253 research outputs found
Thermodynamics of mono and di-vacancies in barium titanate
The thermodynamic and kinetic properties of mono and di-vacancy defects in
cubic (para-electric) barium titanate are studied by means of
density-functional theory calculations. It is determined which vacancy types
prevail for given thermodynamic boundary conditions. The calculations confirm
the established picture that vacancies occur in their nominal charge states
almost over the entire band gap. For the dominating range of the band gap the
di-vacancy binding energies are constant and negative. The system, therefore,
strives to achieve a state in which under metal-rich (oxygen-rich) conditions
all metal (oxygen) vacancies are bound in di-vacancy clusters. The migration
barriers are calculated for mono-vacancies in different charge states. Since
oxygen vacancies are found to readily migrate at typical growth temperatures,
di-vacancies can be formed at ease. The key results of the present study with
respect to the thermodynamic behavior of mono and di-vacancies influence the
initial defect distribution in the ferroelectric phases and therefore the
conditions for aging.Comment: 9 pages, 4 figures, 4 table
Formation and switching of defect dipoles in acceptor doped lead titanate: A kinetic model based on first-principles calculations
The formation and field-induced switching of defect dipoles in acceptor doped
lead titanate is described by a kinetic model representing an extension of the
well established Arlt-Neumann model [Ferroelectrics {\bf 76}, 303 (1987)].
Energy barriers for defect association and reorientation of oxygen
vacancy-dopant (Cu and Fe) complexes are obtained from first-principles
calculations and serve as input data for the kinetic coefficients in the rate
equation model. The numerical solution of the model describes the time
evolution of the oxygen vacancy distribution at different temperatures and
dopant concentrations in the presence or absence of an alternating external
field. We predict the characteristic time scale for the alignment of all defect
dipoles with the spontanenous polarization of the surrounding matrix. In this
state the defect dipoles act as obstacles for domain wall motion and contribute
to the experimentally observed aging. Under cycling conditions the fully
aligned configuration is perturbed and a dynamic equilibrium is established
with defect dipoles in parallel and anti-parallel orientation relative to the
spontaneous polarization. This process can be related to the deaging behavior
of piezoelectric ceramics.Comment: 10 pages, 7 figure
The hiphive package for the extraction of high-order force constants by machine learning
The efficient extraction of force constants (FCs) is crucial for the analysis
of many thermodynamic materials properties. Approaches based on the systematic
enumeration of finite differences scale poorly with system size and can rarely
extend beyond third order when input data is obtained from first-principles
calculations. Methods based on parameter fitting in the spirit of interatomic
potentials, on the other hand, can extract FC parameters from semi-random
configurations of high information density and advanced regularized regression
methods can recover physical solutions from a limited amount of data. Here, we
present the hiPhive Python package, that enables the construction of force
constant models up to arbitrary order. hiPhive exploits crystal symmetries to
reduce the number of free parameters and then employs advanced machine learning
algorithms to extract the force constants. Depending on the problem at hand
both over and underdetermined systems are handled efficiently. The FCs can be
subsequently analyzed directly and or be used to carry out e.g., molecular
dynamics simulations. The utility of this approach is demonstrated via several
examples including ideal and defective monolayers of MoS as well as bulk
nickel
Efficient construction of linear models in materials modeling and applications to force constant expansions
Linear models, such as force constant (FC) and cluster expansions, play a key
role in physics and materials science. While they can in principle be
parametrized using regression and feature selection approaches, the convergence
behavior of these techniques, in particular with respect to thermodynamic
properties is not well understood. Here, we therefore analyze the efficacy and
efficiency of several state-of-the-art regression and feature selection
methods, in particular in the context of FC extraction and the prediction of
different thermodynamic properties. Generic feature selection algorithms such
as recursive feature elimination with ordinary least-squares (OLS), automatic
relevance determination regression, and the adaptive least absolute shrinkage
and selection operator can yield physically sound models for systems with a
modest number of degrees of freedom. For large unit cells with low symmetry
and/or high-order expansions they come, however, with a non-negligible
computational cost that can be more than two orders of magnitude higher than
that of OLS. In such cases, OLS with cutoff selection provides a viable route
as demonstrated here for both second-order FCs in large low-symmetry unit cells
and high-order FCs in low-symmetry systems. While regression techniques are
thus very powerful, they require well-tuned protocols. Here, the present work
establishes guidelines for the design of protocols that are readily usable,
e.g., in high-throughput and materials discovery schemes. Since the underlying
algorithms are not specific to FC construction, the general conclusions drawn
here also have a bearing on the construction of other linear models in physics
and materials science.Comment: 15 pages, 12 figure
Finite-temperature properties of non-magnetic transition metals: Comparison of the performance of constraint-based semi and nonlocal functionals
We assess the performance of nonempirical, truly nonlocal and semi-local
functionals with regard to structural and thermal properties of , , and
non-magnetic transition metals. We focus on constraint-based functionals
and consider the new consistent-exchange van der Waals density functional
version vdW-DF-cx [Phys. Rev. B 89, 035412 (2014)], the semi-local PBE [Phys.
Rev. Lett. 77, 3865 (1996)] and PBEsol functionals [Phys. Rev. Lett. 100,
136406 (2008)] as well as the AM05 meta-functional [Phys. Rev. B 72, 085108
(2005)]. Using the quasi-harmonic approximation structural parameters, elastic
response, and thermal expansion at finite temperatures are computed and
compared to experimental data. We also compute cohesive energies explicitly
including zero-point vibrations. It is shown that overall vdW-DF-cx provides an
accurate description of thermal properties and retains a level of
transferability and accuracy that is comparable to or better than some of the
best constraint-based semi-local functionals. Especially, with regard to the
cohesive energies the consistent inclusion of spin polarization effects in the
atoms turns out to be crucial and it is important to use the rigorous
spin-vdW-DF-cx formulation [Phys. Rev. Lett. 115, 136402 (2015)]. This
demonstrates that vdW-DF-cx has general-purpose character and can be used to
study systems that have both sparse and dense electron distributions.Comment: 10 pages, 5 figure
Implications of the band gap problem on oxidation and hydration in acceptor-doped barium zirconate
Charge carrier concentrations in acceptor-doped proton-conducting perovskites
are to a large extent determined by the hydration and oxidation of oxygen
vacancies, which introduce protons and holes, respectively. First-principles
modeling of these reactions involves calculation of formation energies of
charged defects, which requires an accurate description of the band gap and the
position of the band edges. Since density-functional theory (DFT) with local
and semi-local exchange-correlation functionals (LDA and GGA) systematically
fails to predict these quantities this can have serious implications on the
modeling of defect reactions. In this study we investigate how the description
of band gap and band edge positions affects the hydration and oxidation in
acceptor-doped BaZrO. First-principles calculations are performed in
combination with thermodynamic modeling in order to obtain equilibrium charge
carrier concentrations at different temperatures and partial pressures. Three
different methods have been considered: DFT with both semi-local (PBE) and
hybrid (PBE0) exchange-correlation functionals, and many-body perturbation
theory within the -approximation. All three methods yield similar
results for the hydration reaction, which are consistent with experimental
findings. For the oxidation reaction, on the other hand, there is a qualitative
difference. PBE predicts the reaction to be exothermic while the two others
predict an endothermic behavior. Results from thermodynamic modeling are
compared with available experimental data, such as enthalpies, concentrations
and conductivities, and only the results obtained with PBE0 and , with
an endothermic oxidation behavior, give a satisfactory agreement with
experiments.Comment: 15 pages, 12 figures + supplementary material (2 pages
A variational polaron self-interaction corrected total-energy functional for charge excitations in wide-band gap insulators
We conduct a detailed investigation of the polaron self-interaction (pSI)
error in standard approximations to the exchange-correlation (XC) functional
within density-functional theory (DFT). The pSI leads to delocalization error
in the polaron wave function and energy, as calculated from the Kohn-Sham (KS)
potential in the native charge state of the polaron. This constitutes the
origin of the systematic failure of DFT to describe polaron formation in band
insulators. It is shown that the delocalization error in these systems is,
however, largely absent in the KS potential of the closed-shell neutral charge
state. This leads to a modification of the DFT total-energy functional that
corrects the pSI in the XC functional. The resulting pSIC-DFT method
constitutes an accurate parameter-free {\it ab initio} methodology for
calculating polaron properties in insulators at a computational cost that is
orders of magnitude smaller than hybrid XC functionals. Unlike approaches that
rely on parametrized localized potentials such as DFT+, the pSIC-DFT method
properly captures both site and bond-centered polaron configurations. This is
demonstrated by studying formation and migration of self-trapped holes in
alkali halides (bond-centered) as well as self-trapped electrons in an
elpasolite compound (site-centered). The pSIC-DFT approach consistently
reproduces the results obtained by hybrid XC functionals parametrized by
DFT+ calculations. Finally, we generalize the pSIC approach to hybrid
functionals, and show that in stark contrast to conventional hybrid
calculations of polaron energies, the pSIC-hybrid method is insensitive to the
parametrization of the hybrid XC functional. On this basis, we further
rationalize the success of the pSIC-DFT approach.Comment: 10 pages, 7 figure
A first-principles study of co-doping in lanthanum bromide
Co-doping of Ce-doped LaBr with Ba, Ca, or Sr improves the energy
resolution that can be achieved by radiation detectors based on these
materials. Here, we present a mechanism that rationalizes of this enhancement
that on the basis of first principles electronic structure calculations and
point defect thermodynamics. It is shown that incorporation of Sr creates
neutral -Sr complexes that can temporarily trap
electrons. As a result, Auger quenching of free carriers is reduced, allowing
for a more linear, albeit slower, scintillation light yield response.
Experimental Stokes shifts can be related to different
Ce-Sr- triple complex configurations.
Co-doping with other alkaline as well as alkaline earth metals is considered as
well. Alkaline elements are found to have extremely small solubilities on the
order of 0.1 ppm and below at 1000 K. Among the alkaline earth metals the
lighter dopant atoms prefer interstitial-like positions and create strong
scattering centers, which has a detrimental impact on carrier mobilities. Only
the heavier alkaline earth elements combine matching ionic radii with
sufficiently high solubilities. This provides a rationale for the experimental
finding that improved scintillator performance is exclusively achieved using
Sr, Ca, or Ba. The present mechanism demonstrates that co-doping of wide gap
materials can provide an efficient means for managing charge carrier
populations under out-of-equilibrium conditions. In the present case dopants
are introduced that manipulate not only the concentrations but the electronic
properties of intrinsic defects without introducing additional gap levels. This
leads to the availability of shallow electron traps that can temporarily
localize charge carriers, effectively deactivating carrier-carrier
recombination channels. The principles of this ... [continued]Comment: 13 pages, 10 figures, accepted for publication in the Physical Review
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