52 research outputs found
Convergence and pitfalls of density functional perturbation theory phonons calculations from a high-throughput perspective
The diffusion of large databases collecting different kind of material
properties from high-throughput density functional theory calculations has
opened new paths in the study of materials science thanks to data mining and
machine learning techniques. Phonon calculations have already been employed
successfully to predict materials properties and interpret experimental data,
e.g. phase stability, ferroelectricity and Raman spectra, so their availability
for a large set of materials will further increase the analytical and
predictive power at hand. Moving to a larger scale with density functional
perturbation calculations, however, requires the presence of a robust framework
to handle this challenging task. In light of this, we automatized the phonon
calculation and applied the result to the analysis of the convergence trends
for several materials. This allowed to identify and tackle some common problems
emerging in this kind of simulations and to lay out the basis to obtain
reliable phonon band structures from high-throughput calculations, as well as
optimizing the approach to standard phonon simulations
An unlikely route to low lattice thermal conductivity: small atoms in a simple layered structure
In the design of materials with low lattice thermal conductivity, compounds
with high density, low speed of sound, and complexity at either the atomic,
nano- or microstructural level are preferred. The layered compound MgSb
defies these prevailing paradigms, exhibiting lattice thermal conductivity
comparable to PbTe and BiTe, despite its low density and simple
structure. The excellent thermoelectric performance ( 1.5) in
-type MgSb has thus far been attributed to its multi-valley
conduction band, while its anomalous thermal properties have been largely
overlooked. To explain the origin of the low lattice thermal conductivity of
MgSb, we have used both experimental methods and ab initio phonon
calculations to investigate trends in the elasticity, thermal expansion and
anharmonicity of Mg Zintl compounds with = Mg, Ca, Yb, and
= Sb and Bi. Phonon calculations within the quasi-harmonic approximation reveal
large mode Gr\"uneisen parameters in MgSb compared with isostructural
compounds, in particular in transverse acoustic modes involving shearing of
adjacent anionic layers. Measurements of the elastic moduli and sound velocity
as a function of temperature using resonant ultrasound spectroscopy provide a
window into the softening of the acoustic branches at high temperature,
confirming their exceptionally high anharmonicity. We attribute the anomalous
thermal behavior of MgSb to the diminutive size of Mg, which may be too
small for the octahedrally-coordinated site, leading to weak, unstable
interlayer Mg-Sb bonding. This suggests more broadly that soft shear modes
resulting from undersized cations provide a potential route to achieving low
lattice thermal conductivity low-density, earth-abundant materials.Comment: 11 pages, 9 figure
MicroRNA profiles in hippocampal granule cells and plasma of rats with pilocarpine-induced epilepsy - Comparison with human epileptic samples
The identification of biomarkers of the transformation of normal to epileptic tissue would help to
stratify patients at risk of epilepsy following brain injury, and inform new treatment strategies.
MicroRNAs (miRNAs) are an attractive option in this direction. In this study, miRNA microarrays were
performed on laser-microdissected hippocampal granule cell layer (GCL) and on plasma, at different
time points in the development of pilocarpine-induced epilepsy in the rat: latency, first spontaneous
seizure and chronic epileptic phase. Sixty-three miRNAs were differentially expressed in the GCL
when considering all time points. Three main clusters were identified that separated the control and
chronic phase groups from the latency group and from the first spontaneous seizure group. MiRNAs
from rats in the chronic phase were compared to those obtained from the laser-microdissected GCL
of epileptic patients, identifying several miRNAs (miR-21-5p, miR-23a-5p, miR-146a-5p and miR-
181c-5p) that were up-regulated in both human and rat epileptic tissue. Analysis of plasma samples
revealed different levels between control and pilocarpine-treated animals for 27 miRNAs. Two main
clusters were identified that segregated controls from all other groups. Those miRNAs that are
altered in plasma before the first spontaneous seizure, like miR-9a-3p, may be proposed as putative
biomarkers of epileptogenesis
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Metal phosphides as potential thermoelectric materials
There still exists a crucial need for new thermoelectric materials to efficiently recover waste heat as electrical energy. Although metal phosphides are stable and can exhibit excellent electronic properties, they have traditionally been overlooked as thermoelectrics due to expectations of displaying high thermal conductivity. Based on high-throughput computational screening of the electronic properties of over 48 000 inorganic compounds, we find that several metal phosphides offer considerable promise as thermoelectric materials, with excellent potential electronic properties (e.g. due to multiple valley degeneracy). In addition to the electronic band structure, the phonon dispersion curves of various metal phosphides were computed indicating low-frequency acoustic modes that could lead to low thermal conductivity. Several metal phosphides exhibit promising thermoelectric properties. The computed electronic and thermal properties were compared to experiments to test the reliability of the calculations indicating that the predicted thermoelectric properties are semi-quantitative. As a complete experimental study of the thermoelectric properties in MPs, cubic-NiP2 was synthesized and the low predicted lattice thermal conductivity (∼1.2 W m^(−1) K^(−1) at 700 K) was confirmed. The computed Seebeck coefficient is in agreement with experiments over a range of temperatures and the phononic dispersion curve of c-NiP_2 is consistent with the experimental heat capacity. The predicted high thermoelectric performance in several metal phosphides and the low thermal conductivity measured in NiP_2 encourage further investigations of thermoelectric properties of metal phosphides
Metal phosphides as potential thermoelectric materials
There still exists a crucial need for new thermoelectric materials to efficiently recover waste heat as electrical energy. Although metal phosphides are stable and can exhibit excellent electronic properties, they have traditionally been overlooked as thermoelectrics due to expectations of displaying high thermal conductivity. Based on high-throughput computational screening of the electronic properties of over 48 000 inorganic compounds, we find that several metal phosphides offer considerable promise as thermoelectric materials, with excellent potential electronic properties (e.g. due to multiple valley degeneracy). In addition to the electronic band structure, the phonon dispersion curves of various metal phosphides were computed indicating low-frequency acoustic modes that could lead to low thermal conductivity. Several metal phosphides exhibit promising thermoelectric properties. The computed electronic and thermal properties were compared to experiments to test the reliability of the calculations indicating that the predicted thermoelectric properties are semi-quantitative. As a complete experimental study of the thermoelectric properties in MPs, cubic-NiP2 was synthesized and the low predicted lattice thermal conductivity (∼1.2 W m^(−1) K^(−1) at 700 K) was confirmed. The computed Seebeck coefficient is in agreement with experiments over a range of temperatures and the phononic dispersion curve of c-NiP_2 is consistent with the experimental heat capacity. The predicted high thermoelectric performance in several metal phosphides and the low thermal conductivity measured in NiP_2 encourage further investigations of thermoelectric properties of metal phosphides
How to verify the precision of density-functional-theory implementations via reproducible and universal workflows
In the past decades many density-functional theory methods and codes adopting
periodic boundary conditions have been developed and are now extensively used
in condensed matter physics and materials science research. Only in 2016,
however, their precision (i.e., to which extent properties computed with
different codes agree among each other) was systematically assessed on
elemental crystals: a first crucial step to evaluate the reliability of such
computations. We discuss here general recommendations for verification studies
aiming at further testing precision and transferability of
density-functional-theory computational approaches and codes. We illustrate
such recommendations using a greatly expanded protocol covering the whole
periodic table from Z=1 to 96 and characterizing 10 prototypical cubic
compounds for each element: 4 unaries and 6 oxides, spanning a wide range of
coordination numbers and oxidation states. The primary outcome is a reference
dataset of 960 equations of state cross-checked between two all-electron codes,
then used to verify and improve nine pseudopotential-based approaches. Such
effort is facilitated by deploying AiiDA common workflows that perform
automatic input parameter selection, provide identical input/output interfaces
across codes, and ensure full reproducibility. Finally, we discuss the extent
to which the current results for total energies can be reused for different
goals (e.g., obtaining formation energies).Comment: Main text: 23 pages, 4 figures. Supplementary: 68 page
OPTIMADE, an API for exchanging materials data
: The Open Databases Integration for Materials Design (OPTIMADE) consortium has designed a universal application programming interface (API) to make materials databases accessible and interoperable. We outline the first stable release of the specification, v1.0, which is already supported by many leading databases and several software packages. We illustrate the advantages of the OPTIMADE API through worked examples on each of the public materials databases that support the full API specification
OPTIMADE, an API for exchanging materials data.
The Open Databases Integration for Materials Design (OPTIMADE) consortium has designed a universal application programming interface (API) to make materials databases accessible and interoperable. We outline the first stable release of the specification, v1.0, which is already supported by many leading databases and several software packages. We illustrate the advantages of the OPTIMADE API through worked examples on each of the public materials databases that support the full API specification
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