摘要:

While the Pb containing ferroelectric and antiferroelectric oxides show a large variety in crystal structure, the pulsed neutron atomic pair-distribution function (PDF) studies indicate that their local atomic structures are surprisingly similar to each other. In particular the environment of Pb is nearly independent of composition, with Pb being strongly off-centered in thePbO12cluster, resulting in a large local polarization. A new model is proposed to describe the interplay between the Pb polarization and the random local structural fluctuation. This model explains the relaxor ferroelectricity from a general point of view, while other models such as nano-domain space charge model and the random field model were introduced specifically for hetero-valent systems such asPb(Mg1/3Nb2/3)O3(PMN). ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56286

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

First-principles calculations are performed to investigate alloying and ferroelectric effects in lead zirconate titanate (PZT) with high Ti composition. We find that the main effect of alloying in the paraelectric phase of PZT is the existence of two sets of B–O bonds, i.e., shorter Ti–O bondsvs.longer Zr–O bonds. On the other hand, ferroelectricity leads to the formation of very short covalent Ti–O bonds and to the formation of covalent chains of Pb–O bonds. The covalency in the ferroelectric phase is mainly induced by an enhancement of hybridization between Ti3dand O2p,and between Pb6sand O2p.These hybridizations induce a striking decrease of the effective charges when going from the paraelectric to the ferroelectric phase of PZT. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56287

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Ionic model (SSCAD) calculations were performed for 36 different cation ordered supercell configurations in the pseudobinary perovskite related systemBaTaO3−BaZnO3.Ba(Zn1/3Ta2/3)O3(BZT) in aP3¯m1ordered structure is the only single phase compound that is observed experimentally, and the one that SSCAD calculations predict as having lowest formation energy. It is therefore the presumed ground state at the BZT composition. The SSCAD results are supported by first principles VASP pseudopotential calculations which were performed forBaTaO3,BaZnO3,Ba(Zn1/3Ta2/3)O3,andBa(Zn1/2Ta1/2)O3with NaCl type ordering of Zn and Ta. Finite temperature calculations that are based on the SSCAD results, predict a strongly first-orderP3¯m1→Pm3¯mtransition, but the predictedTCappears to be about an order of magnitude too high. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56288

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

An effective Hamiltonian for the ferroelectric transitions inKNbO3is constructed from first-principles density-functional-theory total-energy and linear-response calculations through the use of a localized, symmetrized basis set of “lattice Wannier functions.” The detailed description provided here provides the foundation for the use of this effective Hamiltonian in classical Monte Carlo, classical molecular dynamics and quantum mechanical simulations, as well as guidance for the construction of effective Hamiltonians for related systems. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56289

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Bulk spontaneous polarization, dynamical charges(Z*)and piezoelectric stress tensor elements of tetragonalPbZr1/2Ti1/2O3(PZT 50/50) with B-site cations ordered along [001] and [111] directions andPbTiO3(PT) were determined from ground-state Berry’s phase calculations using the all-electron Linearized Augmented Plane Wave (LAPW) method. Extensive internal strain contributions to the piezoelectric response in PT are due to large transverse effective charges and sensitivity of atomic positions to lattice strains. Theoretical proper moduli of PT,e15=3.15C/m2,e31=−0.93C/m2ande33=3.23C/m2,agree well with single crystal experimental data. Ordering along the polar [001] direction was found to enhance thee33piezoelectric stress modulus in chemically ordered PZT 50/50. Theoretical proper moduli in this latter material differ significantly from low temperature experimental data, indicating the possibility that the mechanism of piezoelectric response in PZT near the morphotropic phase boundary differs from conventional single crystal piezoelectricity found in PT. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56290

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Monte Carlo simulations with an effective Hamiltonian parametrized from first principles are performed to study the dielectric response ofBaTiO3as a function of temperature, with particular emphasis on the behavior of the dielectric constant near the transition from the ferroelectric tetragonal phase to the paraelectric cubic phase. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56280

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A method for the calculation of the temperature dependence of dielectric and piezoelectric responses, based on the use of a first-principles effective Hamiltonian, is described. Results are presented for the ferroelectric perovskitePbTiO3.While the method includes only the soft-mode contributions to the responses, it is argued to give a good description of the divergences or near-divergences of the response functions near the cubic-tetragonal transition. The expression of the response functions in terms of correlation functions is used to provide a real-space interpretation of the responses which clearly distinguishes betweenPbTiO3and the related materialsBaTiO3andKNbO3.©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56281

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

In this article, we motivate the detailed comparison of the physical properties of individual configurations of a ferroelectric solid solution as a means toward developing first principles models for these systems. We compare energies, dielectric constants&egr;∞,mode effective charges of local polar distortions, and the zero temperature piezoelectric behavior of several orderedPb3GeTe4supercells. Cluster expansions of these properties show the importance of second-neighbor effects, which can be related to symmetry-preserving relaxation and its effect on the symmetry breaking polar instabilities. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56282

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The lattice dynamics ofSrTiO3andPbTiO3have been reinvestigated within the two-dimensional anharmonic polarizability model. Opposite to former calculations, which were based on the self-consistent phonon approximation, in the present calculations the nonlinearly coupled equations of motion are solved exactly for arbitrary q for the dynamical displacement coordinates and associated frequencies. It is found that in both compounds the oxygen ion dynamics are highly anharmonic already far away from the structural instability. The displacement patterns can be related to dynamical stripe and domain formations. The potential associated with these dynamics is calculated and shows periodically fluctuating areas with attractive and repulsive regions. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56283

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Semi-empirical Hartree-Fock calculations using the intermediate neglect of differential overlap (INDO) method, for self-ordered cubic symmetry clusters of seven Nb ions inKTaO3are performed with the aim of verifying the cluster model [1] of second component-induced phase transitions in ferroelectric perovskite matrices. It is shown that such a seven-particle cluster inKTaO3:Nbhas two types of quasidegenerate states of different nature. Namely, the state with the equilibrium full-symmetric dilatation and off-center displacements in [111]-directions of the central Nb-ion in the cluster, and the state with the equilibrium full-symmetric compression without any off-center effect. The consequences of such cluster structures on the multi-well potential are discussed. In particular, an avalanche-like behaviour of the temperature dependence of the ferroelectric order parameter is obtained. The results of semi-empirical INDO calculations of a seven-impurities cluster of Nb-ions inKTaO3confirm the key assumptions of the cluster model of induced phase transitions. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56284

出版商:AIP    

年代:1998

数据来源: AIP