摘要:

A dynamic spherical random bond-random field (SRBRF) model based on the coupled polar cluster picture is formulated. Assuming stochastic flips of the cluster polarization, the Langevin equations of motion are written down, from which the linear and nonlinear dynamic dielectric permittivities are derived. A coupled SRBRF pseudospin phonon Hamiltonian is presented, which models the effects of pressure on the relative stability of the ferroelectric, relaxor-, and paraelectric- like phases and thus the temperature-pressure phase diagram of relaxor systems. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1399694

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

We address the very basic issue of what happens, at a microscopic level, inside a polarized dielectric. We show that the complete information about electronic polarization is embedded in the microscopic polarizationP(ind)(r).Previous studies in the literature have addressed the induced electronic charge density (alias the divergence of our vector field) where the most relevant information is obliterated. The physical meaning ofP(ind)(r)is best understood by imaging that the applied field is adiabatically switched on in time:P(ind)(r)is then proportional to the microscopic transient current flowing through the sample while the field is switched on and the dielectric is polarized. We provide a quantum-mechanical expression forP(ind)(r),and we present first-principle results for two case studies: Si and NaBr. In the case of Si, the (unperturbed) valence charge defines a continuous network of bonds. When a field is switched on, most of the polarization currentP(ind)(r)flows within narrow channels along the bonds, and percolates across the material. Although less dominant, a similar feature occurs even in NaBr. Both materials turn out to be far from the Clausius-Mossotti limit, where the transient current does not cross the cell boundaries. In ferroelectric oxides, which have a mixed ionic/covalent character, the role of percolating transient currents is expected to be dominant. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1399695

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

We study the growth process of ferroelectric materials by kinetic Monte Carlo simulations. An ionic model with long-range Coulomb interactions is used to model the relaxor single crystals. The growth is characterized by thermodynamic processes involving adsorption and evaporation, with solid-on-solid restrictions. An algorithm is developed in order to simulate growth under such a model, for which existing formalism of the kinetic Monte Carlo algorithm is inadequate. We study the growth rates and the order structure of the grown crystals as a function of temperature, chemical composition, and growth orientation. Tests of our algorithm on NaCl gave good results. Preliminary results on growth in Ba-based heterovalent binaries showed 1:2 ordering along the [111] direction over limited scales. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1399696

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

ThePb2MgTeO6perovskite is cubic at high temperature and becomes incommensurately modulated with a rhombohedral,R3¯,average structure at lower temperatures. Both the cubic and the rhombohedral average structure have been investigated using the local density approximation (LDA) of the density functional theory. They are insulators with a 3 eV LDA gap. The electronic band structure is composed of weakly dispersive electronic bands. The peaks in the electronic density of states are assigned to the different atomic and/or molecular orbitals. The ab initio determination of the structure is in reasonable agreement with the experiment, with differences up to 2&percent; for the lattice constants. The relevant unstable modes and their eigenvectors are analyzed. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1399697

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

Isotope effects in ferroelectrics have not attracted much attention in the past, with the exception of the large effect on the transition temperatureTcin hydrogen-bonded ferroelectrics upon deuteration. Reinvestigating this class of compounds experimentally reveals that the exchange of oxygen by its isotope does merely not affectTcof hydrogen-bondedKH2PO4and squaric acid, in contrast to the recently observed isotope induced ferroelectricity inSrTiO3.The new findings in hydrogen-bonded systems are in agreement with theoretical predictions of a vanishing isotope effect in the classical regime and substantially support our earlier conclusions on the coexistence of order-disorder and displacive dynamics in these compounds. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1399698

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

A model of polar clusters and of chemical fluctuation clusters (CFC) in PMN-like relaxors is proposed. The model is mainly based on three phenomena: on a charge transfer state formation corresponding to localized Charge Transfer Vibronic Excitons (CTVE) induced by Mg-Nb disorder, on an appearance of electronic and hole polarons in CFC due to charge compensation, and on Local Configuration Instability (LCI) effects. The origin of co-operative phenomena, of the giant piezoelectric effect and characteristic dielectric losses, including Vogel-Fulcher law as well as peculiarities of EPR and NMR do find an explanation within this model. Another LCI based model of isocharged doped ferroelectric solid solutions (concentrated as well as diluted) is proposed and developed for examples like SBN,Sr1−xCaxTiO3&hthinsp;(SCT),andK1−xLixTa1−yNbyO3&hthinsp;(KLTN)systems. The origin of polar clusters, their interactions, and co-operative phenomenon was considered here taking into account the LCI effect. It is shown that polaron and CTVE-trapping effects including the case of oxygen vacancy clusters play an important role for both types of relaxors discussed. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1399699

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

Quantum fluctuations can strongly influence the low temperature response of a system near a structural phase transition. Among the manifestations of quantum fluctuations at ferroelectric phase transitions are the suppression of the transition temperature,Tc,below its classical value, the emergence of a special critical point—the quantum displacive limit whereTc=0&hthinsp;K,and the ultimate development of a quantum paraelectric state. To study these quantum effects as well as the crossover from the classical to the quantum regime, it is necessary to shiftTcto the appropriate low-temperature range. This is conventionally done by chemical substitution, but we find that high pressure is a “cleaner” variable. After a brief summary of earlier theoretical and experimental work, results onKTaO3and KTN crystals doped with ∼0.05 at &percent; Ca or Ba are presented. The addition of these two dopants introduces dipolar defects into theKTaO3lattice which strongly enhance the dielectric susceptibility of the host lattice in the quantum regime. In a Ca-doped KTN crystal with 2.3 at &percent; Nb, pressure induces a crossover from normal ferroelectric to a relaxor state which on further increase in pressure crosses over to a quantum paraelectric phase. With Ba-dopingKTaO3remains a quantum paraelectric. Because the characteristic energies are small in the quantum regime, the properties are very strongly dependent on pressure and biasing electric fields. The results are discussed in terms of the physics involved. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1399700

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

Slater proposed thatKH2PO4(KDP) has 2 polar and 4 nonpolar (with higher energyEb)H2PO4H-bond configurations. Takagi postulatedH3PO4andHPO4groups with still higher energyEa.Via intrabond H transfer, Takagi pairs form, diffuse independently, and recombine or else annihilate with new partners, allowing H-bond reconfiguration. In both KDP-type ferroelectrics and in proton glasses such asRb1−x(NH4)xH2PO4&hthinsp;(RADP),at higher temperatures there are many Takagi groups. They have short life and are likely to annihilate with new partners, leading to nearly Debye-like dielectric relaxation. Below the ferroelectric transition in KDP, each Takagi pair member requires energyEbto diffuse one step farther from its partner. This energy bias leads to their rapid recombination. In proton glasses with intermediate x, there is no ferroelectric or antiferroelectric transition. Well above the freezing range, the Takagi concentration falls rapidly with temperature and the Takagi group lifetime and path to annihilation grow long. The energy landscape of the annihilation path must be level on the average. However, the random energy steps±Ebor 0 make the highest barrier encountered be proportional (on average) to the square root of the diffusion path length. As temperature falls, this diffusion inhibition makes recombination more likely than annihilation with new partners. The Takagi pair lifetime becomes shorter, but the fluctuations resulting from their diffusion become biased, so their contribution to dielectric and NMR relaxation weakens. In this same temperature range, the Takagi group population temperature dependence grows weak, because many Takagi groups become stranded, finding no annihilation partner. These stranded groups cause relaxation in their vicinity but no relaxation elsewhere, so the crystal is nonergodic on the time scale concerned. 2D NMR shows the mean (averaged over the crystal) time required for a given nucleus to see a change caused by such diffusion.1We propose 4D NMR2to test whether this time is different in different crystal nanoregions because of nonergodicity, and to find the time required to restore ergodicity. Another proposed application of 4D NMR is for proton glass crystals with small x, for which ferroelectric nanoclusters coexist with disordered proton glass regions at low temperatures.3We expect that 4D NMR can find the size and lifetime of such polar nanoclusters. Such experiments could also study polar nanocluster size and lifetime in relaxor ferroelectric and related high-piezoelectric-strain crystals. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1399701

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

Polymers are known for displaying nonexponential relaxation to both electrical and mechanical stresses. A numerical method for integrating fractional differential equations has made it possible to verify the connection between a power-lawacpermittivity function and power-law decay currents resulting from application ofdcstep voltages. The responses show significant dependence on the duration of the applied pulse. The history dependence predicted by the simulations has been confirmed experimentally. The computational techniques should be applicable to a wide range of systems exhibiting nonexponential relaxation. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1399702

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

We study domain pattern formation in ferroelectrics based on a 2-D time-dependent Ginzburg Landau theory. The model includes electrostrictive and elastic effects in the form of a long-range interaction between the polarization fields that is obtained by eliminating the strain fields subject to the elastic compatibility constraint. We simulate a 2-D square to rectangle transition that has four equivalent polarization states in the ferroelectric phase. Starting from an unstable paraelectric state, we simulate the domain pattern evolution in the absence of external electric field. For the case without defects, the final pattern is a twinned state that has only head to tail (uncharged) domain walls. However, for the case with randomly distributed dipolar defects, head to head and tail to tail charged walls are observed. These results are in accordance with recent experiments where charged domain walls have been observed. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1399703

出版商:AIP    

年代:1901

数据来源: AIP