519 research outputs found
Ab initio study on the effects of transition metal doping of Mg2NiH4
Mg2NiH4 is a promising hydrogen storage material with fast (de)hydrogenation
kinetics. Its hydrogen desorption enthalpy, however, is too large for practical
applications. In this paper we study the effects of transition metal doping by
first-principles density functional theory calculations. We show that the
hydrogen desorption enthalpy can be reduced by ~0.1 eV/H2 if one in eight Ni
atoms is replaced by Cu or Fe. Replacing Ni by Co atoms, however, increases the
hydrogen desorption enthalpy. We study the thermodynamic stability of the
dopants in the hydrogenated and dehydrogenated phases. Doping with Co or Cu
leads to marginally stable compounds, whereas doping with Fe leads to an
unstable compound. The optical response of Mg2NiH4 is also substantially
affected by doping. The optical gap in Mg2NiH4 is ~1.7 eV. Doping with Co, Fe
or Cu leads to impurity bands that reduce the optical gap by up to 0.5 eV.Comment: 8 pages, 4 figure
Automation methodologies and large-scale validation for , towards high-throughput calculations
The search for new materials, based on computational screening, relies on
methods that accurately predict, in an automatic manner, total energy,
atomic-scale geometries, and other fundamental characteristics of materials.
Many technologically important material properties directly stem from the
electronic structure of a material, but the usual workhorse for total energies,
namely density-functional theory, is plagued by fundamental shortcomings and
errors from approximate exchange-correlation functionals in its prediction of
the electronic structure. At variance, the method is currently the
state-of-the-art {\em ab initio} approach for accurate electronic structure. It
is mostly used to perturbatively correct density-functional theory results, but
is however computationally demanding and also requires expert knowledge to give
accurate results. Accordingly, it is not presently used in high-throughput
screening: fully automatized algorithms for setting up the calculations and
determining convergence are lacking. In this work we develop such a method and,
as a first application, use it to validate the accuracy of using the
PBE starting point, and the Godby-Needs plasmon pole model
(@PBE), on a set of about 80 solids. The results of the
automatic convergence study utilized provides valuable insights. Indeed, we
find correlations between computational parameters that can be used to further
improve the automatization of calculations. Moreover, we find that
@PBE shows a correlation between the PBE and the
@PBE gaps that is much stronger than that between and
experimental gaps. However, the @PBE gaps still describe
the experimental gaps more accurately than a linear model based on the PBE
gaps.Comment: 12 pages, 11 figure
Ab initio study of magnesium alanate, Mg(AlH4)2
Magnesium alanate Mg(AlH4)2 has recently raised interest as a potential
material for hydrogen storage. We apply ab initio calculations to characterize
structural, electronic and energetic properties of Mg(AlH4)2. Density
functional theory calculations within the generalized gradient approximation
(GGA) are used to optimize the geometry and obtain the electronic structure.
The latter is also studied by quasi-particle calculations at the GW level.
Mg(AlH4)2 is a large band gap insulator with a fundamental band gap of 6.5 eV.
The hydrogen atoms are bonded in AlH4 complexes, whose states dominate both the
valence and the conduction bands. On the basis of total energies, the formation
enthalpy of Mg(AlH4)2 with respect to bulk magnesium, bulk aluminum and
hydrogen gas is 0.17 eV/H2 (at T = 0). Including corrections due to the zero
point vibrations of the hydrogen atoms this number decreases to 0.10 eV/H2. The
enthalpy of the dehydrogenation reaction Mg(AlH4)2 -> MgH2 +2Al+3H2(g) is close
to zero, which impairs the potential usefulness of magnesium alanate as a
hydrogen storage material.Comment: 5 pages, 3 figure
The PseudoDojo: Training and grading a 85 element optimized norm-conserving pseudopotential table
First-principles calculations in crystalline structures are often performed
with a planewave basis set. To make the number of basis functions tractable two
approximations are usually introduced: core electrons are frozen and the
diverging Coulomb potential near the nucleus is replaced by a smoother
expression. The norm-conserving pseudopotential was the first successful method
to apply these approximations in a fully ab initio way. Later on, more
efficient and more exact approaches were developed based on the ultrasoft and
the projector augmented wave formalisms. These formalisms are however more
complex and developing new features in these frameworks is usually more
difficult than in the norm-conserving framework. Most of the existing tables of
norm- conserving pseudopotentials, generated long ago, do not include the
latest developments, are not systematically tested or are not designed
primarily for high accuracy. In this paper, we present our PseudoDojo framework
for developing and testing full tables of pseudopotentials, and demonstrate it
with a new table generated with the ONCVPSP approach. The PseudoDojo is an open
source project, building on the AbiPy package, for developing and
systematically testing pseudopotentials. At present it contains 7 different
batteries of tests executed with ABINIT, which are performed as a function of
the energy cutoff. The results of these tests are then used to provide hints
for the energy cutoff for actual production calculations. Our final set
contains 141 pseudopotentials split into a standard and a stringent accuracy
table. In total around 70.000 calculations were performed to test the
pseudopotentials. The process of developing the final table led to new insights
into the effects of both the core-valence partitioning and the non-linear core
corrections on the stability, convergence, and transferability of
norm-conserving pseudopotentials. ...Comment: abstract truncated, 17 pages, 25 figures, 8 table
A model for the formation energies of alanates and boranates
We develop a simple model for the formation energies (FEs) of alkali and
lkaline earth alanates and boranates, based upon ionic bonding between metal
cations and (AlH4)- or (BH4)- anions. The FEs agree well with values obtained
from first principles calculations and with experimental FEs. The model shows
that details of the crystal structure are relatively unimportant. The small
size of the (BH4)- anion causes a strong bonding in the crystal, which makes
boranates more stable than alanates. Smaller alkali or alkaline earth cations
do not give an increased FE. They involve a larger ionization potential that
compensates for the increased crystal bonding.Comment: 3 pages, 2 figure
First-principles study of the optical properties of MgxTi(1-x)H2
The optical and electronic properties of Mg-Ti hydrides are studied using
first-principles density functional theory. Dielectric functions are calculated
for MgxTi(1-x)H2 with compositions x = 0.5, 0.75, and 0.875. The structure is
that of fluorite TiH2 where both Mg and Ti atoms reside at the Ti positions of
the lattice. In order to assess the effect of randomness in the Mg and Ti
occupations we consider both highly ordered structures, modeled with simple
unit cells of minimal size, and models of random alloys. These are simulated by
super cells containing up to 64 formula units (Z = 64). All compositions and
structural models turn out metallic, hence the dielectric functions contain
interband and intraband free electron contributions. The former are calculated
in the independent particle random phase approximation. The latter are modeled
based upon the intraband plasma frequencies, which are also calculated from
first-principles. Only for the models of the random alloys we obtain a black
state, i.e. low reflection and transmission in the energy range from 1 to 6 eV.Comment: 7 pages, 8 figure
Electronic structure and optical properties of lightweight metal hydrides
We study the electronic structures and dielectric functions of the simple
hydrides LiH, NaH, MgH2 and AlH3, and the complex hydrides Li3AlH6, Na3AlH6,
LiAlH4, NaAlH4 and Mg(AlH4)2, using first principles density functional theory
and GW calculations. All these compounds are large gap insulators with GW
single particle band gaps varying from 3.5 eV in AlH3 to 6.5 eV in the MAlH4
compounds. The valence bands are dominated by the hydrogen atoms, whereas the
conduction bands have mixed contributions from the hydrogens and the metal
cations. The electronic structure of the aluminium compounds is determined
mainly by aluminium hydride complexes and their mutual interactions. Despite
considerable differences between the band structures and the band gaps of the
various compounds, their optical responses are qualitatively similar. In most
of the spectra the optical absorption rises sharply above 6 eV and has a strong
peak around 8 eV. The quantitative differences in the optical spectra are
interpreted in terms of the structure and the electronic structure of the
compounds.Comment: 13 pages, 10 figure
First principles modelling of magnesium titanium hydrides
Mixing Mg with Ti leads to a hydride Mg(x)Ti(1-x)H2 with markedly improved
(de)hydrogenation properties for x < 0.8, as compared to MgH2. Optically, thin
films of Mg(x)Ti(1-x)H2 have a black appearance, which is remarkable for a
hydride material. In this paper we study the structure and stability of
Mg(x)Ti(1-x)H2, x= 0-1 by first-principles calculations at the level of density
functional theory. We give evidence for a fluorite to rutile phase transition
at a critical composition x(c)= 0.8-0.9, which correlates with the
experimentally observed sharp decrease in (de)hydrogenation rates at this
composition. The densities of states of Mg(x)Ti(1-x)H2 have a peak at the Fermi
level, composed of Ti d states. Disorder in the positions of the Ti atoms
easily destroys the metallic plasma, however, which suppresses the optical
reflection. Interband transitions result in a featureless optical absorption
over a large energy range, causing the black appearance of Mg(x)Ti(1-x)H2.Comment: 22 pages, 9 figures, 4 table
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Allele-specific NKX2-5 binding underlies multiple genetic associations with human electrocardiographic traits.
The cardiac transcription factor (TF) gene NKX2-5 has been associated with electrocardiographic (EKG) traits through genome-wide association studies (GWASs), but the extent to which differential binding of NKX2-5 at common regulatory variants contributes to these traits has not yet been studied. We analyzed transcriptomic and epigenomic data from induced pluripotent stem cell-derived cardiomyocytes from seven related individuals, and identified ~2,000 single-nucleotide variants associated with allele-specific effects (ASE-SNVs) on NKX2-5 binding. NKX2-5 ASE-SNVs were enriched for altered TF motifs, for heart-specific expression quantitative trait loci and for EKG GWAS signals. Using fine-mapping combined with epigenomic data from induced pluripotent stem cell-derived cardiomyocytes, we prioritized candidate causal variants for EKG traits, many of which were NKX2-5 ASE-SNVs. Experimentally characterizing two NKX2-5 ASE-SNVs (rs3807989 and rs590041) showed that they modulate the expression of target genes via differential protein binding in cardiac cells, indicating that they are functional variants underlying EKG GWAS signals. Our results show that differential NKX2-5 binding at numerous regulatory variants across the genome contributes to EKG phenotypes
DFT Study of Planar Boron Sheets: A New Template for Hydrogen Storage
We study the hydrogen storage properties of planar boron sheets and compare
them to those of graphene. The binding of molecular hydrogen to the boron sheet
(0.05 eV) is stronger than that to graphene. We find that dispersion of alkali
metal (AM = Li, Na, and K) atoms onto the boron sheet markedly increases
hydrogen binding energies and storage capacities. The unique structure of the
boron sheet presents a template for creating a stable lattice of strongly
bonded metal atoms with a large nearest neighbor distance. In contrast, AM
atoms dispersed on graphene tend to cluster to form a bulk metal. In particular
the boron-Li system is found to be a good candidate for hydrogen storage
purposes. In the fully loaded case this compound can contain up to 10.7 wt. %
molecular hydrogen with an average binding energy of 0.15 eV/H2.Comment: 19 pages, 7 figures, and 3 table
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