Size dependence of dielectric properties and structural metastability in ferroelectrics

The size effect of the dielectric properties and the barrier height was investigated in the ferroelectric solid solution Ba_xSr_1-xTiO₃ system. The decrease of the grain size causes the suppression of the ferroelectricity, and the increase of the relaxation frequency. Barrier heights increase with increasing grain size. The result is analogous to magnetic phase transitions in nanocrystals and other solid-solid phase transitions in nanocrystals. It suggests a general rule that may be of use in the discovery of new metastable phases. An explanation of this phenomenon was given by an electric potential model that agrees well with the experimental results. For Ba_xSr_1-xTiO₃ system, the decrease of xcauses the decrease of the barrier height. Received 3 August 1998 and Received in final form 22 November 1998     

Simulation on dielectric susceptibility and domain evolution of relaxor ferroelectrics

Dielectric susceptibility and domain evolution of the relaxor ferroelectrics have been simulated using the Monte Carlo method upon the Potts-Ising model. The grain size effect and the applied ac field frequency effect on the dielectric susceptibility were theoretically investigated. We found that the dielectric susceptibility increases and the T_m (the temperature at which the dielectric susceptibility reaches the maximum) shifts to lower temperature with increasing average grain size or decreasing frequency. In addition, we obtained the value of the relaxation parameter γ estimated from the linear fit of the modified Curie-Weiss law; its changing trend with increasing average grain size or increasing frequency was well consistent with the experimental observation. From the results of the domain pattern evolution process, we observed the differences between relaxor ferroelectrics and normal ferroelectrics subjected to an applied ac field.     

On the temperature dependence of the magnetic susceptibility of rhodium

A semiphenomenological function for the temperature variation of the magnetic susceptibility X(T) of Rh up to the melting point is presented. It interpolates between Pauli-Stoner (itinerant) and Curie (localized) behaviour. The low temperature expansion of X(T) up to T² can be correlated with the band structure dependent spin fluctuation model of Béal-Monod.     

On temperature dependence of impurity absorption and luminescence bands in dielectric crystals

In terms of the adiabatic approach to determining the states of an impurity dielectric crystal, the temperature dependence of the first moment (maximum) of the optical band that arises upon vibronic transitions in a local center is studied. It is shown that the frequency of the band maximum and absolute temperature are linked with each other by a relation that is analogous to the Wien’s displacement law for equilibrium radiation. This result is consistent with experimental data obtained, in particular for F absorption and luminescence bands in alkali halide crystals.     

Compositional dependence of the Curie temperature in mixed ferroelectrics: effective field approach

An effective field approach is used to investigate the compositon depedence of the Curie temperature T_C(x) for mixed ferroelectric systems of partially deuterated triglycine sulfate (TGS)_1−x(DTGS)_x, and partially deuterated triglycine selenate (TGSe)_1−x(DTGSe)_x. We found that the coupling strength between the two sub systems is weaker than the average value of the effective field within each system, this can account for the non-linearity of composition dependence of the Curie temperature in these mixed ferroelectric systems.     

Investigation of dielectric aging in layered ferroelectrics

The specific features of long-term relaxation of the polarization in the Na_0.5Bi_8.5Ti₂Nb₄O₂₇ and Na_0.5Bi_8.5Ti₂Ta₄O₂₇ ceramic materials with different prehistories are investigated. It is established that, for the Na_0.5Bi_8.5Ti₂Nb₄O₂₇ and Na_0.5Bi_8.5Ti₂Ta₄O₂₇ ceramic materials, a change in the measurement conditions, including heating or cooling to the temperature T _a under investigation, leads to a change in the time dependence ε′(t) describing the dielectric aging of the material at T _a = const. The results obtained are discussed from the viewpoint of the possible existence of defect complexes in disordered ferroelectrics.     

On the investigation of temperature changes of ferroelectrics dielectric constant by studying intensity changes of light emitted at their surface

The temperature dependence of the light intensity emitted by the plasma layer generated at the surface of triglycine sulphate samples at alternating polarization has been investigated. The changes of intensity of the emitted light were found to correspond to the changes of the dielectric constant of triglycine sulphate. This phenomenon can be used to determine the ferroelectric phase-transition temperature and to investigate the changes of the dielectric constant of ferroelectrics by studying the light of plasma generated at their surfaces.     

Electric field dependence of the Curie temperature and microwave absorption in displacive ferroelectrics with impurities

The electric field dependence of the complex dielectric constant in an anharmonic defect ferroelectric crystal is calculated in its paraelectric phase from the Silverman-Joseph-Hamiltonian augmented with fourth-order phonon coordinates using double-time Green’s functions. The field and impurity dependence of the shift in the Curie temperature is discussed. An expression is obtained for the electric field dependence of dielectric loss at microwave frequencies in defect ferroelectrics. An erratum to this article is available at .     

The temperature dependence of quantum distribution functions

In the present article we consider a method for obtaining the quantum distribution function in phase space given a specific choice of operator for a function of the coordinates and moments. The temperature equation is written for this function. We obtain the operator expression for the temperature quantum distribution function in phase space.In conclusion the authors would like to express their thanks to V. A. Kuchin for valuable observations in discussing the study.     

On the temperature dependence of the magnetic susceptibility of enhanced paramagnetic metals: Pure palladium

An expression for the temperature variation of the magnetic susceptibilityX(T) of enhanced paramagnetic transition metals is formulated.X(T) consists of one term interpolating between Pauli-Stoner- und Curie-type behaviour and a superposed second term leading to a low temperature susceptibility maximum. The occurrence of a maximum is accounted for by a phenomenologically derived temperature dependent enhancement factor. This concept yields a smooth curve describing the main features ofX(T) at low and high temperatures characterized by four effective model fitting parameters.Numerical values of these parameters are deduced from experiment for pure palladium as a well known representative.     

Nonlinear dielectric susceptibility of dipole impurities dissolved in the lattice of quantum paraelectrics

A general relation is derived for calculating the nonlinear dielectric susceptibility of a system placed in an external ac magnetic field, which consists of tunneling and interacting dipole impurities described by the Hamiltonian of the transverse Ising model and a lattice of ions whose dynamics is specified in the soft-mode model with the possible ferroelectric phase transition suppressed by zero-point quantum oscillations. It is shown that the relaxation peaks in the dielectric response can be enhanced several times through relaxator interaction with the soft mode. A general expression is obtained to describe the deviation from the Arrhenius law in this case. The results of this work can be employed for interpretation of recent experiments that revealed a strong effect of isovalent impurities at extremely low concentrations on the macroscopic properties of incipient ferroelectrics.     

On the dielectric susceptibility of classical Coulomb systems

This paper reports exact and numerical results on the shape dependence of the dielectric susceptibility of the one-component plasma (O.C.P.) in two dimensions. Some apparently conflicting predictions of phenomenological electrostatics and statistical mechanics are resolved. We prove indeed that, for a disk shaped two-dimensional one-component plasma at the particular temperatureT ₀ =q ² (2K _B )^–1, the Clausius-Mossotti relation is exactly fulfilled. It yields a value of the susceptibility which is twice that given by the second moment Stillinger-Lovett sum rule. Similar results are reported for the strip geometry. These discrepancies are explained in terms of shape dependent versus shape independent thermodynamic limits. We report also exact and numerical results on the size dependence of the dielectric susceptibility of the systems quoted above.     

Electric field dependence of the Curie temperature and microwave absorption in displacive ferroelectrics with impurities II

Expressions for the complex dielectric constant, microwave absorption and the Curie temperature in doped displacive ferroelectrics, subjected to an external electric field are discussed, using the approach made in our previous study. A cross-term of defect parameters with electric field and anharmonic parameters is obtained. The intrinsic parametersB andC are modified by impurity terms. The qualitativeE ² dependence of tan δ is discussed.     

On the temperature dependence of paramagnetic susceptibility in a generalized Néel model of ferrimagnetics

In this paper a general form of the relation for temperature dependence of an inverse paramagnetic susceptibility for a generalized Néel model of ferrimagnetics is derived by a molecular field method and some properties of this form are discussed.In conclusion the author thanks Prof. Dr. J. Bro and Dr. S. Krupika CSc for valuable comments and a discussion of this paper.     

The temperature dependence of the magnetic susceptibility of spin glasses

The temperature dependence of the magnetic susceptibility of spin glasses is calculated exactly on the basis of a magnetic cluster model, following Néel. The susceptibility of a single cluster below its blocking temperature is taken to be negligibly small compared to its value above. A distribution of blocking temperatures may be obtained from observations on spin glasses. Conditions for the sharpness of the susceptibility peak are discussed and the low-temperature variation of the susceptibility is also obtained.     

Pressure and temperature dependence of the static dielectric constant of KBr

The static dielectric constant and the temperature and pressure derivatives of the static dielectric constant of KBr have been measured at several temperatures between 4·3°K and room temperature. The sample was prepared in the form of a three-terminal parallel-plate capacitor and the dielectric constant was determined from measurements of the capacitance with a high precision bridge. The dielectric constant decreases with temperature as does the magnitude of the temperature dependence and the magnitude of the pressure dependence. The data were used to calculate the fixed-volume temperature derivative of the dielectric constant. This quantity, (? ln ε/?T)_v, exhibiting lattice-anharmonicity effects, decreases slightly from its room temperature value as the temperature is lowered, rises to a maximum value at about 33°K and then decreases rapidly at lower temperatures.