摘要:

We employ a new method for studying compositionally disordered ferroelectric oxides. This method is based on the virtual crystal approximation (VCA), in which two or more component potentials are averaged into a composite atomic potential. In our method, we construct a virtual atom with the correctly averaged atomic size and atomic eigenvalues. We have used our new method to study the composition dependent phase transition inPb(Zr1−xTix)O3lying betweenx=0.5andx=0.4.We correctly predict the experimentally determined phase transition from the tetragonal phase to a low-temperature rhombohedral phase between these two compositions. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324444

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Using first-principles calculations based on a variational density functional perturbation theory, we investigate the lattice dynamics of solid solutions of barium and strontium titanates. Averaging the information available for the related pure compounds yields results equivalent to those obtained within the virtual crystal approximation, providing frequencies which are a good approximation to those computed for a (111) ordered supercell. Using the same averaging technique we report the evolution of the ferroelectric and antiferrodistortive instabilities with composition. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324445

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

In this paper we discuss some of the details of the calculation of the macroscopic polarization, via the Berry-phase approach, using a general (numerical) local combination of atomic orbitals (LCAO) as a basis set. Previous implementations with a LCAO basis relied on the use of Gaussian expansions, where analytical formulas exist for the crucial matrix elements. However, our approach, which only requires the matrix elements of the position operator, can be more easily implemented in the case of numerical orbitals. This work is a necessary first step towards our main goal of applying the SIESTA code to the study of ferroelectric alloys. This code, which uses a numerical LCAO basis set, has been highly optimized for the treatment of large systems. We are confident that it will allow us to treat larger systems than the standard plane-waves methods, while keeping a reasonable accuracy. Test results for representative ferroelectric perovskites are presented. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324446

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

We introduce an electrostatic model including charge transfer, which is shown to account for the observed B-site ordering in Pb-based perovskite alloys. The model allows charge transfer between A sites and is a generalization of Bellaiche and Vanderbilt’s purely electrostatic model. The large covalency ofPb2−compared toBa2+results in an environment dependent effective A-site charge. This model successfully reproduces the long range order of both Pb-based and Ba-based complex perovskite alloys. It yields smaller structural energy differences for Pb-based compared to Ba-basedA(B1/3B2.3)O3systems, consistent with first-principles calculations and the fact that Ba-based disorder at higher temperatures than the Pb-based alloys. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324447

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

This paper describes the role of oxygen vacancy self-patterning on fatigue in ferroelectric thin-film memories. It also comments briefly on the related problem of nano-electrode self patterning on the surface of ferroelectric thin films. And it suggests a pedagogically simple explanation of the factor of a million in reduction of the effective Richardson coefficientA*for ferroelectric thin films. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324448

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

A systematic study of the temperature dependence of superlattice reflections in the reciprocal space of a single crystalPb(Mg1/3Nb2/3)O3(PMN) was performed using anomalous x-ray scattering technique. Our studies confirm the existence of the two types of nanoregions in PMN: a) chemically ordered nanodomains, and b) polar nanodomains formed by short-range correlated ionic displacements. No detailed temperature dependence of these superlattice reflections has been reported in the past. From monitoring the superlattice reflections corresponding to polar nanodomains (i.e., &agr; spots) over the temperature range 15 K–800 K, we found freezing temperature(Tf)of correlated atomic displacements was near 200 K. In contrast, chemically ordered nanodomains, which give rise to F type superlattice peaks, exhibited spherical shape over the entire temperature interval 15 K–800 K. This leads us to suspect that chemically ordered regions might be different and independent of polar nanodomains. In this paper we explain the contributions of different atomic displacements to &agr; and F superlattice reflections in PMN. Correlation and competition between Pb displacements and the tilting of oxygen octahedra network are presented to understand the microstructural origin of the relaxor ferroelectric behavior. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324449

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Piezoelectric response induced by rotating the polarization, which occurs when the external field is not in the same direction as the internal spontaneous polarization, is studied using a first-principles approach. The piezo-response of the rhombohedralBaTiO3to the field along the ⟨001⟩ direction is considered. The response is found to be path-dependent, and along the lowest free-energy path the strain increases significantly with the field. Furthermore, the strain response (collinear response) to a field along the polarization direction (i.e., without polarization rotation) is also calculated for comparison, and the piezo-responses with and without rotating polarization are found to be very different. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324450

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

We compare two approaches to better understand the electromechanical properties ofBaTiO3under finite electric fields. In the first approach Linearized Augmented Plane Wave (LAPW) computations of total energies and polarization are performed under zero field, and appropriate for zero temperature, and finite field behavior is modeled by considering the free energy changesE⋅Pinduced by the field. In the second approach the temperature and electric field dependencies of polarization and strain are obtained forBaTiO3by Monte Carlo simulations using an effective Hamiltonian with parameterization based on ultrasoft pseudopotential calculations. The LAPW calculation results in an initial strain response larger than the effective Hamiltonian simulation. The effective Hamiltonian shows a similar but smaller response at low temperatures, but also shows a field-induced intermediate disordered monoclinic phase at higher temperatures. The strain-mode coupling in the effective Hamiltonian is smaller than obtained with LAPW. The overall picture of polarization rotation induced by an applied field is found with the effective Hamiltonian, similar to the LAPW results. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324451

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

We have studied the effect of uniaxial pressure on the ferroelectric properties ofPbTiO3by using a first principles electronic structure method. The pressure is applied in theab-plane. The magnitude of the ferroelectric lattice distortion is found to increase as the pressure is increased. Short Ti-O bonds along thec-direction are found to remain almost constant in length whereas the stretched Ti-O bonds become more stretched because of an increase in the equilibrium lattice constant in thec-direction. The thermodynamic stability of the ferroelectric phase over the paraelectric phase increases as pressure is applied. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324452

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

A wide variety of minerals exhibit ordered crystalline phases in which an underlying periodic structure is modulated by small atomic displacements with a wavelength incommensurate with this periodicity. We describe our recent efforts to use state-of-the-art first-principle calculations for the study of the microscopic mechanisms governing the existence of such phases and their transitions. A database containing more than one hundred incommensurate phases, available on the Web, has been constructed, and representative materials (PbO,AuTe2,K2SO4,&ellip;) have been selected for our purpose. The ABINIT software project, thanks to which we can compute and analyze interatomic force constants and lattice instabilities, is described. We present preliminary results for the representative materials, discuss the difficulties that we face, and contrast them with those encountered in the study of ferroelectric materials. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1324453

出版商:AIP    

年代:1900

数据来源: AIP