22 research outputs found
First-principles investigation of morphotropic transitions and phase-change functional responses in BiFeO3-BiCoO3 multiferroic solid solutions
We present an ab initio study of the BFCO solid solution formed by
multiferroics BiFeO3 (BFO) and BiCoO3 (BCO). We find that BFCO presents a
strongly discontinuous morphotropic transition between BFO-like and BCO-like
ferroelectric phases. Further, for all compositions such phases remain
(meta)stable and retain well-differentiated properties. Our results thus
suggest that an electric field can be used to switch between these structures,
and show that such a switching involves large phase-change effects of various
types, including piezoelectric, electric, and magnetoelectric ones.Comment: Submitte
First-Principles Calculations at Constant Polarization
We develop an exact formalism for performing first-principles calculations
for insulators at fixed electric polarization. As shown by Sai, Rabe, and
Vanderbilt (SRV) [N. Sai, K. M. Rabe, and D. Vanderbilt, Phys. Rev. B {\bf 66},
104108 (2002)], who designed an approximate method to tackle the same problem,
such an approach allows one to map out the energy landscape as a function of
polarization, providing a powerful tool for the theoretical investigation of
polar materials. We apply our method to a system in which the ionic
contribution to the polarization dominates (a broken-inversion-symmetry
perovskite), one in which this is not the case (a III-V semiconductor), and one
in which an additional degree of freedom plays an important role (a
ferroelectric phase of KNO). We find that while the SRV method gives rather
accurate results in the first case, the present approach provides important
improvements to the physical description in the latter cases.Comment: 4 pages, 4 figure
Theoretical study of ferroelectric potassium nitrate
We present a detailed study of the structural behavior and polarization
reversal mechanism in phase III of KNO, an unusual ferroelectric material
in which the nitrate groups rotate during polarization reversal. This material
was one of several studied in a previous work [O. Di\'eguez and D. Vanderbilt,
Phys. Rev. Lett. {\bf 96}, 056401 (2006)] where methods were described for
computing curves of energy versus electric polarization. In the present work we
extend and systematize the previous first-principles calculations on KNO,
and analyze in detail a two-parameter model in which the energy of the system
is written as a low-order expansion in the polarization and the nitrate group
orientation. We confirm that this model reproduces the first-principles results
for KNO very well and construct its parameter-space phase diagram,
describing regions where unusual triple-well potentials appear. We also present
first-principles calculations of KNO under pressure, finding that its
energy-versus-polarization curves change character by developing a
first-derivative discontinuity at zero polarization.Comment: Replaced with extended versio
Translational covariance of flexoelectricity at ferroelectric domain walls
Macroscopic descriptions of ferroelectrics have an obvious appeal in terms of efficiency and physical intuition. Their predictive power, however, has often been thwarted by the lack of a systematic procedure to extract the relevant materials parameters from the microscopics. Here we address this limitation by establishing an unambiguous two-way mapping between spatially inhomogeneous fields and discrete lattice modes. This yields a natural treatment of gradient couplings in the macroscopic regime via a long-wavelength expansion of the crystal Hamiltonian. Our analysis reveals an inherent arbitrariness in both the flexoelectric and polarization gradient coefficients, which we ascribe to a translational freedom in the definition of the polar distortion pattern. Remarkably, such arbitrariness cancels out in all physically measurable properties (relaxed atomic structure and energetics) derived from the model, pointing to a generalized translational covariance in the continuum description of inhomogeneous ferroelectric structures. We demonstrate our claims with extensive numerical tests on 180° domain walls in common ferroelectric perovskites, finding excellent agreement between the continuum model and direct first-principles calculations
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
Polymorphism in Bi-based perovskite oxides: a first-principles study
Under normal conditions, bulk crystals of BiScO , BiCrO, BiMnO,
BiFeO, and BiCoO present three very different variations of the
perovskite structure: an antipolar phase, a rhombohedral phase with a large
polarization along the space diagonal of the pseudocubic unit cell, and a
supertetragonal phase with even larger polarization. With the aim of
understanding the causes for this variety, we have used a genetic algorithm to
search for minima in the surface energy of these materials. Our results show
that the number of these minima is very large when compared to that of typical
ferroelectric perovskites like BaTiO and PbTiO , and that a fine energy
balance between them results in the large structural differences seen. As
byproducts of our search we have identified charge-ordering structures with low
energy in BiMnO , and several phases with energies that are similar to that
of the ground state of BiCrO. We have also found that a inverse
supertetragonal phase exists in bulk, likely to be favored in films epitaxially
grown at large values of tensile misfit strain
First-principles predictions of low-energy phases of multiferroic BiFeO3
We used first-principles methods to perform a systematic search for
potentially-stable phases of multiferroic BiFeO3. We considered a simulation
cell compatible with the atomic distortions that are most common among
perovskite oxides, and found a large number of local minima of the energy
within 100 meV/f.u. of the ferroelectric ground state. We discuss the variety
of low-symmetry structures discovered, as well as the implications of these
findings as regards current experimental (e.g., on thin films displaying {\em
super-tetragonal} phases) and theoretical (on models for BiFeO3's structural
phase transitions) work on this compound.Comment: 14 pages, 9 figures, accepted in PRB (contains small changes in the
text with respect to the first version
First-principles study of epitaxial strain in perovskites
Using an extension of a first-principles method developed by King-Smith and
Vanderbilt [Phys. Rev. B {\bf 49}, 5828 (1994)], we investigate the effects of
in-plane epitaxial strain on the ground-state structure and polarization of
eight perovskite oxides: BaTiO, SrTiO, CaTiO, KNbO, NaNbO,
PbTiO, PbZrO, and BaZrO. In addition, we investigate the effects of
a nonzero normal stress. The results are shown to be useful in predicting the
structure and polarization of perovskite oxide thin films and superlattices.Comment: 10 page
First-principles modeling of strain in perovskite ferroelectric thin films
We review the role that first-principles calculations have played in
understanding the effects of substrate-imposed misfit strain on epitaxially
grown perovskite ferroelectric films. We do so by analyzing the case of
BaTiO, complementing our previous publications on this subject with
unpublished data to help explain in detail how these calculations are done. We
also review similar studies in the literature for other perovskite
ferroelectric-film materials.Comment: 14 pages, submitted to Phase Transition