296 research outputs found
Calculation of the Anisotropic Coefficients of Thermal Expansion: A First-Principles Approach
Predictions of the anisotropic coefficients of thermal expansion are needed
to not only compare to experimental measurement, but also as input for
macroscopic modeling of devices which operate over a large temperature range.
While most current methods are limited to isotropic systems within the
quasiharmonic approximation, our method uses first-principles calculations and
includes anharmonic effects to determine the temperature-dependent properties
of materials. These include the lattice parameters, anisotropic coefficients of
thermal expansion, isothermal bulk modulus, and specific heat at constant
pressure. Our method has been tested on two compounds (Cu and AlN) and predicts
thermal properties which compare favorably to experimental measurement over a
wide temperature range.Comment: 8 pages, 9 figures, 1 tabl
Direct subsurface absorption of hydrogen on Pd(111)
We summarize and discuss some of the available experimental and theoretical
data important for understanding the role played by subsurface sites in
dissociative chemisorption calculations for the H/Pd(111) system. Then we
use a semi-empirical potential energy surface (PES) to model the interaction of
a H molecule impinging on a Pd(111) surface. The London-Eyring-Polanyi-Sato
(LEPS) construction has been extended to make direct subsurface absorption
possible. A 2-dimensional wave packet calculation is used to find qualitative
trends in the direct subsurface absorption and to reveal the time scales
involved. We suggest that a partial in-plane relaxation occurs for the slowest
incoming particles, thus resulting in a higher direct subsurface absorption
probability for low energies.Comment: Journal of Chemical Physics (in press), 19 pages, REVTeX, 4
Postscript figure
Theoretical analysis of oxygen vacancies in layered sodium cobaltate Na_xCoO_{2-\delta}
Sodium cobaltate with high Na content is a promising thermoelectric material.
It has recently been reported that oxygen vacancies can alter the material
properties, reducing its figure of merit. However, experimental data concerning
the oxygen stoichiometry are contradictory. We therefore studied the formation
of oxygen vacancies in Na_xCoO_2 with first principles calculations, focusing
on x = 0.75. We show that a very low oxygen vacancy concentration is expected
at the temperatures and partial pressures relevant for applications.Comment: 4 page
Understanding adsorption of hydrogen atoms on graphene
Adsorption of hydrogen atoms on a single graphite sheet (graphene) has been
investigated by first-principles electronic structure means, employing
plane-wave based, periodic density functional theory. A reasonably large 5x5
surface unit cell has been employed to study single and multiple adsorption of
H atoms. Binding and barrier energies for sequential sticking have been
computed for a number of configurations involving adsorption on top of carbon
atoms. We find that binding energies per atom range from ~0.8 eV to ~1.9 eV,
with barriers to sticking in the range 0.0-0.2 eV. In addition, depending on
the number and location of adsorbed hydrogen atoms, we find that magnetic
structures may form in which spin density localizes on a
sublattice, and that binding (barrier)
energies for sequential adsorption increase (decrease) linearly with the
site-integrated magnetization. These results can be rationalized with the help
of the valence-bond resonance theory of planar conjugated systems, and
suggest that preferential sticking due to barrierless adsorption is limited to
formation of hydrogen pairs.Comment: 12 pages, 8 figures and 4 table
Lattice thermal conductivity of TiZrHfNiSn half-Heusler alloys calculated from first principles: Key role of nature of phonon modes
In spite of their relatively high lattice thermal conductivity
, the XNiSn (X=Ti, Zr or Hf) half-Heusler compounds are good
thermoelectric materials. Previous studies have shown that can
be reduced by sublattice-alloying on the X-site. To cast light on how the alloy
composition affects , we study this system using the phonon
Boltzmann-transport equation within the relaxation time approximation in
conjunction with density functional theory.The effect of alloying through
mass-disorder scattering is explored using the virtual crystal approximation to
screen the entire ternary TiZrHfNiSn phase diagram. The
lowest lattice thermal conductivity is found for the TiHfNiSn
compositions; in particular, there is a shallow minimum centered at
TiHfNiSn with taking values between 3.2 and 4.1 W/mK
when the Ti content varies between 20 and 80\%. Interestingly, the overall
behavior of mass-disorder scattering in this system can only be understood from
a combination of the nature of the phonon modes and the magnitude of the mass
variance. Mass-disorder scattering is not effective at scattering acoustic
phonons of low energy. By using a simple model of grain boundary scattering, we
find that nanostructuring these compounds can scatter such phonons effectively
and thus further reduce the lattice thermal conductivity; for instance,
TiHfNiSn with a grain size of nm experiences a 42\%
reduction of compared to that of the single crystal
Lattice Thermal Conductivity from First Principles and Active Learning with Gaussian Process Regression
The lattice thermal conductivity () is a key materials
property in power electronics, thermal barriers, and thermoelectric devices.
Identifying a wide pool of compounds with low is particularly
important in the development of materials with high thermoelectric efficiency.
The present study contributed to this with a reliable machine learning (ML)
model based on a training set consisting of 268 cubic compounds. For those,
was calculated from first principles using the
temperature-dependent effective potential (TDEP) method based on forces and
phonons calculated by density functional theory (DFT). 238 of these were
preselected and used to train an initial ML model employing Gaussian process
regression (GPR). The model was then improved with active learning (AL) by
selecting the 30 compounds with the highest GPR uncertainty as new members of
an expanded training set. This was used to predict of the 1574
cubic compounds in the \textsc{Materials Project} (MP) database with a
validation R2-score of 0.81 and Spearman correlation of 0.93. Out of these, 27
compounds were predicted to have very low values of (
at 300~K), which was verified by DFT calculations. Some of these have not
previously been reported in the literature, suggesting further investigations
of their electronic thermoelectric properties
Discarded gems: Thermoelectric performance of materials with band gap emerging at the hybrid-functional level
A finite electronic band gap is a standard filter in high-throughput screening of materials using density functional theory (DFT). However, because of the systematic underestimation of band gaps in standard DFT approximations, a number of compounds may be incorrectly predicted metallic. In a more accurate treatment, such materials may instead appear as low band gap materials and could have good thermoelectric properties if suitable doping is feasible. To explore this possibility, we performed hybrid functional calculations on 1093 cubic materials listed in the MATERIALS PROJECT database with four atoms in the primitive unit cell, spin-neutral ground state, and a formation energy within 0.3 eV of the convex hull. Out of these materials, we identified eight compounds for which a finite band gap emerges. Evaluating electronic and thermal transport properties of these compounds, we found the compositions MgSc2Hg and Li2CaSi to exhibit promising thermoelectric properties. These findings underline the potential of reassessing band gaps and band structures of compounds to identify additional potential thermoelectric materials.acceptedVersio
The Jahn-Teller active fluoroperovskites : thermo- and magneto optical correlations as function of the -site
Chromium (II) fluoroperovskites are
strongly correlated Jahn-Teller active materials at low temperatures. In this
paper, we examine the role that the -site ion plays in this family of
fluoroperovskites using both experimental methods (XRD, optical absorption
spectroscopy and magnetic fields) and DFT simulations. Temperature-dependent
optical absorption experiments show that the spin-allowed transitions and
only merge completely for = Na at 2 K. Field-dependent optical
absorption measurements at 2 K show that the oscillating strength of the
spin-allowed transitions in increases with increasing
applied field. Direct magneto-structural correlations which suppress the
spin-flip transitions are observed for below its Ne\'el
temperature. In the spin-flip transitions vanish abruptly below
9 K revealing magneto-optical correlations not linked to crystal structure
changes. This suggests that as the long range ordering is reduced local JT
effects in the individual octahedra take control of the
observed behavior. Our results show clear deviation from the pattern found for
the isoelectronic system. The size of the -site cation
is shown to be central in dictating the physical properties and phase
transitions in , opening up the possibility of varying the
composition to create novel states of matter with tuneable properties
Direct subsurface absorption of hydrogen on Pd(111): Quantum mechanical calculations on a new two-dimensional potential energy surface.
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