447 research outputs found
VO2: A Novel View from Band Theory
New calculations for vanadium dioxide, one of the most controversely
discussed materials for decades, reveal that band theory as based on density
functional theory is well capable of correctly describing the electronic and
magnetic properties of the metallic as well as both the insulating M1 and M2
phases. Considerable progress in the understanding of the physics of VO2 is
achieved by the use of the recently developed hybrid functionals, which include
part of the electron-electron interaction exactly and thereby improve on the
weaknesses of semilocal exchange functionals as provided by the local density
and generalized gradient approximations. Much better agreement with
photoemission data as compared to previous calculations is found and a
consistent description of the rutile-type early transition-metal dioxides is
achieved.Comment: 5 pages, 4 figure
Adsorption of organic molecules at the TiO2(110) surface: the effect of van der Waals interactions
Understanding the interaction of organic molecules with TiO2 surfaces is
important for a wide range of technological applications. While density
functional theory (DFT) calculations can provide valuable insight about these
interactions, traditional DFT approaches with local exchange-correlation
functionals suffer from a poor description of non-bonding van der Waals (vdW)
interactions. We examine here the contribution of vdW forces to the interaction
of small organic molecules (methane, methanol, formic acid and glycine) with
the TiO2 (110) surface, based on DFT calculations with the optB88-vdW
functional. The adsorption geometries and energies at different configurations
were also obtained in the standard generalized gradient approximation (GGA-PBE)
for comparison. We find that the optB88-vdW consistently gives shorter surface
adsorbate-to-surface distances and slightly stronger interactions than PBE for
the weak (physisorbed) modes of adsorption. In the case of strongly adsorbed
(chemisorbed) molecules both functionals give similar results for the
adsorption geometries, and also similar values of the relative energies between
different chemisorption modes for each molecule. In particular both functionals
predict that dissociative adsorption is more favourable than molecular
adsorption for methanol, formic acid and glycine, in general agreement with
experiment. The dissociation energies obtained from both functionals are also
very similar, indicating that vdW interactions do not affect the thermodynamics
of surface deprotonation. However, the optB88-vdW always predicts stronger
adsorption than PBE. The comparison of the methanol adsorption energies with
values obtained from a Redhead analysis of temperature programmed desorption
data suggests that optB88-vdW significantly overestimates the adsorption
strength, although we warn about the uncertainties involved in such
comparisons.Comment: 32 pages, 8 figures; accepted in Surface Scienc
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Substitutional and orientational disorder in organic crystals: a symmetry-adapted ensemble model
Modelling of disorder in organic crystals is highly desirable since it would allow thermodynamic stabilities and other disorder-sensitive properties to be estimated for such systems. Two disordered organic molecular systems are modeled using a symmetry-adapted ensemble approach, in which the disordered system is treated as an ensemble of the configurations of a supercell with respect to substitution of one disorder component for another. Computation time is kept manageable by performing calculations only on the symmetrically inequivalent configurations. Calculations are presented on a substitutionally disordered system, the dichloro/dibromobenzene solid solution, and on an orientationally disordered system, eniluracil, and the resultant free energies, disorder patterns, and system properties are discussed. The results are found to be in agreement with experiment following manual removal of physically implausible configurations from ensemble averages, highlighting the dangers of a completely automated approach to organic crystal thermodynamics which ignores the barriers to equilibration once the crystal has been formed
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The closed-edge structure of graphite and the effect of electrostatic charging
The properties of graphite, and of few-layer graphene, can be strongly influenced by the edge structure of the graphene planes, but there is still much that we do not understand about the geometry and stability of these edges. We present an experimental and theoretical study of the closed edges of graphite crystals, and of the effect of an electric field on their structure. High-resolution transmission electron microscopy is used to image the edge structure of fresh graphite and of graphite that has been exposed to an electric field, which experiences a separation of the graphene layers. Computer simulations based on density functional theory are used to rationalise and quantify the preference for the formation of multiple concentric loops at the edges. A model is also presented to explain how the application of an electric field leads to the separation of the folded edges
La âanomalĂa deambulatoriaâ en la antigua Iberia. âMonosĂĄndalosâ, hĂ©roes heridos y jĂłvenes danzantes
In this paper we analyze some elements of the Iberian iconography related to the general subject of the ambulatory anomaly, which are frequent in the Ancient Mediterranean. These themes are the monosandals, the injuries in the leg of the hero and dances of young people. In our opinion, these Iberian representations are related to ritual practices well documented in the Ancient Mediterranean. This relationship illustrates the high degree of contact between the Iberian and the Mediterranean World and the feasible existence of a common cultural character or koinĂ©.En el presente artĂculo se analizan diversos motivos presentes en la iconografĂa ibĂ©rica, relacionables con el tema mĂĄs general, muy presente en el MediterrĂĄneo antiguo, de la anomalĂa deambulatoria: el monosandalismo, las heridas en la pierna del hĂ©roe y los grupos de jĂłvenes danzantes. Se proponen hipĂłtesis para interpretar estas representaciones presentes en el mundo ibĂ©rico a travĂ©s de diversas prĂĄcticas mĂtico-rituales que encontramos bien documentadas en el MediterrĂĄneo antiguo. Esta relaciĂłn nos hablarĂa del profundo grado de identidad del mundo ibero con el resto de culturas mediterrĂĄneas y con una koinĂ© cultural comĂșn
Electronic structure of porphyrin-based metalâ organic frameworks and their suitability for solar fuel production photocatalysis
Metalâorganic frameworks (MOFs) can be exceptionally good catalytic materials thanks to the presence of
active metal centres and a porous structure that is advantageous for molecular adsorption and
confinement. We present here a first-principles investigation of the electronic structure of a family of
MOFs based on porphyrins connected through phenyl-carboxyl ligands and AlOH species, in order to
assess their suitability for the photocatalysis of fuel production reactions using sunlight. We consider
structures with protonated porphyrins and those with the protons exchanged with late 3d metal cations
(Fe2+, Co2+, Ni2+, Cu2+, Zn2+), a process that we find to be thermodynamically favorable from aqueous
solution for all these metals. Our band structure calculations, based on an accurate screened hybrid
functional, reveal that the bandgaps are in a favorable range (2.0 to 2.6 eV) for efficient adsorption of
solar light. Furthermore, by approximating the vacuum level to the pore centre potential, we provide the
alignment of the MOFs' band edges with the redox potentials for water splitting and carbon dioxide
reduction, and show that the structures studied here have band edges positions suitable for these
reactions at neutral pH.Royal Society for an International Exchange Scheme grantVia our membership of the UK's HPC Materials Chemistry Consortium, which is funded by EPSRC (EP/L000202), this work made use of the facilities of ARCHER, the UK's national high-performance computing services, which are funded by the Office of Science and Technology through EPSRC's High End Computing ProgrammeEuropean Research Council through an ERC Starting Grant (ERC2011-StG-279520-RASPA)MINECO (CTQ2013-48396-P)AndalucĂa Region (FQM-1851)University of Granad
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Cation distribution and mixing thermodynamics in Fe/Ni thiospinels
The structural analogy between Ni-doped greigite minerals (Fe3S4) and the (Fe, Ni)S clusters present in biological enzymes has led to suggestions that these minerals could have acted as catalysts for the origin of life. However, little is known about the distribution and stability of Ni dopants in the greigite structure. We present here a theoretical investigation of mixed thiospinels (Fe1âxNix)3S4, using a combination of density functional theory (DFT) calculations and Monte Carlo simulations. We find that the equilibrium distribution of the cations deviates significantly from a random distribution: at low Ni concentrations, Ni dopants are preferably located in octahedral sites, while at higher Ni concentrations the tetrahedral sites become much more favourable. The thermodynamic mixing behaviour between greigite and polydymite (Ni3S4) is dominated by the stability field of violarite (FeNi2S4), for which the mixing enthalpy exhibits a deep negative minimum. The analysis of the free energy of mixing shows that Ni doping of greigite is very unstable with respect to the formation of a separate violarite phase. The calculated variation of the cubic cell parameter with composition is found to be non-linear, exhibiting significant deviation from Vegardâs law, but in agreement with experiment
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Double-well potential energy surface in the interaction between h-BN and Ni(111)
Density functional theory calculations with non-local correlation functionals, properly accounting for dispersion forces, predict the presence of two minima in the interaction energy between h-BN and Ni(111). These can be described as a physisorbed state with no corrugation of the h-BN structure, and a chemisorbed state exhibiting noticeable corrugation and shorter distance of h-BN to the metallic support. The latter corresponds indeed to the one reported in most experiments. The relative stability of the two minima depends on the specific density functional employed: of those investigated here only the optB86b-vdW yields the correct order of stability. We also demonstrate that the effect of the metal support on the Raman frequency of the chemisorbed boron nitride monolayer cannot be reduced to the associated strain. This is important because the Raman frequency has been proposed as a signature to identify h-BN monolayers from multilayered samples. Our analysis shows that such signatures would be strongly dependent on the nature of the support â h-BN interaction
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