摘要:

We have performed a Diffraction Anomalous Fine Structure (DAFS) study of a single crystal of the relaxor ferroelectricPbMg1/3Nb2/3O3.DAFS measurements were performed at the NbKedge on a half-order Bragg reflection originating from the 1:1 ordered nanodomains. DAFS data analysis provided us with the local structure around Nb atoms in the ordered nanodomains. The Nb-O distance in the ordered region was determined to be 0.052(10) Å shorter than half the lattice parameter of the bulk crystal, in good agreement with one model of the ferroelectric phase transition broadening [JETP84, 994 (1997)]. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56285

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Within anab-initioHF scheme, we use both Berry-phase calculations and supercell calculations in order to compute the dynamical charges for lattice dynamics and the electronic dielectric constant forKNbO3andBaTiO3.Comparison with experimental data indicates that HF provides a description of the electronic properties of this material whose accuracy is of the same order as the LDA one. There are however significant differences between the two sets of results, whose origin is scrutinized. Motivated by the study of surface and domain-boundary properties, we also present some results forBaTiO3slabs, including both genuinely isolated and periodically repeated slabs with different terminations. The capability of dealing with a genuinely isolated slab is a virtue of the localized-basis implementation adopted here. We demonstrate, amongst other things, the nontrivial dynamical-charge neutrality ofBaTiO3[001] surfaces. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56265

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Volumetric electrostrictive coefficient,Qh,ofPbMg1/3Nb2/3O3(PMN) relaxor ferroelectric is investigated using a high-resolution neutron powder diffraction and a conventional strain measurement. Using the results of these experiments we: (1) derive the value ofQhof prototype cubic lattice of PMN, which is equal toQh=(8.3±1.0)×10−2(m4/C2);(2) find the temperature dependence of the volume fraction,&dgr;V,of the polar regions in the material, and estimate that at temperatures around the dielectric permittivity maximum,Tmax,it is equal to&dgr;V≈90&percent;;(3) estimate the magnitude of the contribution of the crystal lattice to the polarization response of PMN at temperatures around and belowTmax.These results are essential for the understanding of the nature of the polarization and strain response of relaxor ferroelectrics. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56266

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Mixed crystalsCs1−x(NH4)xH2PO4of the ferroelectricCsH2PO4(CDP) and the antiferroelectric(NH4)H2PO4were grown withx=0.2(CADP0.2) in solution. The structural properties of the crystal were analyzed by means of x-ray diffraction. Dielectric measurements at several temperatures and frequencies have been performed along the three crystallographic axes in this sample and also in the fully deuterated CADP0.2 sample (DCADP0.2). Dielectric and NMR experiments in a powdered sample were also performed. The shift of the transition temperature as a function ofxand deuteration, and the changes in the properties of the different phases together with the conductivity found in the paraelectric phase will be discussed. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56267

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Using the first-principles linear response method implemented with an LAPW basis [1], we have investigated the lattice dynamics of cubicSrTiO3[2] at the LDA equilibrium lattice parameter. Consistent with the antiferrodistortive (AFD) phase transition to the tetragonal structure at 105 K, we found phonon instabilities along the R-M-R line in the Brillouin zone. In addition, a ferroelectric-type (FE) instability was found at the &Ggr;-point, although no FE structural phase transition is observed. Working in the tetragonal phase, we determine the theoretical equilibrium twist angle for the oxygen octahedra, and show that as the lattice parameter is reduced, the AFD instability is enhanced, while the FE instability is diminished. Furthermore, we find that the tetragonal structure is marginally stable against FE distortions along the c-axis. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56268

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The electronic ground state of a periodic crystalline solid is usually described in terms of extended Bloch orbitals; localized Wannier functions can alternatively be used. These two representations are connected by families of unitary transformations, carrying a large degree of arbitrariness. We have developed a localization algorithm that allows one to iteratively transform the extended Bloch orbitals of a first-principles calculation into a unique set ofmaximally localizedWannier functions. We apply this formalism here to the case of cubicBaTiO3.The purpose is twofold. First, a localized-orbital picture allows a meaningful band-by-band decomposition of the whole Bloch band complex. In perovskites, these Wannier functions are centered on the atomic sites and display clearly as,p,d, or hybrid character. Second, since the centers of the Wannier functions map the polarization field onto localized point charges, the ground state dielectric properties become readily available. We study the Born effective charges of the paraelectric phase ofBaTiO3.We are able to identify not only the contributions that come from a given group of bands, but also the individual contributions from the “atomic” Wannier functions that comprise each of these groups. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56269

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We investigate, within local density functional theory, the structural phase stability of the piezoelectric materialPb(Zr1/2Ti1/2)O3(PZT) under finite applied uniaxial stress. Previous theoretical analyses of piezoelectric properties have examined the behavior of the system near the ground-state structure. However, because some piezoelectrics can accept large (>1&percent;) strains reversibly [J. Appl. Phys.82, 1804 (1997)], these crystals are clearly operating beyond their lowest-order behavior, and insight can be gained from studying piezoelectricity at finite strain. We have studied the structural properties of the (111)-PZT crystal over a range of positive and negative uniaxial strains. For each strain state, we identity two metastable structures: a tetragonal phase and a low-symmetry rhombohedral-like phase. For each phase, we determine the evolution of the magnitude and direction of polarization in the crystal as a function of applied stress. We also analyze the energetic and structural profile of the stress-induced tetragonal-to-rhombohedral phase transition. In particular, we compute the transition stress, and we identify the accompanying atomic motions in terms of larger structural motifs such as cation motion within oxygen cages and oxygen-cage tilting. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56270

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A key to optimizing the growth of the new single-crystal relaxor ferroelectrics [1] is resolving basic questions concerning their structural properties and energetics. We report on initial first-principles total energy and force calculations, examining the energetics of local order in PZN type relaxors. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56291

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The position operator (defined within Schro¨dinger representation as usual) becomes meaningless when the usual Born-von Ka´rma´n periodic boundary conditions are adopted: this fact is at the root of the polarization problem. I show how to define the position expectation value by means of rather peculiar many-body (multiplicative) operator acting on the wavefunction of the extended system. This definition can be regarded as the generalization of a precursor work, apparently unrelated to the polarization problem. For uncorrelated electrons, the present finding coincides with the so-called “single-point Berry phase” formula, which can hardly be regarded as the approximation of a continuum integral, and is computationally very useful for disordered systems. Simulations which are based on this concept are being performed by several groups. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56271

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The problem of a crossover between the classical and quantum behavior is studied. A contribution of the dynamic clusters into the thermodynamic and dynamic properties is discussed. The Berry’s phase in the lattice dynamics is considered on the base of the Hamiltonian path integral. The novel model describing quantum fluctuations in the Josephson junction with the quantum ferroelectric is suggested and analyzed. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56292

出版商:AIP    

年代:1998

数据来源: AIP