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Francesc Illas - One of the best experts on this subject based on the ideXlab platform.
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first principles analysis of the stability and diffusion of oxygen vacancies in metal oxides
Physical Review Letters, 2004Co-Authors: Javier Carrasco, Núria López, Francesc IllasAbstract:Oxygen vacancies in metal oxides are known to determine their chemistry and physics. The properties of neutral oxygen vacancies in metal oxides of increasing complexity (MgO, CaO, � -Al2O3, and ZnO) have been studied using density functional theory. Vacancy formation energies, vacancy-vacancy interaction, and the barriers for vacancy migration are determined and rationalized in terms of the ionicity, the Madelung Potential, and lattice relaxation. It is found that the Madelung Potential controls the oxygen vacancy properties of highly ionic oxides whereas a more complex picture arises for covalent ZnO.
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Unexpected role of Madelung Potential in monoplanar high-Tc cuprate superconductors
Chemical Physics Letters, 2003Co-Authors: Francesc Illas, David Muñoz, Coen De Graaf, Ibério De P. R. MoreiraAbstract:Abstract For a series of monoplanar high- T c superconductor cuprates a quantitative linear relationship between T c and the Madelung Potential at the Cu site is found. The Madelung Potential is calculated using the Ewald summation procedure for a simple ionic model with the crystal structure as the only external input. This correlation opens the possibility to predict T c of new or modified existing monolayered superconducting cuprates from its crystal structure. The existence of such a linear relationship strongly suggests that the role of the whole crystal structure of the material is crucial and should be explicitly included in the models for high- T c superconductors.
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Importance of Madelung Potential in quantum chemical modeling of ionic surfaces
Journal of Computational Chemistry, 1997Co-Authors: Gianfranco Pacchioni, Anna Maria Ferrari, Antonio M. Márquez, Francesc IllasAbstract:The importance of the inclusion of the Madelung Potential in cluster models of ionic surfaces is the subject of this work. We have determined Hartree-Fock all electron wave functions for a series of MgO clusters with and without a large array of surrounding point charges (PC) chosen to reproduce the Madelung Potential. The phenomena investigated include: the reactivity of surface oxygens toward CO2, atomic hydrogen, and H+; the geometry and adsorption energy of water and the vibrational shift of CO adsorbed at Mg2+ sites; the electronic structure and the hyperfine coupling constants of oxygen vacancies, the paramagnetic Fs+ centers. While some clusters give results which are virtually independent of the presence of the PCs, other clusters, typically of small size, give physically incorrect results when the PCs are not included. The embedding of the cluster in PCs guarantees a similar response for clusters of different size, at variance with the bare clusters, where the long range coulombic interactions are not included. © 1997 by John Wiley & Sons, Inc.
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Effect of the Madelung Potential in the structure and bonding of metal-oxide systems: Cu on MgO(100)
Journal of Molecular Catalysis A: Chemical, 1997Co-Authors: Núria López, Francesc IllasAbstract:In this paper we use different cluster models to study the chemistry of the metal-oxide interface. These models permit us to show that the main differences between the CuMgO and CuOMg molecules and the Cu/MgO(100) system arise from the Madelung Potential. A quantitative description requires to explicitly consider the extended nature of the ions surrounding the active site although for different physical reasons. For Mg sites the inclusion of O 2- nearest neighbors permits an adequate representation of the frontier orbitals while for O 2- sites explicit consideration of nearest Mg 2+ cations avoids artificial charge density polarization produced by the point charges simulating the Madelung field. The present study shows that the Cu/MgO(100) interaction is weak, of intermolecular or Van der Waals origin. This implies a Volmer-Weber model for the growth of Cu on MgO(100).
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effect of the Madelung Potential value and symmetry on the adsorption properties of adsorbate oxide systems
Surface Science, 1996Co-Authors: Marcos Fernandezgarcia, José C. Conesa, Francesc IllasAbstract:Abstract An ab initio Hartree-Fock self-consistent field cluster model approach has been used to analyze the effect of surface irregularities located at long distances of NO and CO adsorbed on a monocoordinated position of the Cu2O(111) surface. To perform this study different surface cluster models were used. One of these clusters has C3v symmetry as in the perfect surface. In the two other cluster models the adsorption site local symmetry is still C3v but to simulate surface irregularities the array of point charges used to include the Madelung Potential has Cs symmetry. The NO Cu + interaction is always dominated by the electrostatic effects without important contributions from charge transfer mechanisms. The symmetry change between these two kinds of models does not practically alter the geometrical and vibrational parameters of the bond but affects the strength of the NO Cu 2 O interaction in a non-negligible way. On the other hand the CO Cu + interaction is much larger with substantial contributions from σ-donation and π-backdonation. In this case the symmetry change does not have noticeable effects. The present study shows that, on adsorbate/oxide systems with electrostatic bonds, the energy of interaction may depend not only on the local electrostatic field strength felt by the adsorbate but also on the degree of perfection of the oxide surface. The implications of this result for the analysis of experimental data are discussed.
Jiang Hong - One of the best experts on this subject based on the ideXlab platform.
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Theoretical Investigation of Ta2O5, TaON, and Ta3N5: Electronic Band Structures and Absolute Band Edges
JOURNAL OF PHYSICAL CHEMISTRY C, 2017Co-Authors: Cui Zhi-hao, Jiang HongAbstract:Early transition metal oxides, nitrides, and oxynitrides have attracted a great deal of interest because of their Potential applications in photovoltaics and photocatalysis. In this work, a systematic investigation is conducted of the electronic band structures of the Ta2O5 polymorphs,beta-Ta3N5 and beta-TaON, which are crucial for the understanding of their photocatalytic properties, based on state-of-the-art first-principles approaches. The calculated results imply that many-body perturbation theory in the GW approximation can overcome the severe underestimation of the band gap caused by standard density functional theory (DFT) in the local and semilocal approximations and provide a quantitative agreement with experiment. The effects of the electron-phonon coupling on the electronic band structure are considered by the Frolich model, and especially for epsilon-Ta2O5, a strong electron-phonon coupling is predicted as a result of small high- frequency dielectric constants and large effective masses. Based on an analysis in terms of the phenomenological ionic model, the band-gap difference between three compounds can be physically attributed to not only the well-known energy difference between the O 2p and N 2p orbitals, but also the influences of the Madelung Potential on the conduction-band-energy. By comparing the calculated absolute band edge positions to the redox Potentials for water reduction and oxidation, all three of the compounds are predicted to have Potential photocatalytic properties for unassisted water splitting. In addition, we also analyzed the stability and band gaps of different Ta2O5 polymorphs and found that the beta-Ta2O5, the phase commonly used in theoretical studies, is actually unstable and its unusually small band gap can be attributed to the strong overlap of neighboring atomic orbitals. On the other hand, epsilon-Ta2O5, which is much less well studied compared to beta-Ta2O5, leads to calculated properties that are much niore consistent with the experimental findings for Ta2O5 in general. The theoretical analysis and findings presented in this work have general implications for the understanding of the electronic band structures of other early transition metal compounds.National Natural Science Foundation of China [21373017, 21673005, 21321001]; Hui-Chun Chin and Tsung-Dao Lee Chinese Undergraduate Research Endowment (CURE)SCI(E)ARTICLE63241-325112
Gianfranco Pacchioni - One of the best experts on this subject based on the ideXlab platform.
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17o nmr as a measure of basicity of alkaline earth oxide surfaces a theoretical study
Journal of Chemical Physics, 2019Co-Authors: Farahnaz Maleki, Gianfranco PacchioniAbstract:The surface basicity of the alkaline-earth metal oxides has been investigated by studying the properties of 17O nuclear magnetic resonance (NMR). To this end, we performed density functional theory calculations and determined the 17O chemical shift and the quadrupolar coupling constants of the regular and stepped surfaces of MO (M = Mg, Ca, Sr, and Ba) oxides. The computed average chemical shift (δisoav) for 17O NMR of bulk MgO, CaO, SrO, and BaO is 46, 301, 394, and 636 ppm, respectively, in excellent agreement with the experiment. The 17O NMR chemical shifts correlate linearly with the Madelung Potential in the four oxides. Next, we considered the changes in the 17O chemical shift due to the adsorption of BR3 (R = F and OCH3) and pyrrole as probe molecules. We found that the 17O NMR signal of the O ion directly bound to the probe molecule shifts considerably compared to the clean surface. This is due to a change in the polarization of the O charge distribution due to the molecular adsorption. This change is the largest for BaO, with the strongest bond and the shortest surface-adsorbate distance, and the smallest for MgO, thus showing a direct correlation between 17O NMR and surface basicity. The 17O chemical shift of the basic site correlates linearly also with several properties of the adsorbed molecules, providing a direct measure of the surface basicity.
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Importance of Madelung Potential in quantum chemical modeling of ionic surfaces
Journal of Computational Chemistry, 1997Co-Authors: Gianfranco Pacchioni, Anna Maria Ferrari, Antonio M. Márquez, Francesc IllasAbstract:The importance of the inclusion of the Madelung Potential in cluster models of ionic surfaces is the subject of this work. We have determined Hartree-Fock all electron wave functions for a series of MgO clusters with and without a large array of surrounding point charges (PC) chosen to reproduce the Madelung Potential. The phenomena investigated include: the reactivity of surface oxygens toward CO2, atomic hydrogen, and H+; the geometry and adsorption energy of water and the vibrational shift of CO adsorbed at Mg2+ sites; the electronic structure and the hyperfine coupling constants of oxygen vacancies, the paramagnetic Fs+ centers. While some clusters give results which are virtually independent of the presence of the PCs, other clusters, typically of small size, give physically incorrect results when the PCs are not included. The embedding of the cluster in PCs guarantees a similar response for clusters of different size, at variance with the bare clusters, where the long range coulombic interactions are not included. © 1997 by John Wiley & Sons, Inc.
János G. Ángyán - One of the best experts on this subject based on the ideXlab platform.
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quantum mechanical molecular mechanical self consistent Madelung Potential method for treatment of polar molecular crystals
Journal of Computational Chemistry, 1998Co-Authors: Gyorgy G Ferenczy, Gabor I Csonka, Gabor Narayszabo, János G. ÁngyánAbstract:The self-consistent Madelung Potential SCMP approach for calculating molecular wave functions for a subunit embedded in a symmetrical environment constituted by the copies of the subunit is implemented with semiempirical NDDO model Hamiltonians and supplemented with empirically parameterized dispersion)repulsion interaction Potentials. This model yields sublimation enthalpies in good agreement with available experimental data for a series of molecular crystals, including imidazol, benzimidazole, urea, urethane, dicyaneamide, formamide, uracil, cytosine, maleic anhydride, succinic anhydride, and 1,3,5-triamino-2,4,6-trinitro-benzene. The SCMP-NDDO method, which avoids difficulties concerning the parametrization of charges in the molecular mechanics force fields, is proposed mainly for the treatment of molecular crystals with large unit cells. It might be particularly useful where ´
Jorn Schmedt Auf Der Gunne - One of the best experts on this subject based on the ideXlab platform.
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calculation of nmr parameters in ionic solids by an improved self consistent embedded cluster method
Physical Chemistry Chemical Physics, 2010Co-Authors: Johannes Weber, Jorn Schmedt Auf Der GunneAbstract:A new embedded cluster method (extended embedded ion method = EEIM) for the calculation of NMR properties in non-conducting crystals is presented. It is similar to the Embedded Ion Method (EIM) (ref. 1) in the way of embedding the quantum chemically treated part in an exact, self-consistent Madelung Potential, but requires no empirical parameters. The method is put in relation to already existing cluster models which are classified in a brief review. The influence of the cluster boundary and the cluster charge is investigated, which leads to a better understanding of deficiencies in EIM. A recipe for an improved semi-automated cluster setup is proposed which allows the treatment of crystals composed of highly charged ions and covalent networks. EIM and EEIM results for 19F and 31P shielding tensors in NaF and in four different magnesium phosphates are compared with experimental values from solid state MAS NMR, some of which are measured here for the first time. The quantum part of the clusters is treated at hybrid DFT level (mPW1PW) with atomic basis sets up to 6-311G(3df,3pd). The improved agreement of EEIM allows new signal assignments for the different P-sites in Mg2P4O12, α-Mg2P2O7 and MgP4O11. Conversion equations of the type σ = A + Bδ between calculated absolute magnetic shieldings σ and the corresponding experimental chemical shifts δ are obtained independently from linear regressions of plots of isotropically averaged σ versus δ values on 19 31P signals of small molecules.
