54 research outputs found
Antiferromagnetism at T > 500 K in the Layered Hexagonal Ruthenate SrRu2O6
We report an experimental and computational study of magnetic and electronic
properties of the layered Ru(V) oxide SrRu2O6 (hexagonal, P-3 1m), which shows
antiferromagnetic order with a N\'eel temperature of 563(2) K, among the
highest for 4d oxides. Magnetic order occurs both within edge-shared octahedral
sheets and between layers and is accompanied by anisotropic thermal expansivity
that implies strong magnetoelastic coupling of Ru(V) centers. Electrical
transport measurements using focused ion beam induced deposited contacts on a
micron-scale crystallite as a function of temperature show p-type
semiconductivity. The calculated electronic structure using hybrid density
functional theory successfully accounts for the experimentally observed
magnetic and electronic structure and Monte Carlo simulations reveals how
strong intralayer as well as weaker interlayer interactions are a defining
feature of the high temperature magnetic order in the material.Comment: Physical Review B 2015 accepted for publicatio
Galore: Broadening and weighting for simulation of photoelectron spectroscopy
Galore simplifies and automates the process of simulating photoelectron spectra from ab
initio calculations. This replaces the tedious process of extracting and interpolating crosssectional weights from reference data and generates tabulated data or publication-ready
plots as needed. The broadening tools may also be used to obtain realistic simulated
spectra from a theoretical set of discrete lines (e.g. infrared or Raman spectroscopy)
Efficient calculation of carrier scattering rates from first principles
The electronic transport behaviour of materials determines their suitability
for technological applications. We develop an efficient method for calculating
carrier scattering rates of solid-state semiconductors and insulators from
first principles inputs. The present method extends existing polar and
non-polar electron-phonon coupling, ionized impurity, and piezoelectric
scattering mechanisms formulated for isotropic band structures to support
highly anisotropic materials. We test the formalism by calculating the
electronic transport properties of 16 semiconductors and comparing the results
against experimental measurements. The present work is amenable for use in
high-throughput computational workflows and enables accurate screening of
carrier mobilities, lifetimes, and thermoelectric power.Comment: 11 pages, 4 figures (SI: 21 pages, 14 figures
Anion Distribution, Structural Distortion, and Symmetry-Driven Optical Band Gap Bowing in Mixed Halide Cs₂SnX₆ Vacancy Ordered Double Perovskites
Mixed anion compounds in the Fm-3m vacancy ordered perovskite structure were synthesised and characterised experimentally and computationally with a focus on compounds where A = Cs+. Pure anion Cs2SnX6 compounds were formed with X = Cl, Br and I using a room temperature solution phase method. Mixed anion compounds were formed as solid solutions of Cs2SnCl6 and Cs2SnBr6 and a second series from Cs2SnBr6 and Cs2SnI6. Single phase structures formed across the entirety of both composition series, with no evidence of long range anion ordering observed by diffraction. A distortion of the cubic A2BX6 structure was identified in which the spacing of the BX6 octahedra changes to accommodate the A site cation without reduction of overall symmetry. Optical band gap values varied with anion composition between 4.89 eV in Cs2SnCl6 to 1.35 eV in Cs2SnI6, but proved highly non-linear with changes in composition. In mixed halide compounds it was found that lower energy optical transitions appeared that were not present in the pure halide compounds, and this could be attributed to lowering of the local symmetry within the tin halide octahedra. The electronic structure was characterised by photoemission spectroscopy, and Raman spectroscopy revealed vibrational modes in the mixed halide compounds that could be assigned to particular mixed halide octahedra. This analysis was used to determine the distribution of octahedra types in mixed anion compounds, which was found to be consistent with a near-random distribution of halide anions throughout the structure, although some deviations from random halide distribution were noted in mixed iodide-bromide compounds, where the larger iodide anions preferentially adopted trans configurations
High-throughput determination of Hubbard U and Hund J values for transition metal oxides via linear response formalism
DFT+U provides a convenient, cost-effective correction for the
self-interaction error (SIE) that arises when describing correlated electronic
states using conventional approximate density functional theory (DFT). The
success of a DFT+U(+J) calculation hinges on the accurate determination of its
Hubbard U and Hund's J parameters, and the linear response (LR) methodology has
proven to be computationally effective and accurate for calculating these
parameters. This study provides a high-throughput computational analysis of the
U and J values for transition metal d-electron states in a representative set
of over 2000 magnetic transition metal oxides (TMOs), providing a frame of
reference for researchers who use DFT+U to study transition metal oxides. In
order to perform this high-throughput study, an atomate workflow is developed
for calculating U and J values automatically on massively parallel
supercomputing architectures. To demonstrate an application of this workflow,
the spin-canting magnetic structure and unit cell parameters of the
multiferroic olivine LiNiPO4 are calculated using the computed Hubbard U and
Hund J values for Ni-d and O-p states, and are compared with experiment. Both
the Ni-d U and J corrections have a strong effect on the Ni-moment canting
angle. Additionally, including a O-p U value results in a significantly
improved agreement between the computed lattice parameters and experiment.Comment: 18 pages, 6 figure
Comparison of the tetrahedron method to smearing methods for the electronic density of states
The electronic density of states (DOS) highlights fundamental properties of materials that oftentimes dictate their properties, such as the band gap and Van Hove singularities. In this short note, we discuss how sharp features of the density of states can be obscured by smearing methods (such as the Gaussian and Fermi smearing methods) when calculating the DOS. While the common approach to reach a "converged" density of states of a material is to increase the discrete k-point mesh density, we show that the DOS calculated by smearing methods can appear to converge but not to the correct DOS. Employing the tetrahedron method for Brillouin zone integration resolves key features of the density of states far better than smearing methods
Designing transparent conductors using forbidden optical transitions
Many semiconductors present weak or forbidden transitions at their
fundamental band gaps, inducing a widened region of transparency. This occurs
in high-performing n-type transparent conductors (TCs) such as Sn-doped In2O3
(ITO), however thus far the presence of forbidden transitions has been
neglected in searches for new p-type TCs. To address this, we first compute
high-throughput absorption spectra across ~18,000 semiconductors, showing that
over half exhibit forbidden or weak optical transitions at their band edges.
Next, we demonstrate that compounds with highly localized band edge states are
more likely to present forbidden transitions. Lastly, we search this set for
p-type and n-type TCs with forbidden or weak transitions. Defect calculations
yield unexplored TC candidates such as ambipolar BeSiP2, Zr2SN2 and KSe, p-type
BAs, Au2S, and AuCl, and n-type Ba2InGaO5, GaSbO4, and KSbO3, among others. We
share our data set via the MPContribs platform, and we recommend that future
screenings for optical properties use metrics representative of absorption
features rather than band gap alone
Electroactive nanoporous metal oxides and chalcogenides by chemical design
The archetypal silica- and aluminosilicate-based zeolite-type materials are renowned for wide-ranging applications in heterogeneous catalysis, gas-separation and ion-exchange. Their compositional space can be expanded to include nanoporous metal chalcogenides, exemplified by germanium and tin sulfides and selenides. By comparison with the properties of bulk metal dichalcogenides and their 2D derivatives, these open-framework analogues may be viewed as three-dimensional semiconductors filled with nanometer voids. Applications exist in a range of molecule size and shape discriminating devices. However, what is the electronic structure of nanoporous metal chalcogenides? Herein, materials modeling is used to describe the properties of a homologous series of nanoporous metal chalcogenides denoted np-MX2, where M = Si, Ge, Sn, Pb, and X = O, S, Se, Te, with Sodalite, LTA and aluminum chromium phosphate-1 structure types. Depending on the choice of metal and anion their properties can be tuned from insulators to semiconductors to metals with additional modification achieved through doping, solid solutions, and inclusion (with fullerene, quantum dots, and hole transport materials). These systems form the basis of a new branch of semiconductor nanochemistry in three dimensions
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