1,461 research outputs found
Electronic states and magnetic structure at the Co3O4 (110) surface: a first principles study
Tricobalt tetraoxide (Co3O4) is an important catalyst and Co3O4(110) is a
frequently exposed surface in Co3O4 nanomaterials. We employed
Density-functional theory with on-site Coulomb repulsion U term to study the
atomic structures, energetics, magnetic and electronic properties of the two
possible terminations, A and B, of this surface. These calculations predict A
as the stable termination in a wide range of oxygen chemical potentials,
consistent with recent experimental observations. The Co3+ ions do not have a
magnetic moment in the bulk, but become magnetic at the surface, which leads to
surface magnetic orderings different from the one in the bulk. Surface
electronic states are present in the lower half of the bulk band gap and cause
partial metallization of both surface terminations. These states are
responsible for the charge compensation mechanism stabilizing both polar
terminations. The computed critical thickness for polarity compensation is 4
layers
An ab-initio converse NMR approach for pseudopotentials
We extend the recently developed converse NMR approach [T. Thonhauser, D.
Ceresoli, A. Mostofi, N. Marzari, R. Resta, and D. Vanderbilt, J. Chem. Phys.
\textbf{131}, 101101 (2009)] such that it can be used in conjunction with
norm-conserving, non-local pseudopotentials. This extension permits the
efficient ab-initio calculation of NMR chemical shifts for elements other than
hydrogen within the convenience of a plane-wave pseudopotential approach. We
have tested our approach on several finite and periodic systems, finding very
good agreement with established methods and experimental results.Comment: 11 pages, 2 figures, 4 tables; references expande
Mechanism of ferroelectric instabilities in non d^0 perovskites: LaCrO_3 versus CaMnO_3
The incompatibility of partial d occupation on the perovskite B-site with the
standard charge transfer mechanism for ferroelectricity has been a central
paradigm in multiferroics research. Nevertheless, it was recently shown by
density functional theory calculations that CaMnO_3 exhibits a polar
instability that even dominates over the octahedral tilting for slightly
enlarged unit cell volume. Here, we present similar calculations for LaCrO_3,
which has the same d^3 B-site electron configuration as CaMnO_3. We find that
LaCrO_3 exhibits a very similar, albeit much weaker, polar instability as
CaMnO_3. In addition, while the Born effective charge (BEC) of the Mn^{4+}
cation in CaMnO_3 is highly anomalous, the BEC of Cr^{3+} in LaCrO_3 is only
slightly enhanced. By decomposing the BECs into contributions of individual
Wannier functions we show that the ferroelectric instabilities in both systems
can be understood in terms of charge transfer between TM d and O p states,
analogously to the standard d^0 perovskite ferroelectrics.Comment: 6 pages, 4 figures, 2 table
Towards First-principles Electrochemistry
Chemisorbed molecules at a fuel cell electrode are a very sensitive probe of
the surrounding electrochemical environment, and one that can be accurately
monitored with different spectroscopic techniques. We develop a comprehensive
electrochemical model to study molecular chemisorption at either constant
charge or fixed applied voltage, and calculate from first principles the
voltage dependence of vibrational frequencies -- the vibrational Stark effect
-- for CO adsorbed on close-packed platinum electrodes. The predicted
vibrational Stark slopes are found to be in very good agreement with
experimental electrochemical spectroscopy data, thereby resolving previous
controversies in the quantitative interpretation of in-situ experiments and
elucidating the relation between canonical and grand-canonicaldescriptions of
vibrational surface phenomena.Comment: 10 pages, 2 figure
A Peculiar Family of Jupiter Trojans: the Eurybates
The Eurybates family is a compact core inside the Menelaus clan, located in
the L4 swarm of Jupiter Trojans. Fornasier et al. (2007) found that this family
exhibits a peculiar abundance of spectrally flat objects, similar to
Chiron-like Centaurs and C-type main belt asteroids. On the basis of the
visible spectra available in literature, Eurybates family's members seemed to
be good candidates for having on their surfaces water/water ice or aqueous
altered materials. To improve our knowledge of the surface composition of this
peculiar family, we carried out an observational campaign at the Telescopio
Nazionale Galileo (TNG), obtaining near-infrared spectra of 7 members. Our data
show a surprisingly absence of any spectral feature referable to the presence
of water, ices or aqueous altered materials on the surface of the observed
objects. Models of the surface composition are attempted, evidencing that
amorphous carbon seems to dominate the surface composition of the observed
bodies and some amount of silicates (olivine) could be present.Comment: 23 pages, 2 figures, paper accepted for publication in Icaru
Electrosorption at metal surfaces from first principles
Electrosorption of solvated species at metal electrodes is a most fundamental class of processes in interfacial electrochemistry. Here, we use its sensitive dependence on the electric double layer to assess the performance of ab initio thermodynamics approaches increasingly used for the first-principles description of electrocatalysis. We show analytically that computational hydrogen electrode calculations at zero net-charge can be understood as a first-order approximation to a fully grand canonical approach. Notably, higher-order terms in the applied potential caused by the charging of the double layer include contributions from adsorbate-induced changes in the work function and in the interfacial capacitance. These contributions are essential to yield prominent electrochemical phenomena such as non-Nernstian shifts of electrosorption peaks and non-integer electrosorption valencies. We illustrate this by calculating peak shifts for H on Pt electrodes and electrosorption valencies of halide ions on Ag electrodes, obtaining qualitative agreement with experimental data already when considering only second order terms. The results demonstrate the agreement between classical electrochemistry concepts and a first-principles fully grand canonical description of electrified interfaces and shed new light on the widespread computational hydrogen electrode approach
Transition state method and Wannier functions
We propose a computational scheme for materials where standard Local Density
Approximation (LDA) fails to produce a satisfactory description of excitation
energies. The method uses Slater's "transition state" approximation and Wannier
functions basis set. We define a correction to LDA functional in such a way
that its variation produces one-electron energies for Wannier functions equal
to the energies obtained in "transition state" constrained LDA calculations. In
the result eigenvalues of the proposed functional could be interpreted as
excitation energies of the system under consideration. The method was applied
to MgO, Si, NiO and BaBiO and gave an improved agreement with experimental
data of energy gap values comparing with LDA.Comment: 13 pages, 6 figures, 1 tabl
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