43 research outputs found
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
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
First principles based atomistic modeling of phase stability in PMN-xPT
We have performed molecular dynamics simulations using a shell model
potential developed by fitting first principles results to describe the
behavior of the relaxor-ferroelectric (1-x)PbMg1/3Nb2/3O3-xPbTiO3 (PMN-xPT) as
function of concentration and temperature, using site occupancies within the
random site model. In our simulations, PMN is cubic at all temperatures and
behaves as a polar glass. As a small amount of Ti is added, a weak polar state
develops, but structural disorder dominates, and the symmetry is rhombohedral.
As more Ti is added the ground state is clearly polar and the system is
ferroelectric, but with easy rotation of the polarization direction. In the
high Ti content region, the solid solution adopts ferroelectric behavior
similar to PT, with tetragonal symmetry. The ground state sequence with
increasing Ti content is R-MB-O-MC-T. The high temperature phase is cubic at
all compositions. Our simulations give the slope of the morphotropic phase
boundaries, crucial for high temperature applications. We find that the phase
diagram PMN-xPT can be understood within the random site model.Comment: 27 pages, 9 figure