Prediction of γ‐B28 ELNES with comparison to α‐B12

Using ab initio techniques we have calculated the electron energy loss near edge structure (ELNES) of a new high pressure phase of boron (γ‐B₂₈) and the structurally similar allotrope, α‐B₁₂. The total ELNES spectra are presented as weighted sums of the site specific spectra of the constituent non‐equivalent B atoms. The five different non‐equivalent B sites in γ‐B₂₈ all show rich ELNES spectra and their similarities and differences to the simpler α‐B₁₂ case are detailed. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

First-principles study of the sulfur K and L2,3 edges of transition metal disulfide monolayers,MS2 (M=Mo,W and Re)

By means of the energy loss near edge structure (ELNES) analysis, the electronic structures of layered transition metal disulfides were studied. In the framework of full potential linearized augmented plane wave method, ELNES spectra of sulfur K and L_2,3 edges of layered MoS₂, WS₂ and ReS₂ have been calculated at magic angle conditions, and compared with those of bulks and the only existing experimental fine structure. Compared to the bulks, the energy differences between the main peaks in sulfur K and L_2,3 edges of monolayers decrease due to the slightly larger bond lengths that it can be used as a fingerprint for monolayers. Sulfur K edges in monolayers include some main features originated from electron transition to p_z (π) and p_x+p_y (σ) states and their hybridization. The overall dispersions of the sulfur L_2,3 edges in all cases are similar to the d-symmetry density of states. The first two features in L_2,3 edge of bulks and monolayers can be attributed to electron transition of sulfur 2p to the both unoccupied 3s-like states of sulfur and 4d states of transition metal atoms. Due to the considerable amount of s states at the energy position of a shoulder like structure in L_2,3 edge of both bulks and monolayers, these structures can be assigned to the sulfur 2p electron transition to unoccupied sulfur 3s states. The other features at higher energies are due to the transition of sulfur 2p electrons to the d-symmetry states of sulfur. In addition, due to the considerable energy band gaps, it seems that the use of core–hole approximation is essential for accurate reproduction of ELNES features of transition metal disulfides.     

Synthesis and characterization of mixed-valence barium titanates

A single-crystal barium oxotitanate(III, IV) of approximate composition , containing mixed-valence Ti, was grown from a borate flux. The crystal structure was identified as hollandite type by single-crystal X-ray diffractometry. Electron-energy-loss spectroscopy of Ti L 2,3 and O K edges was used to determine chemical shifts related to the presence of mixed-valence Ti in the crystal. Comparison of Ti L 2,3 and O K energy-loss near-edge structure (ELNES) of with those obtained from a K 1.54 Mg 0.77 Ti 7.23 O 16 single crystal with hollandite structure, containing only Ti 4+ , revealed a shift in the Ti L 2,3 edge by 0.4-0.5 eV towards lower energy losses whereas only slight intensity variations without a detectable energy shift of the edge onset occur at the O K ELNES. In addition, valence-specific multiplet structures of the Ti L 23 ELNES are used as valence fingerprints. The observed fine structures of O K and Ti L 2,3 edges can be used to interpret coordination and bonding in related compounds.     

Core-hole screening effects in the initial state of Auger emission across the transition metal series

Supercell method is used to study the relaxation and screening effects on the initial state of the Auger transition in metals. Our consideration is based on the assumption that when a core-hole exists long enough before the Auger transition occurs, the occupied valence states relax to screen the core-hole which results in a redistribution of the valence electrons, in particular within the atom that contains the core-hole. In order to make the interaction between the core-holes sites at different atoms negligible, the real metal is simulated by supercells repeated periodically. In each supercell one atom is considered to have a core-hole and many others not to have one. The electronic states concerned by the Auger transition are calculated by the self-consistent full-potential linearized augmented plane wave (FLAPW) method. Different responses of the local valence band on the site of the core-hole have been shown depending on whether the d-bands are partially or completely filled. According to the final state rule, the screening to the two holes in the local valence band after the Auger transition has also been considered, as examples, for Ni and Cu metals. The result shows that, with the existence of two holes in them, the states of the local valence band of Cu relax to atomic-like impurity states, while the local valence band of Ni changes to a much narrow band at the bottom of the original band. As examples, L₃VV and M₁VV Auger spectral profiles of Cu have been calculated in reasonably good agreement with the experiment.     

Electron energy loss spectrum of graphane from first-principles calculations

In this study, the energy loss near edge structure (ELNES) of carbon atoms in chair and tricycle conformers of hydrogenated graphene, namely ‘graphane’, has been calculated in the density functional theory using FP-LAPW method, and then, it has been compared with that of graphite and graphene. Using ELNES from chair conformer, the carbon K-edge was found to have a few main features including electron transition from 1s orbital of carbon atom to π*, σ*, and a hybridization of these two states. The first feature in tricycle conformer, however, has contributions of both π* and σ* states. The comparison of ELNES and the unoccupied density of states in each structure also justifies this. The energy difference between π* and σ* features of graphane conformers was decreased relative to it in graphite and graphene. Since the inclusion of core-holes and super-cells is essential for accurate reproduction of features in graphite and graphene, it may be essential as well for the ELNES spectra of graphane conformers.     

Yttrium segregation and intergranular defects in alumina

Intergranular segregation of yttrium is investigated in a rhombohedral twin grain boundary of alumina. The deviation from the twin orientation is compensated by a periodic arrangement of intergranular dislocations. TEM tools (CTEM and HREM, EDXS, EFTEM and EELS/ELNES) have been used to characterize changes in chemical and electronic environments along this twin. Within the experimental limits, no Y is detected in the perfect twin parts. On the other hand, Y segregation occurs very locally in the dislocation cores on about four atomic planes perpendicularly to the GB, which in fact corresponds to the step height associated with the interfacial dislocations. By comparison with an undoped bicrystal, both the dislocation distribution and the Y segregation localization confirm the influence of Y on the dislocation mobility. Furthermore, in the Y-rich defects, high spatial resolution analysis of the energy loss near edge structures (ELNES) of the Al-L₂₃ absorption edge brings some enlightenments about the Al³⁺ cation environment, provided the effects of local radiation damage are fully considered and under control.     

A combined SNMS and EFTEM/EELS study on focused ion beam prepared vanadium nitride thin films

We investigated the diffusion profiles and core-loss fine-structures (ELNES) of thin vanadium nitride films by electron energy-loss spectroscopy (EELS) and energy filtering transmission electron microscopy (EFTEM). The nitride layers have been produced by rapid thermal processing in a NH₃ or N₂ atmosphere and have then been cross-sectioned with a focused ion beam instrument (FIB) under mild milling conditions to maintain crystallography. For the high-resolution electron energy-loss spectroscopy studies (HREELS), a recently developed TEM gun monochromator, implemented into a 200 kV field emission gun column was used in combination with a new post-column spectrometer. It was found that, dependent on substrate and atmosphere, layers with different vanadium and nitrogen content were formed, showing distinct differences in their ELNES. With an energy resolution at the 0.2 eV level and a TEM beam spot size of approximately 2 nm these layers could be unambiguously identified when compared to theoretical ELNES simulations from the literature.     

First-principles energy band calculation for undoped and S-doped TiO2 with anatase structure

The electronic structure of S-doped TiO₂ with an optimized anatase structure was calculated within the framework of the density functional theory (DFT). For the calculation we built four kinds of supercells; type-A and B supercells consist of 12 and 48 atoms and a centered Ti atom is substituted for an S atom, while type-C and D supercells consist of 12 and 48 atoms and a centered O atom is substituted for an S atom. The supercells (type-B and D) were employed to adjust the S-concentration in TiO₂ to an experimental value of a few %. The changes of the lattice parameters are not significant in the type-A and B supercells. The phase transition from the tetragonal to the orthorhombic occurs in the type-C and D supercells. In the small supercell (type-A), S-related states are located in the range of −1.6 to 0 eV, and the S-states are band-like. In contrast, in the large supercell (type-B), S-related states appeared at about 0.9 eV above the top of the valence band, and the S-states are atomic-like. The localization of the S-related states is remarkable in the type-B supercell. In the type-D supercell, the S-related states were merged with the top of the valence band, and as a result the band-gap energy is narrowed by 0.7 eV. Despite a low S-concentration (3%) in the type-D supercell, the S-related states are somewhat band-like.     

Structural systematics in the family of (BEDT-TTF)2X salts

A crystallographic family tree can be developed for the (BEDT-TTF)₂X salts which aids in the systematization and rationalization of the structural and electrical properties of this novel series of materials. Importantly, it is recognized that the ambient-pressure superconductor α-(BEDT-TTF)₂I₃ has a crystal structure coincident with the parent subcell of this crystallographic family tree. Moreover, all first-generation supercells (doubling of any one subcell vector) are exclusively populated by salts with non-centrosymmetric anions of tetrahedral symmetry, and apparently, second-generation supercells (doubling of any two subcell vectors) are populated by salts with centrosymmetric anions of octahedral symmetry.     

X-ray absorption spectroscopy study and photocatalyst application of CuO and Cu0.9Ti0.1O nanoparticles

We report detailed investigations on the electronic structure and photocatalyst application of CuO and Cu_0.9Ti_0.1O nanoparticles (NPs). The NPs were prepared by co-precipitation method and subsequent annealing. Crystal structure and morphology of the NPs were investigated by synchrotron X-ray diffraction and high resolution transmission electron microscope, respectively. The local atomic structure around the Cu atoms was investigated by the extended X-ray absorption fine structure (EXAFS) at the Cu K-edge. Electronic structure determination was done using near edge X-ray absorption fine structure (NEXAFS) at the O K-edge, Cu L-edge, Cu K-edge and Ti L-edge. From the structural and electronic structure investigations, it is inferred that the Ti substitutes the Cu in CuO lattice without forming any secondary phases and the valence state of Cu is not affected by the Ti substitution; however the Cu – O bond length is found to be shorten in the Ti doped sample. As prepared NPs exhibit excellent photocatalyst application toward the degradation of methyl orange (MO) and potassium dichromate (PD) pollutant dyes under the visible light irradiation. The mechanism of the photodegradation of MO and PD pollutants, by the smaller sized CuO and larger sized Cu_0.9Ti_0.1O NPs, is briefly discussed.     

Theoretical ELNES using one-particle and multi-particle calculations

One-, two-, and many-particle calculations for electron-energy-loss near-edge structures (ELNES) are reviewed. The most important point for the ELNES calculation is the proper introduction of the core-hole effect. By introducing the core-hole effect in a sufficiently large supercell, one-particle calculations are applicable to the ELNES of many edges. On the other hand, the two-particle interaction between the excited electron and the core-hole, namely the excitonic effect, is significant in the K edges of very light elements and the L_2,3 edges of Mg and Al. Many-particle interactions, including both electron–electron and electron–hole interactions, are indispensable for the L_2,3 edges of transition metals and the M_4,5 edges of lanthanides, namely white lines. In this review, we present the basics, methodologies, and some applications of one-, two-, and many-particle calculations. In addition, importance of momentum transfer vector in the ELNES calculations for comparison with the experiments is discussed.     

Theoretical prediction of ELNES/XANES and chemical bondings of AlN polytypes

The first principles calculations of ELNES/XANES of AlN polytypes were carried out by the first-principles OLCAO method using large supercells composed of more than 100 atoms. It can quantitatively reproduce the experimental spectra from wurtzite AlN using a 108-atoms supercell. ELNES from rock-salt and zinc-blend AlN were predicted by using 128 atoms supercells. The spectral features of rock-salt phase are different from other phases, whereas that of zinc-blend phase have numerous similarities with that from wurtzite AlN. Characteristic differences between the wurtzite and zinc-blend phases are predicted to appear at the first peak of Al L(2,3) and K edges. The first peak of zinc-blend AlN is broader than that of wurtzite AlN. The same tendency was found in the case of SiC. In order to elucidate the cause of the broadness at the first peak, partial density of states and chemical bondings were investigated. The theoretical analysis revealed that the broadness of the first peak is related to the covalency of the compounds. This result suggests that the spectral features at the first peak of L(2,3) and K edges contain information about the covalency at the illuminated area.     

X-ray absorption investigations of copper resinate blackening in a XV century Italian painting

Investigations of the darkening phenomenon of copper resinate observed in a XV century easel painting were carried out by X-ray absorption spectroscopy (XAS) at the GILDA beamline of the European synchrotron radiation facility (ESRF, Grenoble, France). X-ray absorption near edge structure and extended X-ray absorption fine structure (EXAFS) measurements were collected at the Cu K-edge on an original painting sample, as well as on fresh pigment standards and on painting models. The study was aimed at providing structural information of the oxidation states and the local chemical environment (neighbouring atoms and bond distances) of copper in the unaltered and blackened pigments in order to elucidate the discoloration mechanism. Complementary information on optical and molecular properties of copper resinate were obtained by UV-vis and Fourier transform infrared spectroscopies. EXAFS analysis evidenced that the local chemical environment of Cu in copper resinate can be described using neutral copper acetate as a model. It consists, essentially, of bimetal Cu²⁺ carboxylate complexes with a distorted octahedral coordination. Such a structure is retained in the blackened pigment, although some differences were observed. It has been found that the alteration takes place without change of the valence state of Cu(II) ions, while the formation of the copper oxides CuO and Cu₂O responsible for the embrownment is excluded. On the basis of the XAS results we deduced that discoloration of copper resinate may be related to local modification of the copper coordination structure as evidenced by the observation of an increase of the Cu–Cu and Cu–C distances in the EXAFS spectra. PACS  78.70.Dm; 61.10.Ht     

A perfect lattice approach to nonstoichiometry

Although nonstoichiometry implies the presence of defects in a structure and hence a departure from perfect crystallinity, it is sometimes found that the defects are ordered or, at least, show a tendency to order. In these cases, it becomes possible to model the nonstoichiometry by considering a supercell and using perfect lattice techniques. We illustrate the viability of this approach with two examples. Firstly, we show how factors controlling the long-range ordering of extended defects in transition-metal oxides may be elucidated; indeed, we find that an adequate treatment of this phenomenon requires calculations on supercells. Secondly, we discuss how perfect lattice calculations may be used, in conjunction with diffraction data, to examine possible vacancy ordering schemes in the oxidation of magnetite, Fe₃O₄, to maghemite, the defect spinel structured γ-Fe₂O₃.     

First-principles study on the magnetism and electronic structure in 3d transition metal (X=Sc,V, Cr,Mn, Fe,Ni, Cu) doped CoO

We have studied the electronic structure and magnetism of the single transitional metal element X=Sc, V, Cr, Mn, Fe, Ni, Cu-doped CoO systems by first-principles calculations. At X=Sc, Cr, Cu, the binding energy of the doped systems is lower than pure CoO, suggesting that these systems are energetically stable. In the Sc, V, Cr, Mn, Fe, Ni, Cu-doped 2×2×2 CoO supercells, the total magnetic moments are 3.03, 5.64, 6.80, 7.70, 6.93, 2.30 and 1.96 μ_B, respectively. At X=Cr and Fe, the doped CoO systems are half-metallic with a high spin polarization. The large magnetic moment and high spin polarization in the Cr and Fe-doped CoO are important for the design of the spintronic devices.     

Q-dependent stoner factors and reformulated spin-fluctuation enhancements: V and Pd

Spin-polarized self-consistent LMTO band calculations are performed for supercells containing induced, “frozen” spin-waves. Momentum dependent Stoner factors S(q) are obtained as the local ratio between induced and applied spin splitting. The results are used to calculate the spin-fluctuation enhancement λ_sp on a similar basis as a calculation of the electron phonon coupling. The methods are tested on vanadium and palladium. For the latter a strong reduction of S(q) is found for large q. Obtained values for λ_sp are of reasonable magnitude.     

First-principles calculations of the electronic structure and Mössbauer parameters of Sb-doped

The electronic structure and the ¹¹⁹Sn and¹²¹Sb Mössbauer parameters of Sb-doped SnO₂ were calculated using a first-principles method. We show that substitution of Sn by Sb leads to a small decrease of the distances between the dopant and the six oxygen first-nearest neighbours. The most important modifications in the electronic structure are related to the bottom of the conduction band, which has Sb 5s, Sn 5s and O 2p characters. By considering two supercell sizes and charged supercells we show that antimony oxidation state is Sb⁵⁺ and we explain the origin of the Mössbauer parameters from the local electronic structure of the cations.     

Effect of atomic vibrations in XANES: polarization‐dependent damping of the fine structure at the Cu K‐edge of (creat)2CuCl4

Polarization‐dependent damping of the fine structure in the Cu K‐edge spectrum of creatinium tetrachlorocuprate [(creat)₂CuCl₄] in the X‐ray absorption near‐edge structure (XANES) region is shown to be due to atomic vibrations. These vibrations can be separated into two groups, depending on whether the respective atoms belong to the same molecular block; individual molecular blocks can be treated as semi‐rigid entities while the mutual positions of these blocks are subject to large mean relative displacements. The effect of vibrations can be efficiently included in XANES calculations by using the same formula as for static systems but with a modified free‐electron propagator which accounts for fluctuations in interatomic distances.