110 research outputs found
Toroidal ferroelectricity in PbTiO3 nanoparticles
We report from first-principles-based atomistic simulations that
ferroelectricity can be sustained in PbTiO3 nanoparticles of only a few lattice
constants in size as a result of a toroidal ordering. We found that
size-induced topological transformations lead to the stabilization of a
ferroelectric bubble by the alignment of vortex cores along a closed path.
These transformations, which are driven by the aspect ratio of the
nanostructure, change the topology of the polarization field, producing a rich
variety of polar configurations. For sufficiently flat nanostructures, a
multi-bubble state bridges the gap between 0D nanodots and 2D ultra-thin films.
The thermal properties of the ferroelectric bubbles indicate that this state is
suitable for the development of nanometric devices.Comment: 16 pages, 4 figures. Accepted in Phys. Rev. Let
Multiferroic behavior of Aurivillius Bi4Mn3O12 from first-principles
The multiferroic behavior of the hypothetical Aurivillius compound Bi4Mn3O12
has been explored on the basis of density functional calculations. We find that
the tetragonal paraelectric phase of this material is ferromagnetic, showing
ferroelectric and antiferrodistortive instabilities similar to the ones
observed in its ferroelectric parent compound Bi4Ti3O12 . Our results indicate,
however, that the presence of Mn+4 ions at the B-sites shrinks the cell volume
and consequently the unstable polar mode, associated with the ferroelectric
polarization, is overcame by an antiferrodistortive distortion. In this way,
Bi4Mn3O12 exhibits incipient ferroelectricity at its equilibrium volume. We
show that the ferroelectric state can be favored by strain or partial
substitution of Mn with Ti.Comment: 6 pages, 5 figure
Interface effects in ferroelectric PbTiO ultrathin films on a paraelectric substrate
Interface effects on the ferroelectric behavior of PbTiO ultrathin films
deposited on SrTiO substrate are investigated using an interatomic
potential approach with parameters fitted to first-principles calculations. We
find that the correlation of atomic displacements across the film/substrate
interface is crucial for the stabilization of the ferroelectric state in films
a few unit-cells thick. We show that the minimum film thickness for the
appearance of a spontaneous polarized domain state is not an intrinsic property
of the ferroelectric film but depends on the polarizability of the paraelectric
substrate. We also observe that the substrate displays an induced polarization
with an unusual oscillatory behavior.Comment: 4 page
Relative phase stability and lattice dynamics of NaNbO from first-principles calculations
We report total energy calculations for different crystal structures of
NaNbO over a range of unit cell volumes using the all-electron
full-potential (L)APW method. We employed both the local-density approximation
(LDA) and the Wu-Cohen form of the generalized gradient approximation (GGA-WC)
to test the accuracy of these functionals for the description of the complex
structural behavior of NaNbO. We found that LDA not only underestimates the
equilibrium volume of the system but also predicts an incorrect ground state
for this oxide. The GGA-WC functional, on the other hand, significantly
improves the equilibrium volume and provides relative phase stability in better
agreement with experiments. We then use the GGA-WC functional for the
calculation of the phonon dispersion curves of cubic NaNbO to identify the
presence of structural instabilities in the whole Brillouin zone. Finally, we
report comparative calculations of structural instabilities as a function of
volume in NaNbO and KNbO to provide insights for the understanding of
the structural behavior of KNaNbO solid solutions.Comment: Accepted for publication in Physical Review
Temperature-driven phase transitions in SrBiTaO from first-principles calculations
The phase transition sequence of SrBiTaO is investigated using a
shell model with parameters fitted to first-principles calculations. We show
that the complex interplay between polar and nonpolar instabilities leads to
the presence of two phase transitions, corroborating the existence of an
intermediate orthorhombic paraelectric phase. This phase is characterized by
the rotation of the TaO octahedra around the a-axis. We show that this
phase can be also detected from the dielectric response of the material. The
present approach constitutes a powerful tool for a theoretical prediction of
intermediate phases, not yet observed experimentally, in other Aurivillius
compounds
Surface reconstruction and ferroelectricity in PbTiO thin films
Surface and ferroelectric properties of PbTiO thin films are investigated
using an interatomic potential approach with parameters computed from
first-principles calculations. We show that a model developed for the bulk
describes properly the surface properties of PbTiO. In particular, the
antiferrodistortive surface reconstruction, recently observed from X-ray
scattering, is correctly reproduced as a result of the change in the balance of
long-range Coulombic and short-range interactions at the surface. The effects
of the surface reconstruction on the ferroelectric properties of ultrathin
films are investigated. Under the imposed open-circuit electrical boundary
conditions, the model gives a critical thickness for ferroelectricity of 4 unit
cells. The surface layer, which forms the antiferrodistortive reconstruction,
participates in the ferroelectricity. A decrease in the tetragonality of the
films leads to the stabilization of a phase with non-vanishing in-plane
polarization. A peculiar effect of the surface reconstruction on the in-plane
polarization profile is found.Comment: 6 pages, 5 figure
Effects of the antiferrodistortive instability on the structural behavior of BaZrO by atomistic simulations
Recently, the possibility of a low-temperature non-cubic phase in BaZrO
has generated engaging discussions about its true ground state and the
consequences on its physical properties. In this paper, we investigate the
microscopic behavior of the BaZrO cubic phase by developing a shell model
from calculations and by performing molecular dynamics simulations
at finite temperature and under negative pressure. We study three different
scenarios created by tuning the intensities of the antiferrodistortive (AFD)
instabilities, and consequently, the sequence of phase transitions with
temperature. From a detailed analysis of the cubic phase at atomic scale, we
find that oxygen octahedra are barely distorted, present rotation angles that
may oscillate with significant amplitudes, are AFD correlated with their
closest neighbors on the plane perpendicular to the pseudocubic rotation axis
exhibiting -type ordering, and form instantaneous, dynamic and
unstable domains over time. Our simulations support the existence of
nanoregions with short-range ordering in cubic BaZrO associated with
experimentally observed anomalies and unveil that they can exist regardless of
whether or not structural phase transitions related with AFD distortions occur
at lower temperatures.Comment: 10 pages, 8 figure
Microscopic scale investigation of piezoelectric properties of lead-free alkaline niobates
A first-principles-based technique is used to investigate the electromechanical properties oflead-free alkaline niobates. In particular, we study the structural and piezoelectric properties of LixK1-x NbO3 which is taken as a prototypical solid solution. We show that the system develops an orthorhombic to tetragonal polymorphic phase transition at room temperature for a Li concentration of 4.5%. We showed that not only the phase coexistence region is relevant for the enhancement of the piezoelectric properties but also the off-center Li impurities play an important role in the improvement of the electromechanical response.Fil: Machado, Rodrigo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Física de Rosario (i); ArgentinaFil: Sepliarsky, Marcelo Claudio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Física de Rosario (i); ArgentinaFil: Stachiotti, Marcelo Gabriel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Física de Rosario (i); Argentin
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