Saulius Lapinskas

Microwave study of the soft ferroelectric mode in Sn2P2S6 crystals

Abstract This paper presents the results of the investigation of dielectric dispersion in semiconductive ferroelectric Sn2P2S6 crystals over the frequency range 1 kHz to 78.5 GHz. The main dielectric dispersion in Sn2P2S6 is caused by the soft ferroelectric Bu mode and in the vicinity of the Curie point it occurs in the millimeter region. The frequency of the soft mode in the paraelectric phase varies according to vs = 35 (T-Tc )½ GHz on approaching the Curie point. The soft mode is strongly overdamped. Close to Tc the relative damping is y/vs = 14. The dielectric contribution of the soft mode is equal to the static dielectric permittivity and it explains its whole temperature-dependence.

High Frequency Measurements of Ferroelecrics and Related Materials in Coaxial Line

In this contribution we present calculations, which allow extending measurement range compared with the previous formalisms. In this model is accounted not only no homogeneous distribution of the electromagnetic field in the sample, but also additional modes, which generate in the vicinity of the sample in the dynamical capacitor. The examples of measurements of relaxor ceramics presented in the frequency range 10 MHz−3 GHz and compared between two different methods. As it is shown this method allows to measure more precisely at higher frequencies, especially ferroelectrics with high values of the dielectric permittivity such as relaxor ceramics.

Soft mode behavior in ferroelectrics Sn2P2S6 and Sn2P2Se6 near phase transitions

Abstract The paper presents the results of the microwave dielectric investigations of the semiconductive ferroelectrics Sn2P2S6 and Sn2P2Se6. The dielectric dispersion is caused by the ferroelectric overdamped Buu mode witch softens up to the millimeter range close to Tc. In Sn2P2S6 the soft Bu mode alone is responsible for the high value and the temperature-dependence of ϵ(0). In the incommensurate phase of Sn2P2Se6 the phason causes the main dielectric dispersion at mi crowaves.

Microwave and millimetre-wave dielectric response of Rb1-x(ND4)D2PO4 dipolar glass

The microwave and millimetre-wave dielectric response of Rb0.5(ND4)0.5D2PO4 dipolar glass has been studied around the frustration temperature Tf?120?K. It was found that above the frustration temperature Tf the soft relaxational deuteron mode is responsible for the whole dielectric dynamics below the phonon range. Its frequency decreases from that of submillimetre waves to that of microwaves following the classical law ?R = A(T-Tf), with A = 0.75?GHz?K-1, but in the vicinity of Tf the soft mode gradually shifts towards the glass-like dispersion predominantly related to the diffusion of Takagi defects according to the Vogel-Fulcher law. From the experimental results, the distribution of the relaxation times and of the local polarization at various temperatures is calculated. It is shown that the local polarization distribution function obtained from the dielectric response ?*(?,T) coincides well with that obtained from the NMR results.

Determination of the two dimensional distribution of the attempt relaxation times and activation energies from temperature dependence of dielectric dispersion

We present a method for numerical calculation of two dimensional distributions of the attempt relaxation times and activation energies from the temperature dependence of the experimental dielectric permittivity dispersion. We introduce empirical attempts to account for broad and/or asymmetric dispersions with the idea of using a weighted collection of Debye relaxation times. Then we present a modification of the aforementioned idea including attempt relaxation time and activation energy using the Arrhenius law, which significantly complicates the computation of the aforementioned distribution. Incorporating the activation energy and the attempt relaxation time into the equation transforms the discretized matrix equations into tensor equations. We rework the tensor equations into simpler matrix equations, thus permitting us to solve the presented discretized integral equation by using existing Least Distance Problem solving methods. Also, we present a regularization method and a way to choose the regularization parameter based on a best fit criterion. In the end we discuss the method showing some simulated results and experimental results. We then point out some problems involved in the calculations and propose methods to reduce their significance.

Dielectric investigations and theoretical calculations of size effect in lead titanate nanocrystals

The dielectric properties of nanograin ferroelectric lead titanate crystals are presented. The PbTiO3 samples were prepared by pressing nanopowders into plates and were studied experimentally by dielectric permittivity measurements in a wide frequency and temperature range. The TC dependence obtained showed a critical change of behavior with increasing mean nanoparticle size in the 9-nm region. The theoretical calculations based on Monte Carlo simulation were performed to describe the behavior of this material. It was shown that the distribution of nanoparticle sizes in the sample taken into account with the Monte Carlo method describes the dielectric properties of PbTiO3 nanocrystals quite well

Simulation of Relaxation Times Distribution for Relaxors using Distribution of Three-Dimensional Ising-Type Clusters

Using n-fold way dynamic algorithm we have studied the dynamical decay of the autocorrelation function of 3D clusters composed of Ising-type dipoles. Trying to reproduce the situation in real ferroelectric materials, we (i) instead of Glauber dynamics, assumed that the spin before the flip has no a priori knowledge of its final energy and (ii) used open boundary conditions assuming that in real experiment, due to existence of impurities, the system consists of finite domains of different sizes. In particular, we followed the behavior of the longest (“ergodic”) relaxation time of a finite domain, the time which is related to flips of a cluster polarization below transition temperature T c. We found that ergodic relaxation time scales with Lϵ ν and is proportional to exp[const(Lϵ ν)3.2]. This result is supported by calculation of surface tension obtained from probability distribution of polarization at T < T c. Accounting for the contributions of the longest relaxation times, we obtained the relaxation times distribution function characterized by two (sharp and robust) maxima in its relaxation time dependence. Our results qualitatively confirm the assumption that the non-Debye relaxation in dipolar glasses and relaxors might be caused by distribution of sizes of clusters, consisting of ordinary Ising-type dipoles.

Relaxation Times Obtained From Dynamical Decay Function of 1D and 3D Ising Model

We calculated the dynamical decay of the autocorrelation function of 1D and 3D systems of Ising spins. The calculations were performed by Monte Carlo method for different size domains and at different temperature. Analyzing autocorrelation functions for a transition mechanism, when spin flip dynamics depends on initial energy before the flip only, two main relaxation times were distinguished for each size. When dynamics depends on energy difference before and after the flip, only one relaxation time, (corresponding to the main mode) which depends on domain size, is obtained. These results strongly suggest that non-Debye relaxation in dipolar glasses and relaxors might be caused by domain size distribution.

Two Dimensional Distribution of Relaxation Times

In this paper the method of calculation of two dimensional distribution function from dielectric spectra is presented. Simulation showed, that it is possible to obtain distribution function from experimental dielectric spectra.

Dielectric Investigations and Theoretical Calculations of Size Effect in Lead Titanate Nanocrystals

In this paper the dielectric properties of nanosize ferroelectric lead titanate crystals are presented. The PbTiO3 samples were prepared by pressing nanopowder into the plates and have been studied experimentally by dielectric permittivity measurements in a wide frequency and temperature range. The obtained TC dependence shows a critical change of behavior with increasing of mean nanoparticle size in a 9 nm region. The theoretical calculations based on Monte Carlo simulation were performed to describe such behavior of this material. It was shown that taking into account distribution of nanoparticle sizes in sample is possible to describe dielectric properties of PbTiO3 nanocrystals rather well.