176 research outputs found
Lattice effects on the formation of oxygen vacancies in perovskite thin films
We use first-principles methods to investigate the effects of collective
lattice excitations on the formation of oxygen vacancies in perovskite thin
films. We find that phonons play a crucial role on the strain-mediated control
of defect chemistry at finite temperatures. In particular, zero-temperature
oxygen vacancy formation trends deduced as a function of epitaxial strain can
be fully reversed near room temperature. Our first-principles calculations
evidence a direct link between the lattice contribution to the oxygen vacancy
free energy and the volume expansion that the system undergoes when is
chemically reduced: The larger the resulting volume expansion, the more
favorable thermal excitations are to point defect formation. However, the
interplay between the vibrational vacancy entropy, or equivalently, chemical
expansion, and epitaxial strain is difficult to generalise as this can be
strongly influenced by underlying structural and magnetic transitions. In
addition, we find that vacancy ordering can be largely hindered by the thermal
lattice excitations.Comment: 5 pages, 5 figure
The Role of Density Functional Theory Methods in the Prediction of Nanostructured Gas-Adsorbent Materials
With the advent of new synthesis and large-scale production technologies,
nanostructured gas-adsorbent materials (GAM) like carbon nanocomposites and
metal-organic frameworks are becoming increasingly more influential in our
everyday lives. First-principles methods based on density functional theory
(DFT) have been pivotal in establishing the rational design of GAM, a factor
which has tremendously boosted their development. However, DFT methods are not
perfect and due to the stringent accuracy thresholds demanded in modelling of
GAM (i.e., exact binding energies to within ~0.01 eV) these techniques may
provide erroneous conclusions in some challenging situations. Examples of
problematic circumstances include gas-adsorption processes in which both
electronic long-range exchange and nonlocal correlations are important, and
systems where many-body energy and Coulomb screening effects cannot be
disregarded. In this critical review, we analyse recent efforts done in the
assessment of the performance of DFT methods in the prediction and
understanding of GAM. Our inquiry is constrained to the areas of hydrogen
storage and carbon capture and sequestration, for which we expose a number of
unresolved modelling controversies and define a set of best practice simulation
principles. Also, we identify the subtle problems found in the generalization
of DFT benchmark studies performed in model cluster systems to real materials,
and discuss effective approaches to circumvent them. The increasing awareness
of the strengths and imperfections of DFT methods in the simulation of
gas-adsorption phenomena should lead in the medium term to more precise, and
hence even more fruitful, ab initio engineering of GAM.Comment: 27 pages, 10 figures, review articl
In the search of new electrocaloric materials: Fast ion conductors
We analyse the effects of applying an electric field on the critical
temperature, Ts, at which superionicity appears in archetypal fast ion
conductor CaF2 by means of molecular dynamics simulations. We find that the
onset of superionicity can be reduced by about 100K when relatively small
electric fields of ~50KV/cm are employed. Under large enough electric fields,
however, ionic conductivity is depleted. The normal to superionic phase
transition is characterised by a large increase of entropy, thereby sizeable
electrocaloric effects can be realised in fast ion conductors that are
promising for solid-state cooling applications.Comment: 2 pages, 2 figure
First-principles modeling of three-body interactions in highly compressed solid helium
We present a new set of three-body interaction models based on the
Bruch-McGee (BM) potential that are suitable for the study of the energy,
structural and elastic properties of solid 4He at high pressure. Our ab initio
three-body potentials are obtained from the fit to total energies and atomic
forces computed with the van der Waals density functional theory method due to
Grimme, and represent an improvement with respect to previously reported
three-body interaction models. In particular, we show that some of the
introduced BM parametrizations reproduce closely the experimental equation of
state and bulk modulus of solid helium up to a pressure of ~ 60 GPa, when used
in combination with standard pairwise interaction models in diffusion Monte
Carlo simulations. Importantly, we find that recent predictions reporting a
surprisingly small variation of the kinetic energy and Lindeman ratio on
quantum crystals under increasing pressure are likely to be artifacts produced
by the use of incomplete interaction models. Also, we show that the
experimental variation of the shear modulus, C44, at P < 25 GPa can be
quantitatively described with the new set of three-body BM potentials. At
higher pressures, however, the agreement between our C44 results and
experiments deteriorates and thus we argue that higher order many-body terms in
the expansion of the atomic interactions probably are necessary in order to
better describe elasticity in very dense solid 4He.Comment: 11 pages, 7 figure
Electrostatic engineering of strained ferroelectric perovskites from first-principles
Design of novel artificial materials based on ferroelectric perovskites
relies on the basic principles of electrostatic coupling and in-plane lattice
matching. These rules state that the out-of-plane component of the electric
displacement field and the in-plane components of the strain are preserved
across a layered superlattice, provided that certain growth conditions are
respected. Intense research is currently directed at optimizing materials
functionalities based on these guidelines, often with remarkable success. Such
principles, however, are of limited practical use unless one disposes of
reliable data on how a given material behaves under arbitrary electrical and
mechanical boundary conditions. Here we demonstrate, by focusing on the
prototypical ferroelectrics PbTiO3 and BiFeO3 as testcases, how such
information can be calculated from first principles in a systematic and
efficient way. In particular, we construct a series of two-dimensional maps
that describe the behavior of either compound (e.g. concerning the
ferroelectric polarization and antiferrodistortive instabilities) at any
conceivable choice of the in-plane lattice parameter, a, and out-of-plane
electric displacement, D. In addition to being of immediate practical
applicability to superlattice design, our results bring new insight into the
complex interplay of competing degrees of freedom in perovskite materials, and
reveal some notable instances where the behavior of these materials depart from
what naively is expected.Comment: 13 pages, 9 figure
The Limit of Mechanical Stability in Quantum Crystals: A Diffusion Monte Carlo Study of Solid 4He
We present a first-principles study of the energy and elastic properties of
solid helium at pressures below the range in which is energetically stable. We
find that the limit of mechanical stability in hcp 4He is = -33.82 bar,
which lies significantly below the spinodal pressure found in the liquid phase
(i.e., -9.6 bar). Furthermore, we show that the pressure variation of the
transverse and longitudinal sound velocities close to do not follow a
power law of the form , in contrast
to what is observed on the fluid.Comment: 4 pages, 4 figure
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