dynamics of hydrogen-bonded ferroelectrics with two interacting subsystems (protons and ordered ferroelectric) in the vicinity of phase transition. As a results, the shift of the phase transition point is obtained. The results are compared with experimental data on microwave irradiation of triglycine sulphate.

Low-Temperature Pyroelectricity

The current state of the set of problems associated with the study and application of the pyroelectric effect at low (T<30 K) temperatures is outlined. The reasons for the qualitatively different temperature dependences of the total pyroelectric coefficient γσ(T) in linear pyroelectrics and ferroelectrics are discussed. An analysis is given of the reasons why the theoretical temperature dependences obtained for the primary pyroelectric coefficient γε(T) adequately describe the experimental γσ(T) dependences for all materials studied. In this connection, the correctness of determining the secondary pyroelectric coefficient γS(T) from the macroscopically measured coefficients of thermal expansion is considered. The potential of thermodynamically nonequilibrium polar media and low-temperature pyroelectric materials is substantiated. The review consists of the following sections: (1) an introduction; (2) the theory of low-temperature pyroelectricity; (3) experimental procedures; (4) discussion of the experimental data: (A) perfect single crystals, (B) the problem of the secondary pyroelectric coefficient, (C) imperfect single crystals; (5) pyroelectric materials for low-temperature applications; and (6) a conclusion.

Dielectric Response of Holmium Formate Crystallohydrate at 100mHz–10MHz

The dielectric response of Holmium formate crystallohydrate was studied in the frequency range of 10−1–107 Hz at 20–70°C. Experimental data were analyzed within formalisms of dielectric permittivity, conductivity and dielectric modulus. Specific features of dielectric properties and conductivity which were detected are typical for crystallohydrates and ferroelectrics with hydrogen bonds. It is assumed that such anomalies are caused by changes in the dynamics of protons that participate in hydrogen bonds in the crystal structure.

3D dependence of the dielectric dispersion in a BaTiO3 single crystal

The 3D dependences ɛ′(log f, T) and tanδ(logf, T) of a perfect BaTiO3 single crystal grown by the Remeika method have been studied in the ranges f = 1–2 × 107 Hz and T = −80–130°C. These dependences characterize a transition from the paraelectric phase (121.5°C) as a near-antiferroelectric transition followed by the transition to the tetragonal phase at ∼79.5°C. According to a number of signs, the range 121.5–79.5°C corresponds to a metastable phase typical of first-order phase transitions. The unexpected result of this work has been discussed with invoking the hypothesis on the BaTiO3 structure in the paraelectric phase, according to which it consists of three antiferroelectric states oriented along the crystallographic axes. Using the dielectric properties of BaTiO3 as an example, the method of direct correct determination of the temperatures of the structural transformations from the anomaly of tanδ(logf, T) has also been demonstrated.