Xα Molecular Orbital Calculations Research Articles

Optimization of First-Principles Calculation Conditions for Ce3+ in Perovskite-Type Oxides

The 4f-5d transition of Ce3+ in crystals is used in a variety of optical materials, including scintillators and phosphors. It also has the advantages that the wavelength can be controlled by the coordination environment and that Ce3+ is relatively inexpensive among rare earth elements. For the theoretical design of novel Ce3+-doped phosphors, prediction of 4f-5d transition energy by first-principles molecular orbital calculations using model clusters is effective. In general, increase of the cluster size improves the accuracy of calculations and reduces the difference between the experimental and calculated values while very large clusters have the disadvantage of requiring a huge amount of calculation time. Hence, understanding the cluster size dependence is important to determine the efficient and effective cluster size for the first-principles calculations.In this study we focused on Ce3+-doped in YAlO3, and performed first-principles calculations of the 4f-5d transition energies for model clusters with different sizes. At first, the local structures around the Y3+ site of YAlO3 were cut out from the crystal structure. Then the central Y3+ ions of these clusters were replaced by Ce3+ ions. Then the transition energy of Ce3+ in YAlO3 was calculated by the relativistic discrete-variational Xα (DV-Xα) molecular orbital calculations using the Slater's transition state method. The detailed comparison of the results showed that for clusters larger than a certain size, the error between the experimental and calculated values became negligibly small. For comparison, the lattice relaxation was also taken into account by modifying the model clusters using two different methods: the method based on the Shannon's crystal radii and the structural optimization using the CASTEP code, and the results were compared in detail.

Investigation of Unoccupied Electronic States near the Fermi Level of Polysilane using Resonant Auger Spectroscopy

Unoccupied electronic states near the Fermi level of poly(dimethylsilane) were probed using Si 1s X-ray absorption spectroscopy (XAS) and Si KL2,3L2,3 resonant Auger spectroscopy (RAS). The measured resonance peaks of XAS spectra near Si K-edge have been assigned in comparison with the discrete variational (DV)-Xα molecular orbital calculations. The rapid delocalization of Si 1s core-excited electron through the empty conduction band was observed along the polymer chain with the energy-dependent RAS measurement, and the electron delocalization time was estimated based on the core-hole clock method.

Study of the Electronic Structure and Magnetic Properties of ε-Fe2O3 by First-Principles Calculation and Molecular Orbital Calculations

ε-Fe2O3 is known to exhibit a large coercive field of 20 kOe at room temperature. In this work, we examine the electronic structure and magnetic properties of ε-Fe2O3 using first-principles calculation and discrete variational (DV)-Xα molecular orbital calculation. The first-principles calculation shows that ε-Fe2O3 is a charge-transfer type insulator with a valence band of O2p and a conduction band of Fe3d. The optical transition is an indirect transition from Γ to S point. The density of states (DOS) of the four nonequivalent Fe sites (FeA, FeB, FeC, and FeD) indicates that ε-Fe2O3 has ferrimagnetic ordering due to α spins on FeB and FeC and β spins on FeA and FeD. The charge density map of the occupied Fe3d band displays a strong hybridization between Fe3d and O2p. Molecular orbital calculation for each Fe site also supports the existence of a strong Fe3d–O2p hybridization. Such a strong hybridization induces nonzero orbital angular momentum L on Fe3d through the partial charge transfer from O2p to Fe3d. The appearance of L causes a large magnetic anisotropy through spin–orbit interaction, which induces the large coercive field.

Chemical State Analysis of Entrapped Nitrogen in Carbon Nanohorns Using Soft X-ray Emission and Absorption Spectroscopy

The chemical state of nitrogen in nitrogen-entrapped carbon nanohorns (N-CNHs) synthesized by submerged arc discharge in liquid nitrogen was determined by soft X-ray emission and absorption spectroscopy using synchrotron radiation. X-ray absorption spectra (XAS) and X-ray emission spectra (XES) in the CK and NK regions of the N-CNHs were measured at the Advanced Light Source. From fingerprint analysis in NK-XAS and by referring to nitrogen-containing aromatic compounds, the suggested chemical state of nitrogen in N-CNHs is imine. Theoretical analysis of NK-XES using the discrete variational Xα molecular orbital calculations elucidated that the local structure of N-CNHs is chemisorption of N2, forming pentagonal and/or hexagonal rings with carbon atoms at the graphitic layer edges, which assume an imine structure. Therefore, the entrapped nitrogen in N-CNHs is chemisorbed at the edges of the graphitic layers.

Evaluating Seebeck coefficient of Na xCoO 2 from molecular orbital calculations

We propose the evaluations of Seebeck coefficient of Na x CoO 2 ( x = 0.38, 0.67, 0.76, 0.86, and 0.95) systems by Boltzmann theory: Mott expression and Fermi energy from the discrete variational (DV)-Xα molecular orbital calculation. This approach was based on molecular orbital calculations using the Na 8Co 21O 72, Na 14Co 21O 72, Na 16Co 21O 72, Na 18Co 21O 72, and Na 20Co 21O 72 clusters to obtain the Fermi energy at the Co3d orbital of Na x CoO 2 metallic for evaluating the temperature dependence of Seebeck coefficient values. The evaluated Seebeck coefficient of the Na x CoO 2 systems increases with increasing temperature as well as with increasing x value and temperature.

Electronic Structure Analysis of Iron(III)-Porphyrin Complexes by X-ray Absorption Spectra at the C, N and Fe K-Edges

X-ray absorption near edge structure (XANES) measurements at the C, N, and Fe K absorption edges were performed for iron(III)-tetraphenylporphyrin (FeTPP), iron(III)-tetrakis(p-carboxyphenyl)porphyrin (FeTCPP), and iron(III)-tetrakis(p-sulfonatophenyl)porphyrin (FeTSPP). The spectral shapes differ in the Fe K XANES, but not in C and N K XANES among FeTPP, FeTCPP, and FeTSPP. Crosschecks of XANES data for C, N, and Fe K absorption edges in combination with discrete variational (DV)-Xalpha molecular orbital (MO) calculations indicate that each p-electron-withdrawing group on four meso-phenyl substitutes in an Fe(III)-porphyrin complex brings about a unique electron state through the complex because of the electron-withdrawal strength, itself. Consequently, they affect the positive charge of the center Fe(III) ion.

Soft x-ray absorption spectra of amorphous carbon

Information about the local structure of amorphous carbon (a-C) films was obtained by comparing soft x-ray absorption spectra in the CK region of sputtered a-C films to reference carbon compounds such as hydrofullerene (C60H36), highly oriented pyrolytic graphite (HOPG), and carbon black. The spectra of the a-C films exhibited fine structures, which consisted of at least different six portions. Comparing the spectral features of the a-C films with the reference compounds and the spectral analysis by discrete variational (DV)-Xα molecular orbital calculations indicated that the fine structures of the a-C films are due to a hybrid of sp2 and sp3 carbon atoms.

Carbonizing Processes of Titanium Thin Films by Carbon-Implantation

Observations of the crystallographic structural changes of deposited Ti films (100 nm thick) during C-implantation were made by using transmission electron microscope (TEM) for the purpose to elucidate the carbonizing processes of Ti films. The Ti films were deposited by an electron beam heating method onto thermally cleaned NaCl (001) surfaces held at room temperature (RT). Both the 200 kV conventional TEM and the 400 kV analytical high resolution TEM combined with ion accelerators were used. The deposited Ti films consisted of mainly (03\\cdot 5)-oriented hcp-Ti and (110)-oriented CaF2 type TiHx (x ∼ 1.5). Carbon ions (C+) with 26 keV were implanted into the Ti films held at RT in the 400 kV TEM. The H atoms which constituted TiHx were released from the as-deposited Ti films during the early stage of C-implantation. In the C-implanted Ti film (C/Ti=0.54), there coexisted (001)- and (110)- oriented NaCl-type TiCy and a small amount of hcp-Ti. The results of TEM observation indicated that (001)- and (110)-oriented TiCy were epitaxially formed by the transformation of (03\\cdot 5)-oriented hcp-Ti and (110)-oriented TiHx, respectively. In the cases, the hcp-Ti sublattice is transformed into fcc-Ti sublattice of TiCy with inheritance of square atomic arrangement of hcp-Ti. The hcp-fcc transformation was discussed with the results of a self-consistent charge discrete variational (DV)-Xα molecular orbital (MO) calculation. [DOI: 10.1380/ejssnt.2005.319]

Nitriding transformation of titanium thin films by nitrogen implantation

Changes of electronic structure during nitrogen implantation into heated Ti thin films, which were accompanied by crystallographic structural transformation, were studied by applying in situ electron energy loss spectroscopy (EELS) combined with self-consistent charge discrete variational (DV)-Xα molecular orbital (MO) calculations. Hydrogen atoms constituting TiH x were released from the films with heating at temperatures more than about 100 °C. The H-released unstable fcc-Ti was transformed into hcp-Ti. The energy loss peak observed by EELS for TiH x shifted to lower energies with the release of H atoms during heating. Nitrogen ions of N 2 + with 62 keV were implanted into the hcp-Ti films held at 350 °C. The TiN y films were “epitaxially” formed by the transformation of hcp-Ti to fcc-Ti sublattice, partially inheriting the atomic arrangement of the hcp-Ti and accompanying the occupation of octahedral sites of the fcc-Ti sublattice by N atoms. The energy loss peak for hcp-Ti films observed by EELS during N implantation gradually shifted to higher energies with increasing the dose. The shift meant increase in the density of electrons occupying the hybridized valence band. The formation mechanism of TiN y was discussed with the results of the calculated overlap populations of Ti–Ti and Ti–N bonds.

Soft X-ray emission spectroscopy of polycyclic aromatic hydrocarbons

High-resolution C K X-ray emission spectra of polycyclic aromatic hydrocarbons (PAH) were measured using synchrotron radiation. The main peak energies in the PAH X-ray spectra shifted to a higher energy region as the ratio of hydrogenated outer carbon atoms to the non-hydrogenated inner carbon atoms increased. Discrete variational (DV)-Xα molecular orbital calculations provided theoretical confirmation that the spectral features depend on the ratio of hydrogenated/non-hydrogenated carbon atoms, which suggests that the features around the main peaks provide the information of the degree of hydrogenation in PAH compounds.

Electronic Structures of Crystalline and Aqueous Solutions of LiBr, NaBr, KBr, and KBrO3: In Situ Br L-Edge Near-edge X-ray Absorption Fine Structure

The electronic structures of crystalline and aqueous solutions of LiBr, NaBr, KBr, and KBrO3 were studied using in-situ Br L-edge near-edge X-ray absorption fine structure (NEXAFS) under ambient conditions. The direct observation of the ligand-field potential (10Dq) allows the determination of their dependence on the interatomic distances between Br and the first near neighbor in crystalline LiBr, NaBr, KBr, and KBrO3 and the effect of hydration in the corresponding aqueous solutions. DV−Xα molecular-orbital calculations show that for both crystalline and aqueous solutions of KBr, the transitions occur from Br 2p to the unoccupied states containing mainly 4d orbitals of Br. The 5s and 5p orbitals of Br and 3d orbitals of K also contribute to the unoccupied states in addition to the 4d orbitals of neighbor Br due to the orbital mixing in crystalline KBr.

Angle-resolved soft X-ray emission and absorption spectroscopy of hexagonal boron nitride

Angle-resolved soft X-ray emission and absorption spectra in the BK and NK regions of hexagonal BN were measured using polarized synchrotron radiation. The take-off/incident-angle-dependence on the spectral features in both X-ray emission and absorption is clearly observed. The configuration of the σ and π orbitals, which were calculated using discrete variational (DV)-Xα molecular orbital calculations, explains the angle-resolved soft X-ray emission and absorption spectra. The relative peak intensity of the 394-eV peak in the NK X-ray emission provides useful information about the BN layer ordering.

Structural Characterization of Chemical Species in Solution by a Theoretical Analysis of XANES Spectra

A theoretical analysis method using a discrete variational Xα (DV-Xα) molecular orbital (MO) calculation, based on electron transition from an inner orbital to an unoccupied orbital, is introduced to analyze X-ray absorption near-edge structure (XANES) spectra in a lower energy region below the X-ray absorption edge. The computational procedure is described in detail and the evaluation of an obtained result is also explained. Applications of this method are presented for Cu(II) ions in liquid ammonia, macrocyclic Cu(II) complex in aqueous solution, and Al(III) ions in aqueous solution.

SOFT X-RAY ABSORPTION SPECTRA IN THE 0 K REGION OF MICROPOROUS CARBON AND SOME REFERENCE AROMATIC COMPOUNDS

To analyze the oxidation states of the graphitic surface of microporous carbon, soft X-ray absorption spectra in the 0 K region have been obtained for microporous carbon and various aromatic compounds. The aromatic molecules studied are substituted with one or more of the following oxygenated functional groups: hydroxy (–OH), carboxy (–COOH), carbonyl (>C=O), formyl (–CH=O), and ether (–O–). From comparison of the soft X-ray absorption spectra of microporous carbon and of reference aromatic compounds, the most probable chemical bonding states of oxygen in microporous carbon are found to be –COOH and >C(H)=O. Spectral features in the soft X-ray absorption spectra of microporous carbon are well explained by the O2p density of states in these oxygenated functional groups from discrete variational (DV)-Xα molecular orbital calculations.

Soft X-ray emission spectra in the O K region of oxygen incorporated in microporous carbon

Soft X-ray emission spectra in the O K region of oxygen incorporated in microporous carbon were measured using synchrotron radiation, in order to observe directly the oxygen on the graphitic surface in micropores and obtain information about its chemical bonding states. The O K X-ray emission spectra of the microporous carbon exhibited a main peak at 526 eV with a low-energy tail, and the reference compounds also exhibited similar spectral features. A spectral feature analysis of the reference compounds using discrete variational Xα molecular orbital calculations showed that the measured 526-eV peaks of the microporous carbon may be due to O2p orbitals and the low-energy tails be to the hybridized O2p/O2s orbitals.

Li K-edge spectra of lithium halides

We measured the Li K-edge spectra of lithium halides by means of total electron yield method (TEY), using the soft x-ray Beam Line 2 (BL2) of the compact synchrotron radiation facility at Ritsmeikan University in Japan. In lithium halides, the spectra have a sharp peak at about 60eV and a broad peak at the higher energy side. Various peak structures that appear in the absorption spectra are assigned to the corresponding Li-1s to valence band free orbitals transitions, which have been calculated by the Discrete Variational (DV)-Xalpha molecular orbital calculation method.

Electronic Structure of Clean and H-adsorbed HfC(111) Surface.

The electronic structure of a clean and H-adsorbed HfC(111) surface have been studied by angle-resolved photoemission spectroscopy and Discrete-Variational Xα molecular orbital calculation. A sharp peak in normal-emission spectra of the clean surface is observed at just below the Fermi level, and the peak is ascribed to the emission from a surface state composed of 5d orbitals of surface Hf atoms. Hydrogen adsorbs dissociatively on the HfC(111) surface forming a (1×1) overlayer at the saturation coverage. H-induced bands are observed at 6-7 eV and around the Fermi level. The calculation for the Hf13C13H3 cluster shows that these bands should be ascribed to a H 1s-Hf 5d bonding state and an extrinsic surface state composed of Hf 5d orbitals formed through the modification of the surface potential by H-adsorption, respectively.

In-Situ Transmission Electron Microscopy and Electron Energy Loss Spectroscopy Observation of TiN Grown by N-Implantation

Nitrogen ions (N2+) with 62 keV were implanted into evaporated-Ti films in the 400 kV analytical and high resolution transmission electron microscopy (TEM) combined with ion accelerators at JAERI-Takasaki. Observations by in-situ TEM equipped with electron energy loss spectroscopy, along with the discrete variational Xα molecular orbital calculations, revealed changes in the crystallographic and electronic structures of evaporated-Ti films due to N-implantation. A (001)-oriented NaCl-type TiNy is epitaxially formed by the transformation of (03·5)-oriented hcp-Ti to (001)-oriented fcc-Ti and by the occupation of N in the octahedral (O-) sites, whereas a (110)-oriented TiNy is formed by nitriding a (110)-oriented TiHx. The release of H from the TiHx occurs preferentially rather than the occupation of N in the O-sites of fcc-Ti sublattice. The loss peak due to volume plasmon of areas where TiHx has grown in the as-grown Ti film shifts to lower loss energy in the early N-implanting stage, while that of areas, where hcp-Ti has grown, gradually shifts to higher loss energies with increasing N dose. Analysis of Mulliken bond overlap populations determines that occupation of N in the O-sites gives rise to weakening Ti-Ti bonds and formation of Ti-N covalent bonds. The growth process of TiNy accompanied by changes of the crystallographic and electronic structures of Ti films due to N-implantation is discussed.

Haldane Gap System: Electronic Structures and Magnetic Properties

The electronic structure of typical s = 1 Haldane gap compounds such as [Ni(NH2(CH2)3NH2)2N3]ClO4 (NINAZ), [Ni(NH2CH2C(CH3)2CH2NH2)2N3]ClO4 (NDMAZ) and [Ni(NH2CH2C(CH3)2CH2NH2)2N3]PF6(NDMAP) have been investigated by means of discrete variational (DV)-Xα molecular orbital calculation. The DV-Xα calculations of these compounds reveal the importance of the overlap population analysis of bridging N3 − ligands, suggesting the tendency of the well-balance intramolecular overlap dispersion in the bridged N3 − molecule is correlated with the larger Haldane gap Eg .

Soft X-ray emission and absorption spectroscopy of hydrofullerene

Abstract High-resolution soft X-ray emission and absorption spectra in the C K region of hydrofullerene, which probably consists mainly of C 60 H 36 , were measured using highly brilliant synchrotron radiation to identify its molecular structure and electronic structure. Comparison with the C2p-DOS spectra, calculated by discrete variational (DV) Xα molecular orbital calculations, of three possible C 60 H 36 isomers, showed that the measured X-ray spectra approximately agreed with the calculated C2p-DOS spectra of the T-symmetry isomer. The X-ray spectral features were explained by the electronic structure hybridized between the sp 2 and sp 3 carbon atoms of the T-symmetry C 60 H 36 .