Crystal Orbital Overlap Population Research Articles

Covalent bonding andbandgap formation in transition-metal aluminides: di-aluminidesof group VIII transition metals

In this paper we study the electronic structure, electrondensity distribution and bonding mechanism in transition-metal(TM) di-aluminides Al2TM formed by metals of group VIII (TM = Fe,Ru,Os) and crystal structures of TM di-silicidesC11b (MoSi2), C40 (CrSi2) and C54 (TiSi2). Apeculiar feature of the electronic structure of these TMdi-aluminides is the existence of a semiconducting gap at theFermi level. A substitution of a 3d TM by 4d or 5dmetal enhances the width of the gap. From the analysis of thecharge-density distribution and the crystal-orbitaloverlap population we conclude that the bonding between atomshas strong covalent character. This is confirmed not only fromthe enhanced charge density halfway between atoms, but also by aclear bonding-antibonding splitting of the electronic states.Groups of bonding and antibonding states corresponding to aparticular bonding configuration of atoms are separated by agap. As such a gap is observed in all bonding configurationsamong atoms in the unit cell it results in a gap in the totaldensity of states. The bandgap exists at a certain electron peratom ratio e/A≈4.67 and also occurs in TMdi-aluminides of groups VII and IX. For group VIII TMdi-aluminides the Fermi level falls just in the gap.

Electronic Properties and Chemical Bonding of Orthorhombic Chromium Carbide

The electronic properties of an orthorhombic phase of chromium carbide are calculated by the extended Huckel tight-binding method. We found in this phase similar trends in band structure and total and p-d orbital projected DOS with respect to other crystalline phases calculated previously. The bonding nature is analyzed in terms of the crystal orbital overlap population (COOP). The results show a high degree of metal-non-metal hybridized states contributing to the metallic nature of bonding.

The Ternary Compounds Pd13ln5.25Sb3.75 and Pdln1.26Sb0.74: Crystal Structure and Electronic Structure Calculations

The new ternary compound Pd13In5.25Sb3.75 was found. Its crystal structure was determined using a CCD diffractometer at room temperature. Evaluations and refinements finally yielded a C-centered monoclinic structure (space group, C2/c; Pearson symbol, mC88, Z=4) with a=15.189(2) Å, b=8.799(1) Å, c=13.602(2) Å, and β=123.83(1)°. For the entire data set of 3706 independent reflections residual values are R=0.0461 and Rw=0.0789. The structure was found to be isotypic to Pd13Pb9 with In and Sb on the Pb sites. The existence of a further ternary compound, which was already described as Pd3In4Sb2, could be confirmed. Its composition range was determined by EPMA to be PdIn1.2–1.3Sb0.8–0.7. It does not melt congruently and we were not able to find suitable single crystals. However, we were able to prepare the pure ternary compound in order to perform X-ray powder diffraction using a Guinier image plate technique. The entire diffraction spectrum was refined by full profile Rietveld method using the program Fullprof. The α-PdSn2 structure type (space group, I41/acd; Pearson symbol, t148, Z=16), proposed for this compound, was confirmed and the lattice parameters are a=6.4350(1) Å and c=24.3638(3) Å. The residual values were Rp=5.34 and Rwp=6.70. The tetragonal PdSn2 structure type is a mixed variant of the CaF2 type and the CuAl2 type structure. Also in this ternary compound we assumed a random contribution of In and Sb over the 16e and 16f positions. The electronic structures of both compounds were investigated by extended Hückel calculations. Crystal orbital overlap populations show extended bonding interactions between the main group elements. The bonding interactions of the main group elements are almost optimized at the experimentally observed In/Sb ratio of the ternary compound. The In/Sb ratio in Pd13In5.25Sb3.75 can thus be rationalized on the basis of the electronic structure.

Local electronic states in the topmost surface layer probed by metastable atom electron spectroscopy: N2adsorbed and condensed onNi(111)

Electron emission spectra obtained by thermal collisions of ${\mathrm{He}}^{*}{(2}^{3}S)$ metastable atoms with ${\mathrm{N}}_{2}$ molecules on a Ni(111) surface in the chemisorbed, physisorbed, and condensed phases were measured together with the gas-phase spectrum. In the gas-phase collision where the metastable atoms approach randomly oriented ${\mathrm{N}}_{2}$ molecules, Penning ionization rates for the $3{\ensuremath{\sigma}}_{g}$ and $2{\ensuremath{\sigma}}_{u}$ states are almost identical reflecting their spatial extent, while that for the $1{\ensuremath{\pi}}_{u}$ state is rather small. In the saturated overlayer at \ensuremath{\sim}50 K where the ${\mathrm{N}}_{2}$ molecules are chemisorbed with the molecular axes perpendicular to the surface, the $3{\ensuremath{\sigma}}_{g}$-derived state is drastically enhanced relative to the $2{\ensuremath{\sigma}}_{u}$-derived state. This indicates that, upon chemisorption, two \ensuremath{\sigma} states are strongly modified in space by mixing with each other to yield a strong charge localization, i.e., an outer N atom in the $3{\ensuremath{\sigma}}_{g}$-derived state gains considerable charge whereas an outer N atom in the $2{\ensuremath{\sigma}}_{u}$-derived state loses it. The orbital mixing was confirmed by crystal orbital overlap population obtained using density functional theory within a generalized gradient approximation. Our spectra at \ensuremath{\sim}20 K show that the physisorbed ${\mathrm{N}}_{2}$ molecules are located on the chemisorbed species with a parallel orientation in the monolayer region and then condense to form a multilayer. We also found a characteristic band shift and narrowing with increasing layer thickness and these findings are discussed in terms of two final-state relaxation effects.

The electronic structure and bonding of H pairs at Σ=5 BCC Fe grain boundary

The H–Fe interaction at a grain boundary (GB) in BCC Fe was studied using qualitative electronic structure calculations in the framework of the atom superposition and electron delocalization molecular orbital (ASED-MO) theory. Calculations were performed using an Fe 196 cluster to simulate the 36.9° [1 0 0] {0 1 3} symmetrical tilt GB structure. The most stable positions for one H atom and two H atoms at the GB core were determined. The total energy of the cluster decreases when the H atoms are at that location, making it a possible site for H accumulation. An analysis of the orbital interaction reveals that H–Fe bonding involves mainly the Fe 4s and H 1s orbitals. In general, H drain charge from the first neighbor Fe atoms. The crystal orbital overlap population (COOP) curves gives a measure of Fe–Fe bond weakening due to H segregate at GB. Some Fe–Fe bonds in the GB plane shows a 60% decrease in the overlap population when H is present. H–H interaction was also analyzed. Although some H–H association is reveled no bond is formed between the impurity atoms.

Electronic structures of gold nanowires

The electronic properties of the 7-1 structure of a gold nanowire, which is the simplest structure among possible helical multi-shell nanowires, are examined. Density of states (DOS) and crystal orbital overlap population (COOP) analyzes at the extended Hückel level of theory show that the contribution of the inner gold-atom row to the electric conduction and structural strength is small. The outer tube is responsible for both the structural stability and the electric conduction. The gold atoms of the nanowire are charged when the nanowire has an inner row. There is a wavy negative charge distribution in the outer tube when the outer tube has a helical structure.

Investigation of the bonding and magnetic properties of U3Cu4Ge4 and structurally related systems

Electronic and magnetic properties of the ferromagnets UCu2Ge2 (122), UGe2 (122), and of the newly found U3Cu4Ge4 (344) intermetallic system are self-consistently calculated within the local spin density functional theory using the augmented spherical wave (ASW) method. The influence of hybridisation on the chemical bonding and magnetic behaviour is discussed from the densities of states (DOS) as well as from the crystal orbital overlap population (COOP) leading to suggest comparative influences between electronic effects of the hybridisation as with respect to the structural changes when the constituents of (122) and (12) are built within U3Cu4Ge4. From the spin-only magnetic results the important contribution of the orbital effects is analysed.

Spatial electron distribution of CO adsorbed on Ni(100) and Ni(111) surfaces probed by metastable impact electron spectroscopy

Electron emission spectra obtained by thermal collisions of He*(2 3S) metastable atoms with CO on Ni(100) in the c(2×2) structure and on Ni(111) in the c(4×2) structure were measured to probe directly the spatial electron distribution. For a systematic comparison, the metastable spectra of free CO, condensed CO on Ni(111), and gaseous Cr(CO)6 were also measured under the same beam conditions. Our data showed that the relative ionization cross sections for the CO 4σ-, 1π-, and 5σ-derived states depend drastically on the molecular orientation of CO with respect to the metastable beam, reflecting the local electron density of CO in the impact region. Moreover, it was found that the 4σ- and 5σ- derived states of CO at hollow sites on Ni(111) are strongly modified in space by mixing with each other, where considerable charge transfer occurs from the C site to the O site in the 5σ-derived state and in the opposite way in the 4σ-derived state. In contrast, such a strong charge redistribution was not seen in the cases of terminal CO on Ni(100) and Cr(CO)6. These findings were in good accordance with the crystal orbital overlap population obtained by density functional theory through a generalized gradient approximation.

Local Electron Distribution of CO Adsorbed on Ni(100) and Ni(111) Surfaces Studied by Metastable Atom Electron Spectroscopy.

Electron emission spectra caused by thermal collisions of He* (23S) metastable atoms with Ni(100)c(2×2)-CO and Ni(111)c(4×2)-CO were measured to probe the local electron distribution. Our data showed that the 4σ- and 5σ-derived states of CO at hollow sites on Ni(111) are strongly modified in space by mixing with each other, where considerable charge transfer occurs from the C atom to the O atom in the 5σ-derived state and in the opposite way in the 4σ-derived state. In contrast, such a heavy charge redistribution was not seen in the case of terminal CO on Ni(100). These findings were in good accordance with the crystal orbital overlap population obtained by density functional theory within the generalized gradient approximation.

Crystal Orbital Overlap Population Analysis of the Capacity Fading of Metal‐Substituted Spinel Lithium Manganate LiMn2 O 4

The capacity fading with charge‐discharge cycles, which is a problem in the practical use of spinel lithium manganate for the positive electrode material of a lithium ion rechargeable battery, is analyzed using extended Hückel band calculations. The strength of the manganese‐oxygen bond is characterized by crystal orbital overlap population (COOP) analyses. The manganese atom in lithium manganate is substituted with first‐row transition metal atoms, and the COOP is calculated and analyzed. The COOP of substituted lithium manganate is compared with the experimental data for the capacity performance in charge‐discharge cycles. Both showed very good agreement. We understand by examining the Mn‐O COOP that the suppression of capacity fading with manganese substitution in lithium manganate by Fe, Co, and Ni has relevance to an increase in the strength of the Mn‒O bond. COOP is a good index for the capacity fading with charge‐discharge cycles for the substituted lithium manganate. It is applicable to the search and examination for the best composition in the lithium manganate material system whose properties are influenced by the Mn‒O bond. © 2000 The Electrochemical Society. All rights reserved.

True space group of spinel-type chromium oxides and sulfides, and ternary lithium chlorides – band structure calculations

The results of band structure (extended Hückel procedure) and cohesive energy calculations of spinel-type chromium oxides and sulfides, and of lithium vanadium chloride were presented. They confirm that the D3dto C3vsymmetry reduction of the octahedrally coordinated metal ions (Cr, V) of the respective oxides and of Li2VCl4(space group F-43m instead of Fd-3m) is caused by relatively strong metal-metal bonds with crystal orbital overlap populations (COOP) up to 10% of those of the metal-oxygen bonds despite M-M distances ≅300pm. In the case of spinel-type chromium sulfides, the Cr-Cr interactions are antibonding in nature and, hence, no symmetry reduction occurs.

Étude ab initio des structures électroniques et magnétiques des systèmes YFe 2 et YFe 2 H 3 au sein de la théorie de la fonctionnelle de la densité (DFT)

Abstract First principles investigation of the electronic and magnetic properties of YFe2 and YFe2H3 within the density functional theory (DFT). The electronic and magnetic properties of YFe2 and of its hydride system YFe2H3 are studied within the local spin density functional theory. Analyses of the density of states (DOS) and of the crystal orbital overlap populations (COOP) allowed us to provide an explanation for the role played by hydrogen as to the increase of the magnetisation as well as to the chemical bond with the metal constituents Y and Fe. © 2000 Academie des sciences / Editions scientifiques et medicales Elsevier SAS Laves phases / alloys / hydrides / magnetism / DFT / LSDA / chemical-bonding

Local spin density functional investigations of the chemical bonding and of the magnetism in some uranium ternary intermetallic systems: How physics and chemistry can meet in the solid state

The electronic and magnetic structures of different uranium-based ternary intermetallic systems are self-consistently calculated within local spin density functional theory using the augmented spherical wave method. The influence of hybridization on the chemical bonding and on the magnetic behavior is discussed from the densities of states as well as from the crystal orbital overlap population. From this the author addresses the mechanisms of chemical bonding and of the onset of magnetism. The original concept of building blocks between different intermetallic systems is discussed.

Electronic and structural properties of extended-chain compounds and polymers

Results of density functional calculations on infinite, periodic chains are reported. The method that is applied is based on linearized muffin-tin orbitals as basis functions, although the full potential and not only its muffin-tin part is included in the calculations. Special emphasis is put on analyzing the interatomic interactions by means of crystal-orbital overlap or Hamilton populations (COOP and COHP, respectively). As examples of conjugated polymers, trans-polyacetylene and polycarbonitrile are studied. Here, in particular, the existence of a bond length alternation is discussed. Subsequently, PtS{sub 2} (both without and with K counterions) and NbSe{sub 3} chains are considered. For the former, the single-chain calculations are supplemented with calculations on the crystalline compounds, and it is shown how single-chain effects are responsible for the structural properties whereas interchain effects have to be included in order to account for all the electronic properties. Parts of the results are explained through an analysis of the COOP and COHP. For NbSe{sub 3} the three different structures occurring in the crystalline material are considered, and the implications of the results for the existence of charge density waves as well as the importance of spin-orbit couplings are discussed. Finally, HF as an example of an extendedmore » hydrogen-bonded system is examined, and it is demonstrated how the electronic interactions change when the covalent and hydrogen bonds are interchanged as it occurs in charge transport via solitons.« less

Semiempirical band structure calculations on skutterudite-type compounds

Semiempirical band structure calculations were performed on several skutterudite-type compounds by using the extended Huckel method. Starting with the molecular orbital calculations on isolated P4 and As4 rings, the reason for the band dispersions of the skutterudites was found to be the interactions between the nonmetal atoms. Both the intermolecular and the intramolecular interactions between the phosphorus atoms are stronger than those between the arsenic atoms. Hence, the dispersion of the bands in CoP3 is larger than that in CoAs3. The COOP (crystal orbital overlap population) integrals of the intramolecular P-P bonds reveal the relation between the valence electron count and the observed bond lengths. The P-P bonds in the skutterudite-type compounds like TP3 (T = Co, Rh, Ir) become stronger by reduction as in NiP3 and weaker by oxidation as in RT4X12 (X = P, As, Sb; R = alkaline earth or rare earth metals) because the bands near the Fermi level are bonding. The electronic reason for the geometric distortion of the Ge2Y2 (Y = S, Se) units of mixed skutterudites TGe1.5Y1.5 is caused by an electron pair gap on germanium, which corresponds to low electron density perpendicular to the ring plane on the germanium atoms.

Ab initio analysis of electron energy loss spectra for complex oxides

The local electronic structures in ionic materials are analysed in terms of the site- and angular-momentum-projected densities of states, which are obtained from self-consistent ab initio band-structure calculations. The unoccupied conduction-band states are compared to the energy-loss near-edge spectra (ELNES), which are measured by analytical transmission electron microscopy with high spatial resolution. A novel tool for the interpretation of this ELNES data is presented: The projection of the calculated density of states onto an optimised atomic-orbital basis set allows a neighbourhood-sensitive electron-distribution analysis based on crystal orbital overlap populations. This method is applied to spinel, MgAl 2O 4, both in its regular and in its inverse structure. An analysis of the O–K-edge gives evidence of a distinction between the influences of tetrahedrally coordinated and octahedrally coordinated cations on the near-edge structure.

Why has 1T-TaTe 2 not yet been synthetized? A DFT contribution.

Periodical Density Functional (DFT) calculations were done on a hypothetical 1T-TaTe 2 phase in order to understand why this compound has not yet been synthetized. The combined analysis of the density of states and of the crystal orbital overlap populations have disclosed bands interaction that could be at the origin of the non existence of the 1T-TaTe 2 phase.

The adsorption of methyl on Rh 10 clusters: selective C–H bond weakening

The adsorption of methyl on an Rh 10 cluster was studied by ab initio density functional calculations as a model for the Rh(1/1/1) surface. The surface was modeled by a three-layer, 10 atom Rh cluster. The most stable binding site was found to be the threefold site, and the most stable geometry was a methyl species tilted 20° from the surface normal towards the onefold site of the Rh. Crystal orbital overlap populations were calculated and showed that the tilted methyl was energetically preferred because of a bonding interaction between the d states of the Rh and one of the C–H antibonding orbitals in the methyl. Electron donation from the d states to this C–H orbital causes a selective weakening of the bond, possibly explaining experimental vibrational data in which a softened C–H stretch mode is observed. A discussion of the advantages and the limitations of the cluster approximation is also presented.

Relative stabilities, bulk moduli and electronic structure properties of different ultra‐hard materials investigated within the local spin density functional approximation

The relative stabilities of the following phases: graphite-C3N4, α-C3N4, β-C3N4, cubic-C3N4 and pseudo-cubic-C3N4 have been determined using density functional theory in its local density approximation. In particular three calculational methods were imployed: augmented spherical wave, linear muffin-tin orbitals and full-potential linearized augmented plane-wave. The main objective of this work was the prediction of the hardness for a series of C3N4 phases (α, β, cubic and pseudo-cubic) as well as for the cubic BN (c-BN) structure. To this purpose total energy calculations were performed for different unit cell volumes and the resulting data were fitted to a polynomial function in order to determine the equilibrium lattice constants (aeq and ceq), bulk moduli (B0) and pressure derivatives (B0′). Even though the different methods do not produce comparable energy trends, all methods are in agreement in predicting equilibrium volume, bulk modulus and pressure derivatives. Further, for the graphite-based structures the influence of hybridisation on the chemical bonding and stability is discussed in terms of the site projected densities of states as well as the crystal orbital overlap population. For the hexagonal and orthorhombic phases the electronic properties are also discussed by means of a density of states analysis.

Local spin density functional investigations of a manganite with perovskite-type derived structures

The electronic and magnetic structures of the perovskite CaMnO 3 are self-consistently calculated assuming two crystal structures at the same formula unit volume within the local spin density functional theory and the augmented spherical wave (ASW) method. From the comparisons of energy differences between the different magnetic states the ground state configuration is an insulator with G-type ordering. This result together with the magnitudes of the magnetic moments are in agreement with experiment. The influence of mixing between Mn and O is found spin dependent from the analysis of the crystal orbital overlap population (COOP) which enable to describe the chemical bond. The calculations underline a feature of a half metallic ferromagnet which could be connected with the colossal magnetoresistance (CMR) property of related compounds.