Cluster Model Calculations Research Articles

Electronic Origin of Giant Magnetic Anisotropy in MultiferroicLuFe2O4

We investigated the orbital anisotropy of LuFe(2)O(4) using the Fe L(2,3)- and O K-edge x-ray absorption spectroscopy (XAS) and cluster model calculations. X-ray magnetic circular dichroism reveals a surprisingly large orbital magnetic moment (m(o) approximately 0.8 micro(B)/f.u.), which originates the giant magnetic anisotropy. The polarization dependent XAS enables us to identify the orbital states and occupations, different from the band calculation predictions. These findings were examined by using the cluster model analysis, which also explains the orbital magnetic moment as well as the total moment (2.9 micro(B)/f.u.). Taking into account the charge order, we also determined the spin structure.

Quantum Chemical Investigation of Cluster Models for TiO2 Nanoparticles with Water-Derived Ligand Passivation: Studies of Excess Electron States and Implications for Charge Transport in the Gratzel Cell

We present hybrid DFT calculations for large TiO2 cluster models in the gas phase and in solution. Two clusters are investigated, one derived from the anatase bulk structure and the second from rutile. The surfaces are passivated with hydroxyl and water ligands, and continuum solvation is used to model bulk solvent in a subset of calculations. The geometrically optimized bonding patterns, structures, and electronic properties are similar in the two clusters. The distinction between anatase and rutile is minor at this small size. The HOMO and LUMO of the clusters are delocalized, and qualitatively resemble those observed in bulk for both the anatase and rutile derived species. When an additional electron is added, the wave function is again delocalized and there is little change in geometry, and hence minimal polaronic self-trapping. Removal of a surface ligand, creating a defect in that location, does lead to localization of the wave function, but it is unclear whether this actually occurs in real nanocrystalline TiO2 systems. Our results suggest that modeling of electron transport in TiO2 nanocrystal photovoltaic cells may require the presence of electrolyte ions to stabilize localized trapping states.

Silylene Defect at the Dihydrogen Terminated (100) Si Surface

Density functional calculations for both periodic slabs and different size cluster models of the hydrogen-terminated (100) surface of silicon are used to study a new configuration, formed by a silylene center interacting with vicinal silicon dihydrides through nonconventional hydrogen bonds. A comparison between slab-model and cluster-model approaches to modeling surface silylene defect formation processes is presented. The cluster models are used to analyze the structure and bonding of the silylene with a Lewis acid and base, showing the Zwitterionic nature of the defect. The silylene is also demonstrated to behave as a strong Brønsted acid. The stabilization of the silylene defect via interaction with species unavoidably present in the HF(aq)-etching solution is investigated. Finally, the negative chemical shift observed by X-ray photoelectron spectroscopy in the HF(aq)-etched (100) Si surface is attributed to the occurrence of silylene defect.

Hydrogen-Bonding-Assisted Self-Doping in Tetrathiafulvalene (TTF) Conductor

The synthesis, characterization, and carrier generation mechanism of self-doping in a tetrathiafulvalene (TTF) conductor, ammonium tetrathiafulvalene-2-carboxylate (TTFCOO(-)NH(4)(+)), are described together with molecular orbital characteristics. Insulating TTFCOOH changes into a hole-doped conductor TTFCOO(-)NH(4)(+) with a conductivity of sigma = 2.0 x 10(-4) S/cm (300 K), upon salt formation with NH(3). A radical species, TTF(*+)COO(-)NH(4)(+), is generated via protonation of the TTF moiety as demonstrated by UV-vis, ESR, and (1)H NMR spectra. The X-ray crystallographic structure of TTFCOO(-)NH(4)(+) reveals supramolecular arrays of TTFCOO(-) moieties with short S...S contact, assisted by the one-dimensional hydrogen-bonding network composed of the ammonium and carboxylate ions. Molecular orbital calculations of cluster models show that the singly occupied molecular orbital (SOMO) of TTF(*+)COO(-)NH(4)(+) in the supramolecular array is not at the highest energy level, which is characterized as a quasi-closed-shell state. The ab initio periodic calculation with a one-dimensional boundary condition reveals that TTF(*+)COO(-)NH(4)(+) behaves as a dopant leading to the semiconducting behavior of the stacked TTF moieties assembled by the hydrogen-bonding network. Namely, TTFCOO(-)NH(4)(+) can be described as a "hydrogen-bonding-assisted self-doped conductor". The contribution of the hydrogen-bonding interaction to the electron conduction is experimentally supported by a large isotope effect in the ac conductivity of TTFCOO(-)NH(4)(+) at low temperature.

STAU-CATALYZED BIG-BANG NUCLEOSYNTHESIS AND NUCLEAR CLUSTER MODEL

Three-body cluster-model calculations are performed for the new types of big-bang nucleosynthesis (BBN) reactions that are calalyzed by a supersymmetric (SUSY) particle stau, a scalar partner of the tau lepton. If a stau has a lifetime ≳ 103s, it would capture a light element previously synthesized in standard BBN and form a Coulombic bound state. The bound state, an exotic atom, is expected to induce various reactions, such as (αX-) + d → 6 Li + X-, in which a negatively charged stau (denoted as X-) works as a catalyzer. Recent literature papers have claimed that some of these stau-catalyzed reactions have significantly large cross sections so that inclusion of the reactions into the BBN network calculation can change drastically abundances of some elements, giving not only a solution to the 6 Li -7 Li problem (calculated underproduction of 6 Li by ~ 1000 times and overproduction of 7 Li +7 Be by ~ 3 times) but also a constraint on the lifetime and the primordial abundance of the elementary particle stau. However, most of these literature calculations of the reaction cross sections were made assuming too naive models or approximations that are unsuitable for those complicated low-energy nuclear reactions. We use a few-body calculational method developed by the authors, and provides precise cross sections and rates of the stau-catalyzed BBN reactions for the use in the BBN network calculation.

Electronic Structure of Nd2-xCexCuO4 and La2-xSrxCuO4 Deduced from Cu 2 p Photoemission and Photoabsorption

The electronic structures of Nd 2- x Ce x CuO 4 and La 2- x Sr x CuO 4 are studied using numerical diagonalization calculations for cluster models, Cu 5 O 16 , Cu 7 O 20 , and Cu 8 O 16 . The Fermi energy of Nd 2 CuO 4 used in the experiment is assumed to be pinned at the bottom of the upper Hubbard band, while that of La 2 CuO 4 is assumed to be pinned at the top of the Zhang–Rice singlet band. This assumption enables us to consistently understand the doping dependence of the Cu 2 p photoemission and photoabsorption spectra in Nd 2- x Ce x CuO 4 and La 2- x Sr x CuO 4 on the basis of the so-called “crossing the gap” picture. The present calculation of Cu 2 p photoemission spectrum also shows that core hole screening caused by charge transfer between neighboring CuO 4 plaquettes, in other words, nonlocal screening (NLS), is important in Nd 2 CuO 4 as well as in La 2 CuO 4 .

Cluster model calculations for charge states of a silicon vacancy

To investigate the charge states of a silicon vacancy, we introduce a cluster model which includes both the Coulomb interaction U between electrons in the dangling bond and the coupling g between the electrons and Jahn-Teller phonons, and solve the model by using the numerical diagonalization method. It is found that, for U > 0 and g = 0, the ground state of the neutral charge state V0 is spin singlet (S = 0) and orbital doublet. When g is varied for a finite U, the ground state changes to the orbital triplet state with S = 1 at an intermediate coupling gc1, and finally changes to the orbital triplet state with S = 0 at a strong coupling gc2. The obtained result for g > gc2 is consistent with the low temperature elastic softening observed in non-doped crystalline silicon.

All-electron CI calculations of 3d transition-metalL2,3 XANES using zeroth-order regular approximation for relativistic effects

X-ray-absorption near-edge structures (XANES) at 3d transition-metal (TM)L2,3 edges are computed using the all-electron configuration interaction (CI) method. Slaterdeterminants for the CI calculations are composed of molecular orbitals obtained bydensity functional theory (DFT) calculations of model clusters. Relativistic effects aretaken into account by the zeroth-order regular approximation (ZORA) usingtwo-component wavefunctions. The theoretical spectra are found to be strongly dependenton the quality of the one-electron basis functions. On the other hand, a different choice ofthe exchange–correlation functionals for the DFT calculations does not exhibit visiblechanges in the spectral shape. Fine details of multiplet structures in the experimental TML2,3 XANES of MnO, FeO and CoO are well reproduced by the present calculations when theone-electron basis functions are properly selected. This is consistent with ourprevious report showing good agreement between theoretical and experimental TML2,3 XANES when four-component relativistic wavefunctions were used.

Real-time icosahedral to fcc structure transition during CoPt nanoparticles formation

Nucleation and growth of supported CoPt nanoparticles were studied in situ and in real time by combined grazing incidence small-angle x-ray scattering (GISAXS) and grazing incidence x-ray diffraction (GIXD). GISAXS provides morphological features of nanoparticles as a function of size, shape and correlation distance between particles, while GIXD allows the determination of the atomic structure. We focus on the formation of ultrasmall CoPt nanoparticles, in the 1–4 nm size range at 500○C. The structural analysis method based on the Debye equation is coupled with cluster model calculations performed by Monte Carlo simulations using a semi-empirical tight-binding potential to interpret diffraction spectra and structural transitions. Our results show that the cluster structure evolution during the growth is size-dependent and composition-dependent, yielding an icosahedral to fcc structure transition.

Investigation of local symmetry effects on the electronic structure of manganites: HexagonalYMnO3versus orthorhombicLaMnO3

The local site symmetry effects on electronic structure of manganites were investigated by using theoretical analyses in terms of the cluster model calculations for the $\text{O}\text{ }K$- and $\text{Mn}\text{ }{L}_{2,3}$-edge x-ray-absorption spectroscopy (XAS) results of hexagonal ${\text{YMnO}}_{3}$ and orthorhombic ${\text{LaMnO}}_{3}$. ${\text{YMnO}}_{3}$, which has an unusual ${\text{MnO}}_{5}$ $({D}_{3h})$ site symmetry, exhibits remarkably different electronic structure and XAS line shape from those of ${\text{LaMnO}}_{3}$ with the conventional ${\text{MnO}}_{6}$ $({D}_{4h})$ site symmetry, despite the same ${\text{Mn}}^{3+}$ $(3{d}^{4})$ trivalent ionic state. We developed an algebraic formalism, which takes into account the crystal-field energies and hybridization strengths of the ${D}_{3h}$ $\text{Mn}\text{ }3d$ orbital in ${\text{YMnO}}_{3}$. The deduced values are very consistent with the first-principles local-density approximation plus Hubbard U calculation results. The cluster model calculations involving the different site symmetries well reproduce the $\text{O}\text{ }K$-edge and $\text{Mn}\text{ }{L}_{2,3}$-edge spectra of both manganites. In the analyses of the $\text{O}\text{ }K$-edge spectra, we also took into account the $\text{Y}\text{ }4d$ and $\text{La}\text{ }4f/5d$ orbital states.

Theoretical calculations of effective exchange integrals by spin projected and unprojected broken-symmetry methods II: Cluster models of Jahn–Teller distorted K 2CuF 4 solid

In the strong correlated perovskite-type copper fluoride of K 2CuF 4, the degree of freedom in d orbitals of copper cation should be considered to understand the magnetism. We performed the cluster model calculations under consideration of octahedral fluorine anions around copper cation, in order to investigate the magnetism. At the amplitude of Jahn–Teller distortion, the alternate d z 2 - x 2 type orbital ordering and the experimental effective exchange integral ( J ab ) value were reproduced. The calculated potential energy curves were discussed, from the viewpoint of the theoretical total energy in the Jahn–Teller distorted structure proposed by Kanamori.

The reactivity of CO2 with K atoms adsorbed on MgO powders

In this combined quantum chemical and EPR study we have investigated the formation of CO(2)(-) radicals by contact of CO(2) molecules with a K precovered MgO surface. K atoms have been deposited on polycrystalline MgO samples, and then exposed to CO(2). The typical EPR signal of the isolated K atoms disappears when the reaction with CO(2) takes place and a new paramagnetic species attributed to CO(2)(-) is observed. DFT cluster model calculations show that there is a spontaneous electron transfer from the adsorbed K atom to the CO(2) molecule, with formation of K(+)CO(2)(-) surface complexes. These species have the same electronic characteristics and spin distribution of gas-phase M(+)CO(2)(-) (M = Li, Na, K) molecules, but are stabilized by the presence of the ionic surface. The most stable MgO sites where the adsorption of CO(2) occurs and the computed EPR properties are discussed.

Formation of CO2− Radical Anions from CO2 Adsorption on an Electron-Rich MgO Surface: A Combined ab Initio and Pulse EPR Study

We report the results of DFT cluster model calculations on the formation of carboxylate radical anions, CO2−, by reaction of CO2 with an electron rich MgO surface. The theoretical results are used in conjunction with new pulse electron paramagnetic resonance (EPR) experiments to interpret recently reported experimental data obtained with continuous wave EPR (CW-EPR) (Chiesa, M.; Giamello, E. Chem. Eur. J. 2007, 13, 1261). Three cases have been considered: (1) interaction of CO2 with oxide anions at low-coordinated sites and formation of surface carbonates, CO32−; (2) interaction of CO2 with “free” electrons trapped at low-coordinated sites of the MgO surface with formation of adsorbed CO2−; and (3) interaction of CO2 with (H+)(e−) centers and formation of CO2− species bound near an adsorbed proton (OH group). CO2 prefers to form surface carbonates and only once most or all the low-coordinated O sites of the surface have reacted it will interact with trapped electrons to form CO2−. Several adsorption sites...

Theoretical study of different speciation of mercury adsorption on CaO (0 0 1) surface

Adsorption of mercury and mercury chloride on the surface of CaO surface has been investigated by means of density functional theory (DFT) cluster model calculations. Different embedding clusters (shell models, point charges cluster and bare cluster) have been compared. The calculation functions and basis used for mercury were optimized to provide the reliable fundamental understanding of gas–solid adsorption system. A small binding energy of −21.47 kJ/mol was found when the elemental mercury molecule is coordinated to the O 2− anion, which inferred mercury was weakly physisorbed at the surface of the CaO. HgCl 2 adsorption is more complex. When the mercury chloride molecular axis was parallel to the surface, the Hg atom oriented to the O 2− anion and had a large binding energy, which meant that mercury chloride adsorbed on surface of CaO is mainly chemisorption. The present calculations indicated that CaO injection could substantially reducing gaseous mercury chloride, but have no apparent effect on the mercury, which was compatible with the available experimental results. Besides obtaining structures and energies, the temperature effect on adsorption were also studied. The results suggested that the high temperature has adverse effect on the adsorption when the temperature is above 280 °C because of the nature of adsorption.

Effect of surface site on the spin state of first-row transition metals adsorbed on MgO: Embedded cluster model and hybrid density functional theory calculations

The interaction of first-row transition-metal atoms with low-coordinated oxygen atoms and oxygen vacancies of the MgO surface at low coverage has been studied systematically using an embedded-cluster model approach and hybrid density functional theory calculations. It was found that the interaction with these defects is much stronger than with the regular sites but in general insufficient to change the number of unpaired electrons in the free metal atom. Nevertheless, the larger interaction at these sites reduces the energy required to switch from high spin to low spin. These findings are in agreement with previous work on the adsorption of transition-metal atoms on regular anionic sites of the MgO(001) surface. Our results show that the spin state of adsorbed metal atoms on oxide supports needs to be explicitly taken into account.

Photoemission evidence for a Mott-Hubbard metal-insulator transition inVO2

The temperature $(T)$-dependent metal-insulator transition (MIT) in ${\text{VO}}_{2}$ is investigated using bulk sensitive hard-x-ray $(\ensuremath{\sim}8\text{ }\text{keV})$ valence-band, core-level, and $\text{V}\text{ }2p\ensuremath{-}3d$ resonant photoemission spectroscopies (PESs). The valence-band and core-level spectra are compared with full-multiplet cluster model calculations including a coherent screening channel. Across the MIT, $\text{V}\text{ }3d$ spectral weight transfer from the coherent ($3{d}^{1}\underset{̱}{C}$ final) states at Fermi level to the incoherent ($3{d}^{0}+3{d}^{1}\underset{̱}{L}$ final) states, corresponding to the lower Hubbard band, leads to gap formation. The spectral shape changes in $\text{V}\text{ }1s$ and $\text{V}\text{ }2p$ core levels as well as the valence band are nicely reproduced from cluster model calculations, providing electronic structure parameters. Resonant PES finds that the $3{d}^{1}\underset{̱}{L}$ states resonate across the $\text{V}\text{ }2p\ensuremath{-}3d$ threshold in addition to the $3{d}^{0}$ and $3{d}^{1}\underset{̱}{C}$ states. The results support a Mott-Hubbard transition picture for the first-order MIT in ${\text{VO}}_{2}$.

Evolution of the spectral weight in the Mott-Hubbard seriesSrVO3-CaVO3-LaVO3-YVO3

We studied the Mott-Hubbard series ${\text{SrVO}}_{3}{\text{-CaVO}}_{3}{\text{-LaVO}}_{3}{\text{-YVO}}_{3}$ with high-energy photoemission. The features in the experimental spectra were interpreted using cluster model calculations. The valence-band photoemission results show very interesting trends across the Mott-Hubbard series: (i) From ${\text{SrVO}}_{3}$ to ${\text{CaVO}}_{3}$, the spectral weight is transferred from the coherent to the incoherent feature. (ii) From ${\text{CaVO}}_{3}$ to ${\text{LaVO}}_{3}$, the coherent structure disappears, opening the insulating band gap. (iii) Finally, from ${\text{LaVO}}_{3}$ to ${\text{YVO}}_{3}$, the bandwidth of the remaining incoherent feature further decreases. There is also a considerable $\text{V}\text{ }3d$ spectral weight contribution mixed in the $\text{O}\text{ }2p$ band region in all cases. These results suggest that the $\text{O}\text{ }2p$ states play an important role in the Mott-Hubbard transition. Some of the changes in the spectra are unexpected and cannot be explained by the current Mott-Hubbard theories. The calculation reproduces not only the coherent and incoherent structures in the $\text{V}\text{ }3d$ band but also the main features in the $\text{O}\text{ }2p$ band. In addition, the same model explains the charge-transfer satellites observed in the $\text{V}\text{ }2p$ core-level spectra.

Boron‐doped diamond: Investigation of the stability of surface‐doping versus bulk‐doping using cyclic cluster model calculations

Boron-doped bulk diamond and the boron-doped hydrogen terminated (001) surface of diamond were investigated using the cyclic cluster model. Structure and stability of the hydrogen-terminated (001) surface were calculated and compared with experimental and other theoretical results from the literature. Boron-doping was modeled by substitution of a carbon atom by a boron atom in different positions with increasing distance from the surface up to boron-doped bulk diamond. In agreement with experiments on nanoclusters, boron is most stable in the first surface layers. (c) 2008 Wiley Periodicals, Inc. J Comput Chem, 2008.

Electronic structure of BiSeI clusters

The valence band (VB) energy levels of BiSeI crystals have been calculated and compared to the X-ray photoelectron spectra of SbSI crystals. A molecular cluster consisting of 20 molecules of BiSeI has been used to calculate the total VB density of states by the density functional theory (DFT) method. The spectrum of total VB density of states of the (BiSeI) 20 cluster weighted by atomic photoemission cross-sections has been compared with the experimental X-ray photoelectron VB spectrum of SbSI type crystals. The cluster model calculation has showed that the core level energy splitting depends on the difference of ionic charges of the same atoms at the edges of the (BiSeI) 20 cluster.

Cluster model and band structure calculations ofV2O5: ReducedV5+symmetry and many-body effects

We studied the electronic structure of the ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ compound using cluster model calculations. The calculation included the reduced ${\mathrm{V}}^{5+}$ symmetry and the strong $\mathrm{V}\phantom{\rule{0.2em}{0ex}}3d--\mathrm{O}\phantom{\rule{0.2em}{0ex}}2p$ covalence effects. The many-body effects are taken into account using the configuration interaction method. The ground state of ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ is highly covalent and dominated by the $3{d}^{1}\underset{}{L}$ $(^{1}A_{1})$ configuration, where $\underset{}{L}$ denotes a ligand hole. The ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ material is in the charge-transfer regime and the band gap is due to $p\text{\ensuremath{-}}d$ transitions. The first removal state is given by the $3{d}^{0}\underset{}{L}$ $(^{2}A_{1})$ configuration, and the first addition state is formed by the $3{d}^{1}$ $(^{2}E)$ configuration. The results of the cluster model are in good agreement with band structure calculations. The calculation results are also in good agreement with photoemission and x-ray absorption spectra. The $\mathrm{V}\phantom{\rule{0.2em}{0ex}}2p$ core-level spectra exhibit many-body effects despite the nominal $3{d}^{0}$ occupancy. These effects and the reduced ${\mathrm{V}}^{5+}$ symmetry are crucial to describe the electronic structure of ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$.