92 research outputs found
Coulomb interaction and ferroelectric instability of BaTiO3
Using first-principles calculations, the phonon frequencies at the
point and the dielectric tensor are determined and analysed for the cubic and
rhombohedral phases of BaTiO. The dipole-dipole interaction is then
separated \`a la Cochran from the remaining short-range forces, in order to
investigate their respective influence on lattice dynamics. This analysis
highlights the delicate balance of forces leading to an unstable phonon in the
cubic phase and demonstrates the extreme sensitivity of this close compensation
to minute effective charge changes. Within our decomposition, the stabilization
of the unstable mode in the rhombohedral phase or under isotropic pressure has
a different origin.Comment: 9 pages, 4 tables, 1 figur
First-principles study of lattice instabilities in Ba_xSr_(1-x)TiO_3
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.Comment: 9 pages, 2 figures, Proceedings for Fundamental Physics of
Ferroelectrics, Aspen (CO), Feb. 13-20, 200
Avoiding the polarization catastrophe in LaAlO3 overlayers on SrTiO3(001) through a polar distortion
A pronounced uniform polar distortion extending over several unit cells
enables thin LaAlO3 overlayers on SrTiO3(001) to counteract the charge dipole
and thereby neutralize the "polarization catastrophe" that is suggested by
simple ion-counting. This unanticipated mechanism, obtained from density
functional theory calculations, allows several unit cells of the LaAlO3
overlayer to remain insulating (hence, fully ionic). The band gap of the
system, defined by occupied O states at the surface and unoccupied Ti 3d
states at the interface in some cases 20 \AA distant, decreases with
increasing thickness of the LaAlO3-film before an insulator-to-metal transition
and a crossover to an electronic reconstruction occurs at around five
monolayers of LaAlO3.Comment: 5 pages, 4 figures, submitted for publicatio
Lattice dynamics of BaTiO3, PbTiO3 and PbZrO3: a comparative first-principles study
The full phonon dispersion relations of lead titanate and lead zirconate in
the cubic perovskite structure are computed using first-principles variational
density-functional perturbation theory, with ab initio pseudopotentials and a
plane-wave basis set. Comparison with the results previously obtained for
barium titanate shows that the change of a single constituent (Ba to Pb, Ti to
Zr) has profound effects on the character and dispersion of unstable modes,
with significant implications for the nature of the phase transitions and the
dielectric and piezoelectric responses of the compounds. Examination of the
interatomic force constants in real space, obtained by a transformation which
correctly treats the long-range dipolar contribution, shows that most are
strikingly similar, while it is the differences in a few key interactions which
produce the observed changes in the phonon dispersions. These trends suggest
the possibility of the transferability of force constants to predict the
lattice dynamics of perovskite solid solutions.Comment: 9 pages, 2 figures (one in colors), revised version (small changes
essentially in Sec. III
Wannier-based definition of layer polarizations in perovskite superlattices
In insulators, the method of Marzari and Vanderbilt [Phys. Rev. B {\bf 56},
12847 (1997)] can be used to generate maximally localized Wannier functions
whose centers are related to the electronic polarization. In the case of
layered insulators, this approach can be adapted to provide a natural
definition of the local polarization associated with each layer, based on the
locations of the nuclear charges and one-dimensional Wannier centers comprising
each layer. Here, we use this approach to compute and analyze layer
polarizations of ferroelectric perovskite superlattices, including changes in
layer polarizations induced by sublattice displacements (i.e., layer-decomposed
Born effective charges) and local symmetry breaking at the interfaces. The
method provides a powerful tool for analyzing the polarization-related
properties of complex layered oxide systems
Incipient ferroelectricity in 2.3% tensile-strained CaMnO3 films
Epitaxial CaMnO3 films grown with 2.3% tensile strain on (001)-oriented
LaAlO3 substrates are found to be incipiently ferroelectric below 25 K. Optical
second harmonic generation (SHG) was used for the detection of the incipient
polarization. The SHG analysis reveals that CaMnO3 crystallites with in-plane
orientation of the orthorhombic b axis contribute to an electric polarization
oriented along the orthorhombic a (resp.\ c) axis in agreement with the
predictions from density functional calculations
Interplay of couplings between antiferrodistortive, ferroelectric, and strain degrees of freedom in monodomain PbTiO/SrTiO superlattices
We report first-principles calculations on the coupling between epitaxial
strain, polarization, and oxygen octahedra rotations in monodomain
(PbTiO)/(SrTiO) superlattices. We show how the
interplay between (i) the epitaxial strain and (ii) the electrostatic
conditions, can be used to control the orientation of the main axis of the
system. The electrostatic constrains at the interface facilitate the rotation
of the polarization and, as a consequence, we predict large piezoelectric
responses at epitaxial strains smaller than those that would be required
considering only strain effects. In addition, ferroelectric (FE) and
antiferrodistortive (AFD) modes are strongly coupled. Usual steric arguments
cannot explain this coupling and a covalent model is proposed to account for
it. The energy gain due to the FE-AFD coupling decreases with the periodicity
of the superlattice, becoming negligible for .Comment: 5 pages, 4 figure
Ab initio phonon dispersion curves and interatomic force constants of barium titanate
The phonon dispersion curves of cubic BaTiO_3 have been computed within a
first-principles approach and the results compared to the experimental data.
The curves obtained are very similar to those reported for KNbO_3 by Yu and
Krakauer [Phys. Rev. Lett. 74, 4067 (1995)]. They reveal that correlated atomic
displacements along chains are at the origin of the ferroelectric
instability. A simplified model illustrates that spontaneous collective
displacements will occur when a dozen of aligned atoms are coupled. The
longitudinal interatomic force constant between nearest neighbour Ti and O
atoms is relatively weak in comparison to that between Ti atoms in adjacent
cells. The small coupling between Ti and O displacements seems however
necessary to reproduce a ferroelectric instability.Comment: 12 pages, 4 figure
Electron localization : band-by-band decomposition, and application to oxides
Using a plane wave pseudopotential approach to density functional theory we
investigate the electron localization length in various oxides. For this
purpose, we first set up a theory of the band-by-band decomposition of this
quantity, more complex than the decomposition of the spontaneous polarization
(a related concept), because of the interband coupling. We show its
interpretation in terms of Wannier functions and clarify the effect of the
pseudopotential approximation. We treat the case of different oxides: BaO,
-PbO, BaTiO and PbTiO. We also investigate the variation of the
localization tensor during the ferroelectric phase transitions of BaTiO as
well as its relationship with the Born effective charges
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