Electronic Structure

The calculation of vibrational properties of materials from their electronic structure is an important goal for materials modeling. A wide variety of physical properties of materials depend on their lattice-dynamical behavior: specific heats, thermal expansion, and heat conduction; phenomena related to the electron-phonon interaction such as the resistivity of metals, superconductivity, and the temperature dependence of optical spectra, are just a few of them. Moreover, vibrational spectroscopy is a very important tool for the characterization of materials. Vibrational frequencies are routinely and accurately measured mainly using infrared and Raman spectroscopy, as well as inelastic neutron scattering. The resulting vibrational spectra are a sensitive probe of the local bonding and chemical structure. Accurate calculations of frequencies and displacement patterns can thus yield a wealth of information on the atomic and electronic structure of materials.

Materials with the olivine LixMPO4 structure form an important new class of materials for rechargeable Li batteries. There is significant interest in their electronic properties because of the importance of electronic conductivity in batteries for high rate applications. The density of states of LixMPO4 (x = 0, 1 and M = Fe, Mn) has been determined with the ab initio GGA+U method, appropriate for these correlated electron systems. Computed results are compared with the optical gap of LiFePO4, as measured using UV-Vis-NIR diffuse reflectance spectroscopy. The results obtained from experiment (3.8-4.0 eV) and GGA+U computations (3.7 eV) are in very good agreement. However, standard GGA, without the same level of treatment of electron correlation, is shown to make large errors in predicting the electronic structure. It is argued that olivines are likely to be polaronic conductors with extrinsically determined carrier levels and that their electronic conductivity is therefore not simply related to the band gap.

Using Hartree-Fock, B3LYP and MP2 treatments, the optimal geometries and corresponding electronic structures of tetrahedral [Pb 4 (m 3 -OH) 4 ] 4C , [Pb 4 O(m 3 -OH) 4 ] 2C , [Pb 4 (m 2 -OH) 6 ] 2C and [Pb 4 O(m 2 -OH) 4 ] complexes are investigated. All these complexes of T d symmetry are stable. Direct Pb-Pb and O-O interactions are weakly antibonding in all the systems under study. The clusters are held together exclusively by relatively weak Pb-O(OH) and Pb-O bonds, which are stronger in m 2 -OH bridged complexes (despite longer Pb-Pb distances). The non-existence of [Pb 4 (m 2 -OH) 6 ] 2C and [Pb 4 O(m 2 -OH) 6 ] complexes in real solutions/solids might be explained by their high reactivity and/or solvent/solid state effects of polar environment. q

Gold nanoparticles protected by a binary mixture of thiolate molecules have a ligand shell that can spontaneously separate into nanoscale domains. Complex morphologies arise in such ligand shells, including striped, patchy, and Janus domains. Characterization of these morphologies remains a challenge. Scanning tunneling microscopy (STM) imaging has been one of the key approaches to determine these structures, yet the imaging of nanoparticles' surfaces faces difficulty stemming from steep surface curvature, complex molecular structures, and the possibility of imaging artifacts in the same size range. Images obtained to date have lacked molecular resolution, and only domains have been resolved. There is a clear need for images that resolve the molecular arrangement that leads to domain formation on the ligand shell of these particles. Herein we report an advance in the STM imaging of gold nanoparticles, revealing some of the molecules that constitute the domains in striped and Janus gold nanoparticles. We analyze the images to determine molecular arrangements on parts of the particles, highlight molecular "defects" present in the ligand shell, show persistence of the features across subsequent images, and observe the transition from quasimolecular to domain resolution. The ability to resolve single molecules in the ligand shell of nanoparticles could lead to a more comprehensive understanding of the role of the ligand structure in determining the properties of mixed-monolayer-protected gold nanoparticles.

The excitation spectra for various impurity emissions have been measured in MgO:Al, MgO:OH, MgO:Cr crystals and MgO:F, MgO:LiF ceramics at 8 K using synchrotron radiation of 5±32 eV. Processes of radiative recombination at the impurity centres of various structure dier signi®cantly for two stages of multiplication of electron±hole (e±h) pairs with threshold photon energies at 20 and 25 eV. The multiplication processes have been analysed taking into account the peculiarities of the electronic structure of MgO.

It was previously shown that the addition of 1 equiv of a strong acid to [Mn IV (salpn)(µ-O)] 2 , 1, generates the oxo/hydroxo complex [{Mn IV (salpn)} 2 (µ-O,µ-OH)](CF 3 SO 3), 2, which emphasized the basicity of the µ 2-O 2units in the [Mn IV (µ-O)] 2 dimers. We now demonstrate the inherent nucleophilicity of those µ 2-O 2units by showing that the addition of methyl triflate to 1 results in formation of the oxo/methoxo-bridged Mn IV dimer [{Mn IV (salpn)} 2 (µ-O,µ-OCH 3)](CF 3 SO 3), 3. EXAFS analysis of 3 demonstrates that alkylation of an oxo bridge results in the same structural modification of the [Mn IV (µ-O)] 2 core as an oxo bridge protonation. Electrochemical and spectroscopic comparisons of 3 to 2 indicate that 3 is a good electronic structure analogue for 2 without the complication of proton lability and hydrogen bonding. Indeed, 2 and 3 react nearly identically with hydrogen peroxide and with strong acids. In contrast, the products of their reactions with amines, acetate, and triphenylphosphine are dramatically different. The proton lability of 2 results in simple proton transfer, circumventing the slower redox reactions of these substrates with 3. Isotopic labeling, kinetic, and EPR-monitored radical trap studies lead to a proposed reduction-oxidation mechanistic scheme for the reactions of 3 with amines and triphenylphosphine. The Mn III product of this reaction, [Mn III (salpn)(Ph 3 PO)](CF 3 SO 3), was isolated and crystallographically characterized as a dimerized complex. The redox nature of the reactions is confirmed by trapping of a reduced Mn intermediate which is identified by EPR spectroscopy. Comparison of the reactions of 2 and 3 demonstrates the dramatic effect of proton lability and hydrogen bonding on reactivity, and suggests how metalloenzymes may regulate active site reactivity to produce very different catalytic activities with similar active site structures. Furthermore, it also emphasizes that caution should be used when the reactivity of model compounds with easily and rapidly dissociable protons is assessed.

Ab inifio calculations are used to provide bond lengths, harmonic frequencies, and dissociation energies of low-lying electronic states for LiX, LiX+, and LiX-(with X = Li through F and Na through Cl). Most of these species represent hitherto experimentally unknown molecules or ions, which provides the focus of the work presented here. All of these species are stable to dissociation and the anions are stable to loss of an electron. Differences among the electronic structures of the valence isoelectronic LiX; and HX, LiX+, and HX+; and LiX-and HXspecies are analyzed. Optimized geometries, dissociation energies, ionization potentials, and electron affinities were calculated for the following ground states of the respective species: rZ+ for Li2('1:)

Due to their bandgap engineering capabilities for optoelectronics applications, the study of nano-graphene has been a topic of interest to researchers in recent years. Using a first-principles study based on density functional theory (DFT) and thermal DFT, we investigated the electronic structures and optical properties of bilayer graphene quantum dots (GQDs). The dielectric tensors, absorption spectra, and the refractive indexes of the bilayer GQDs were obtained for both in-plane and out-of-plane polarization. In addition, we calculated the absorption spectra via time-dependent DFT (TD-DFT) in the linear response regime. The TDDFT results show that a blue shift occurs in the absorption spectrum, which is consistent with the experimental results. In this investigation, we consider triangular and hexagonal GQDs of various sizes with zigzag and armchair edges. Our simulations show that unlike monolayer GQDs, for which light absorption for out-of-plane polarization occurs in the ultraviolet wavelength range of 85-250 nm, the out-of-plane polarization light absorption peaks in the bilayer GQDs appear in the near-infrared range of 500-1600 nm, similar to those in bilayer graphene sheets. The out-of-plane polarization light absorption peaks in the near-infrared range make bilayer GQDs suitable for integrated optics and optical communication applications.

Density Functional calculations have proven to be a powerful tool to study the ground state of many materials. For finite temperatures the situation is less ideal and one is often forced to rely on models with parameters either fitted to zero temperature first principles calculations or experimental results. This approach is especially unsatisfactory in inhomogeneous systems, nano particles, or other systems where the model parameters could vary significantly from one site to another. Here we describe a possible solution to this problem by combining classical Monte Carlo calculations-the Wang-Landau method[2] in this case-with a firs principles electronic structure calculation, specifically our locally selfconsistent multiple scattering code (LSMS)[3]. The combined code shows superb scaling behavior on massively parallel computers. The code sustained 1.836 Petaflop/s on 223,232 cores of the Cray XT5 jaguar system at Oak Ridge.

The synthesis and characterization of a series of new olefin-substituted closo-1,12-[C 2 B 10 H 12 ] compounds is described. The reaction of deprotonated closo-1,12-dicarbadodecaborane with 7-methoxycycloheptatriene yields the cycloheptatriene-substituted carborane compound, closo-[1-(1-C 7 H 7)-12-(H)-C 2 B 10 H 10 ] (3). Deprotonation of 3 with butyllithium and subsequent reaction with 3,4-dimethyl-2-cyclopenten-1-one was found to yield the bisolefin-substituted carbinol compound [1-(1-C 7 H 7)-12-(C 5 H 4-1-(OH)-3,4-(CH 3) 2)-C 2 B 10 H 10 ] (5) in good yield. Acidic dehydration of 5 quantitatively converted it into the simple bis-olefin cage compound [1-(1-C 7 H 7)-12-(C 5 H 3-3,4-(CH 3) 2)-C 2 B 10 H 10 ] (6). Finally, thermal treatment of 6 in refluxing toluene was employed to prepare the isomerized cycloheptatriene compound [1-(4-C 7 H 7)-12-(C 5 H 3-3,4-(CH 3) 2)-C 2 B 10 H 10 ] (7) in high yield. These compounds represent the first metal-free cyclopentadiene and bis-olefin large cluster species reported in which the C 5 ring is attached directly to the cage. The complete characterization of products by multinuclear NMR (1 H, 11 B, and 13 C), infrared, UV-visible, and mass spectroscopic analyses is presented. The X-ray crystal structures of 3 and 7 are also reported. X-ray data for 3: triclinic system, space group P1 h with cell constants a) 6.6807(3) Å, b) 10.2939(3) Å, c) 10.3962(4) Å, R) 89.342(2)°,) 74.610(2)°, γ) 83.373(2)°, Z) 2, R1) 0.0487 (wR2 (all data)) 0.1272). X-ray data for 7: monoclinic system, space group P2 1 /c with cell constants a) 7.7207(7) Å, b) 15.8730(14) Å, c) 15.5493(13) Å,) 99.146(2)°, Z) 4, R1) 0.0761 (wR2 (all data)) 0.2050).

The de Haas-van Alphen effect was observed in the underdoped cuprate YBa2Cu3O6.5 via a torque technique in pulsed magnetic fields up to 59 T. Above a field of approximately 30 T the magnetization exhibits clear quantum oscillations with a single frequency of 540 T and a cyclotron mass of 1.76 times the free electron mass, in excellent agreement with previously observed Shubnikov-de Haas oscillations. The oscillations obey the standard Lifshitz-Kosevich formula of Fermi-liquid theory. This thermodynamic observation of quantum oscillations confirms the existence of a well-defined, closed, and coherent, Fermi surface in the pseudogap phase of cuprates.

A model is proposed to study the electronic structure of slightly curved graphene sheets with an arbitrary number of pentagon-heptagon pairs and Stone-Wales defects based on a cosmological analogy. The disorder induced by curvature produces characteristic patterns in the local density of states that can be observed in scanning tunnel and transmission electron microscopy. PACS numbers: 75.10.Jm, 75.10.Lp, 75.30.Ds The recent synthesis of single or few layers of graphite allows to test the singular transport properties predicted in early theoretical studies and experiments . The discovery of a substantial field effect [8] and of ferromagnetic behavior[9] allows to envisage graphene as a reasonable replacement of nanotubes in electronic applications. Disorder plays a very important role in the electronic properties of low dimensional materials.

State-resolved two-pulse correlation measurement is useful to study the time scale and mechanism of photo-induced desorption as well as electronic structures of admolecules before desorption. In photodesorption of nitric oxide molecules from Pt(1 1 1) at 250 K induced by 80 fs, 620 nm laser pulses, the yields and translational temperatures reached a maximum at zero delay time. In contrast, the rotational temperature of X ¼ 3=2 molecules reached a maximum at a delay time of AE1 ps, while that of X ¼ 1=2 molecules was enhanced at around zero delay. In addition, the ratio of total yields of X ¼ 1=2 molecules to that of X ¼ 3=2 molecules increased at short delay times. All of these results can be explained by the model of rotational heating, including spin-orbit (SO) coupling, where the difference in the correlation time was found to be due to the SO structure in the adsorbed phase. : S 0 0 3 9 -6 0 2 8 ( 0 2 ) 0 1 6 5 9 -X

Research on amorphous carbon (a-C) to date has focused on the distinction between the sp 2 and sp 3 phases and understanding the properties on the basis of the sp 2 -C bonded component. Recently, sufficient information on the sp 2 -bonded clusters and nanoforms of carbon has helped to identify the importance of sp 2 -C over sp 3 -C, especially in transport properties and encouraged many groups to exploit this knowledge for device design. However, at present, few studies dedicated purely to understanding the transport properties and electronic structure of the family of a-C films as a whole is available. In this paper, we try to identify the key issues in using a-C as an unconventional semiconducting material and try to elaborate on how to overcome these hurdles in order to utilize this extremely versatile material for active device fabrication. D (S. Bhattacharyya). Thin Solid Films 482 (2005) 94 -98 www.elsevier.com/locate/tsf

A theoretical study based on the density functional theory of the adsorption of Mn monomers and dimers on a Au-(111) surface is presented. As necessary preliminary steps, the bulk and clean surface electronic structure are calculated, which agree well with previous reports. Then, the electronic structure of the Mn adatom, chemisorbed on four different surface geometries, is analyzed. It is found that the most stable geometry is when the Mn atom is chemisorbed on threefold coordinated sites. Using this geometry for a single adatom a second Mn atom is chemisorbed and the most stable dimer geometrical structure is calculated. The lowest-energy configuration corresponds to the molecule lying parallel to the surface, adsorbed on two topological equivalent threefold coordinated sites. It is also found that the lowest-energy magnetic configuration corresponds to the antiferromagnetic arrangement with individual magnetic moments of 4.64μ B . Finally, it is concluded that the dimer is not stable and should fragment at the surface.

We use a combination of first principles many-body methods and the numerical renormalizationgroup technique to study the Kondo regime of cobalt-porphyrin compounds adsorbed on a Cu surface. We find the Kondo temperature to be highly sensitive to both molecule charging and distance to the surface, which can explain the variations observed in recent scanning tunneling spectroscopy measurements. We discuss the importance of many-body effects in the molecular electronic structure controlling this phenomenon and suggest scenarios where enhanced temperatures can be achieved in experiments.

The V K-edge spectrum of a V-bearing grossular garnet (tsavorite) single crystal has been measured by high-energy resolution fluorescence detected X-ray absorption spectroscopy (HERFD-XAS). Firstprinciples calculations based on the density functional theory (DFT) reproduce the experimental spectra well, including the angular dependence of the pre-edge. The latter arises from pure electric quadrupole transitions since vanadium is incorporated in a centrosymmetric site, which inhibits the electric dipole transitions. An interpretation of the spectral features is given using a monoelectronic description of the transitions involved in the X-ray absorption process. The assignment of experimental pre-edge peaks in terms of 1s-3d transitions makes it possible to determine the magnitude of the 3d-splitting, which is similar to the 10 Dq crystal-field value determined from optical absorption spectroscopy. DFT calculations also indicate a full structural relaxation around V 3+ substituting Al 3+ in the garnet structure. The angular dependence only causes weak variations of the pre-edge intensity, which supports the use of V-bearing grossular garnet as a reference compound for octahedral V 3+ .

The electronic structures of HgO and Tl 2 O 3 have been investigated by valence and core-level x-ray photoemission, x-ray absorption, and x-ray emission spectroscopies. Valence-band photoemission under Al K␣ excitation is dominated by the metal 5d partial density of states and thus provides a sensitive probe of shallow core mixing into the O 2p valence-band states. Conversely O K shell emission is determined by the O 2p partial density of states and therefore allows the extent of corresponding mixing of O 2p character into the shallow core states to be measured. The experimental work is supported by band-structure calculations carried out within the framework of density-functional theory. It is shown that the bonding in HgO involves significant mixing between O 2p states and both Hg 6s and shallow core 5d states: the calculated O 2p partial density of states mirrors the O K shell emission spectrum and reveals significant O 2p character within the shallow core Hg 5d states. There is, however, little direct on-site mixing between the Hg 6s and 5d states. In Tl 2 O 3 , the hybridization of the deeper metal 5d states with O 2p states is much less pronounced than in HgO. Moreover, the states at the bottom of what is conventionally regarded as the O 2p valence band are found in fact to have very strong Tl 6s atomic character. The photoemission spectrum of Tl 2 O 3 shows a well-defined metallic Fermi edge: the shape of the structure around the photoemission onset suggests that the metallic nature of Tl 2 O 3 arises from an occupation of states above the main valence-band edge, probably arising from oxygen vacancy defects. The conduction electrons of Tl 2 O 3 are strongly perturbed by ionization of Tl core levels, giving rise to distinctive plasmon satellites in core x-ray photoemission spectroscopy.

We study the dependence of magnetic interactions and Curie temperature in Ni (1+x) MnSb system on the Ni concentration within the framework of the density-functional theory. The calculation of the exchange parameters is based on the super-cell and frozen-magnon approaches. The Curie temperatures, TC , are calculated within the random-phase approximation. In agreement with experiment we obtain decrease of the Curie temperature with increasing Ni content.

Recently, we reported a four-coordinate titanium complex containing a terminal phosphinidene functionality. 1b Ti-phosphinidenes are expected to be unstable, because of the hard-soft contrast among these elements. The recent isolation of the first complex containing a TidP bond was in part accomplished by stabilizing kinetically this reactive functionality and by applying an R-hydrogen migration strategy using a nucleophilic alkylidene ligand. 1b,5,6 Inspired by this result, we applied an analogous strategy for preparing a family of four-coordinate complexes with a thus far unknown terminal VdP functionality. We have also determined that these radical species are stabilized not only kinetically but also via a pseudo Jahn-Teller effect of second order. 7

rxn cr and KKR cPA calculations are performed for the pseudobinary alloy system Ti(Fe,Co1_,), including the pure binary phases. The results indicate that, in accordance wüh "ipe.imental findings, the magnetic behaviour of the pseudobinary system is entirely governed tiy antistructure atoms.

We present symmetry-breaking and electronic-structure phase diagrams for two-center molecules with one and two electrons in the limit of a space of large dimensions. For one electron, the phase diagram in the internuclear distance ± nuclear charge (R ± Z) plane has two different stable phases. One corresponds to the electron equidistant from the two nuclei; the other where the electron is localized on one of the nuclei. The phase diagram for two electrons with two equally charged centers shows three different stable phases corresponding to different electronic-structure configurations. This phase diagram is characterized by a bicritical point. When the charges are unequal, the phase diagram shows only two stable phases, covalent and ionic. This phase diagram is charac-terized by a tricritical point, where the first-order transition line meets with the second-order transition line. The role of the interelectron Coulombic repulsion in giving rise to different electronic structures and the distinction between a continuous deformation of one structure into another versus a discontinuous, so-called firstorder, transition, where two isomers can coexist, are emphasized. The connection to the spectroscopic notion of intersecting potential energy curves is discussed.

X-ray absorption near edge stru ture spectroscopy has been used to establish the chemical shifts of the technetium K edge in a range of compound \ containing Tc in a variety of formal oxidation states. The edge positions span 19.9 eV from Tc metal to N€L+Tc04. Strong correlation between chemical shift and formal oxidation state is observed. Extended X-ray absorptron fine structure (EXAFS) spectroscopy of Tc2(CO)10 indicates that multiple scattering along the Tc-C-gvector is more important than direct Tc. 0 scattering. TcOz is shown by EXAFS to possess a distorted rutile structure with a closest Tc-Tc distance of 2.61 A. This is rationalized in terms of the Goodenough model for bonding in transition metal dioxides.

The newly developed full-potential linearized augmented plane wave (LAPW) and local orbitals (lo) based on standard APW methods are briefly introduced, and the structure and magnetic properties of R(Fe, Si)12 compounds (R = Y, Nd) are calculated using the method. The distribution of Si at different sites is analyzed based on total energy of one crystal unit with structure having been optimized. The characters of magnetic moments, total density of states (TDOS) and partial density of states (PDOS) for different crystal sites Si occupies are obtained and analyzed. The results show that the total magnetic moments of RFe10Si2 (R = Y, Nd) are larger than those of RFe10M 2 (M = Ti, V, Cr, Mn, Mo and W) and the hybridization mechanism is seen as follows. Si(8j) reduce the magnetic moments of Fe at three sites, however, Si(8f) mainly reduce the magnetic moments of Fe(8i) and Fe(8j) atoms. The Curie temperature is markedly enhanced by the introduction of Si atoms according to spin fluctuation of DOS at Fermi level.

The polarized Cu K-edge X-ray Absorption Near Edge Structure (XANES) of La2CuO4 has been interpreted by the multiple scattering approach. The size of the cluster of neighbouring atoms having good agreement with the XANES experimental data is determined by 45 atoms surrounding the absorbing Cu. The polarized spectra can be predicted in term of a one-electron dipole (Δl=+1) transition Cu 1s→ϵp, probing the unoccupied p-like (l=1) density of states projected on a Cu site with orbital angular momentum ml=0 in the E∥z spectra, and the ml=1 for the E⊤c spectra. Thus we show that the electronic structure of the high energy conduction bands, beyond the Cu 3d band, of La2CuO4 are well described in terms of the one-electron approximation. It is shown that XANES spectra are consistent with the contraction of the Cu-apex oxygen distance with doping.Final state effects induced by the core hole have been indentified: (i) the core transitions take place in the fully relaxed potential, (ii) the satellite at 7 eV above the main K-XANES peak in both polarizations is assigned to a multielectron shake up excitation. Finally the shoulder on the rising absorption edge, present in many Cu compounds and usually assigned to a shake down multi-electron excitation, is shown to be due to a one-electron transition to a state delocalized over a large cluster.

We investigate the electronic structure of carbon nanotubes functionalized by adsorbates anchored with single C-C covalent bonds. We find that, despite the particular adsorbate, a spin moment with a universal value of 1.0 $\mu_B$ per molecule is induced at low coverage. Therefore, we propose a mechanism of bonding-induced magnetism at the carbon surface. The adsorption of a single molecule creates a dispersionless defect state at the Fermi energy, which is mainly localized in the carbon wall and presents a small contribution from the adsorbate. This universal spin moment is fairly independent of the coverage as long as all the molecules occupy the same graphenic sublattice. The magnetic coupling between adsorbates is also studied and reveals a key dependence on the graphenic sublattice adsorption site.

Absorption due to conduction intersubband transitions is studied in n-type s-Si/SiGe multiquantum wells ͑MQW͒ of different well widths and barrier composition grown by UHV-chemical vapor deposition ͑CVD͒. The measured intersubband transition energies are compared with the theoretical results of a tight-binding model which provides the electronic band structure of the complete MQW system throughout the whole Brillouin zone. Our findings demonstrate both the high quality of the CVD grown MQWs and the effectiveness of the adopted tight-binding model in describing band profiles and electronic structures of SiGe multilayer systems. In particular we have evaluated the conduction band offsets in the investigated structures.

We report UPS, work-function and LEED results on the interfacial reaction between evaporated Ta and Si(111)(7• surface under ultrahigh vacuum conditions. At room temperature, a disordered chemisorbed phase is formed at low coverage, 0 _< 1 monolayer (ML), and is characterized by an UPS Ta 5d peak shifted at about -1.2 eV as compared to bulk Ta. For 1 < 0 ~ 4 ML, although AES and XPS indicate a 1 Ta : 2 Si composition, the valence band spectra are still different from the TaSi 2 one. At higher coverage, the electronic structure of polycrystalline Ta is progressively recovered. Annealing of the deposits leads to silicide formation by interdiffusion and surface atomic ordering at temperatures from 650 ~ to 850 o C, depending on the thickness. All the deposits in the 0.8-100 ML range, including the smallest ones, are stable until -500 ~ and the silicide formation reaction begins only at higher temperature, reflecting an activation energy barrier. For 0 ~ 5 ML, clustering of the TaSi 2 phase occurs. For thicker deposits, continuous TaSi/overlayers are obtained. The electronic structure of this silicide is discussed in relation with existing models.

Electronic structure calculations of cubic SrTiO3 and SrHfO3 are presented. The full-potential linear augmented-plane-wave method is used and exchange-correlation effects are treated by the local-density approximation. The tendency to ferroelectricity of both compounds is explored and compared by displacing the transition metal atom (Ti or Hf) towards one of the oxygens (001 direction). The calculations show that ferroelectricity is favored in SrTiO3 with respect to SrHfO3 and that this fact may be correlated with the degree of hybridization between transition metal d-O p bands as has been found for other related systems. Also a detailed discussion of the calculated electric field gradients is presented.

In the framework of density functional theory (DFT), the structural and magnetic properties of FeN mono nitride have been investigated using the all electrons augmented spherical wave method (ASW) with a generalized gradient GGA functional for treating the effects of exchange and correlation. Calculation of the energy versus volume in hypothetic rocksalt (RS), zinc-blende (ZB) and wurtzite (W) types structures shows that the RS-type structure is more stable than the others. Spin polarized calculation results at equilibrium volume indicate that the ground state of RS-FeN is ferromagnetic with a high moment, while ZB-FeN and W-FeN are non magnetic. The influence of distortions on the stability is taken into account by considering FeN in two different face-centred tetragonal structures (fct): fct-rocksalt and fct-zincblende. The magnetovolume effects with respect to Slater-Pauling-Friedel model are discussed. The electronic structures analyzed from site and spin projected density of states are reported. A discussion of the structural and magnetic properties of FeN is given with respect to N local environment of Fe.

The radical cation P840" was studied in frozen suspensions of Chlorobium limicolu f. sp. thiosulphatophilum membranes using ENDOR and Special TRIPLE spectroscopies. The spectra show that P840" arises from a bacteriochlorophyll a 'special' pair with a highly symmetrical distribution of electron spin density between the constituent bacteriochlorophylls. Special TRIPLE spectroscopy has resolved the separate contributions of the two halves of the pair and revealed small deviations from a 1 : 1 electron spin density distribution. Nevertheless P840" appears to come the closest yet to the symmetrical 'dimer' originally proposed for the structure of the primary donor radical cation (P870") in purple non-sulphur photosynthetic bacteria.

The crystal chemistry of Ni-and Co-bearing manganese oxides (lithiophorite and asbolane) has been investigated by X-ray Absorption Spectroscow (XAS). The Mu oxides come from tbe lateritic weathering profiles of the ultrabasites of New Cakdonia. The distinct bebaviours of Ni and Co concern both oxidation states and local structures. The electronic structure and short range order around Co atoms do not depend on the natme of the Coconed phase. Co atoms are trivaknt and 6-fold c~~~~. Co=@, OH) and Co-(Co, Mn) interatomic distant dexived from EXAMS are equal to those found around Mn atoms which rules out the possibiity of an adsorption of Co atoms dire&y above and beIow vacancies of MnQ, layers, The high structural order around Co contrasts with the structural disorder observed around Mn. Cobalt atoms do not occupy specific Mn sites and are not randomly distributed witbin the octahedml Mn layers. Unlike Co, Ni exhibis distinct surroundings in both phasea. In asbolane, Ni atoms build partial Ni(OH)2 layers. Ni-OH distaste am lower as compared with the free Ni hydroxide because of the fotmation of hydrogen bonds between Ni(OHh and Ma02 layers. In li#io~o~~ Ni atoms are Iocated in the ham layer (AI(OH Both chemical composition and stmctural considerations militate for a Ni for Li substitution in iithiophorite. Finally, evidence is given for the existence of a mixwi-layering between lithiophorhe and a&lane and the chemical vttriatiom generally observed in these Mn oxides are interpretad as a variable proportion of (Mn, (3x0, OHh, Ni(OH), and (Al, Li, NiXOH) layers. ~ODLJCi=ION THE MANOAWSE OXIDES have long been w as important carriers of minor elements, some of which (Co, Ni, Cu, @I) are collcentrated up to several petwnt. Transition meWbearing manganese oxides have been

Copper sulfide (Cu2-xS) is a class of low-cost, environment friendly p-type semiconductor, where electronic structure and the thus induced optoelectronic properties can be significantly varied through the creation of copper deficiency. To this end, varying composition of Cu2-xS (i.e., Cu2S, Cu1.96S, Cu1.8S, Cu1.8S+ Cu1.6S and CuS) films were grown here by using a low temperature molecular solution based deposition method, following which a wide range of characterization tools were used to understand their microstructure, electronic structure and optoelectronic properties. The hole concentration of these films are found to vary from 3.32 × 1019 cm−3 to 2.54 × 1022 cm−3 as Cu2-xS composition changes from Cu2S to CuS. This is because of the induced Cu deficiency in Cu2-xS films with decreasing Cu/S-molar ratio, which reduced the Cu d-band width in the valence band, thus pushing the Fermi level deep into the valence band. This leads the optical and transport gap to increase from 1.36 eV to 2.23 eV and 1.31 eV to 2.02 eV respectively with increasing copper deficiencies from Cu2S to CuS. Moreover, in this work, both the valence and conduction band edge positions are found to shift negatively with increasing Cu deficiency in these films.

Crystal structures of three derivatives of phenylpropionic acid, 2, 3 and 4 with hydroxyl, methyl and methoxy substitutions at the 2, 4 positions have been determined from X-ray powder diffraction data and their electronic structures were calculated at the DFT level. The optimized molecular geometries agree closely to that obtained from the crystallographic analysis. Intermolecular O-H. . .O hydrogen bonds generate R 2 2 (8) rings, which are further connected through O-H. . .O and C-H. . .O hydrogen bonds into two-dimensional framework in 2 and 4, and a step like architecture in 3. The HOMO-LUMO energy gap (>4.0 eV) indicates a high kinetic stability of the three compounds.

The electronic ground states of [n]cyclacenes, as well as short-zigzag nanotubes, computed at unrestricted broken spin-symmetry density functional theory (UBS-DFT), were found to be open-shell singlets, rather than triplets. Computations for [6]cyclacene at complete active space self-consistent field (CASSCF) and multireference perturbation theory (MRMP2) levels support this conclusion. Along with strain, the radical character of the open-shell singlet with antiferromagnetically coupled electron spins may contribute to the difficulties in synthesizing [n]cyclacenes.

Analysis of the relationships between TM-C and CtO distances of the more than 20 000 structures reported to the CSD has revealed new experimentally derived insights into the bonding in these systems. The databases of structures were investigated by a combination of DFT and statistical methods. The different abilities of transition metals to both accept and donate electrons are reflected in differences in their data sets. There are significant changes in gradient of the curves representing TM-C versus CtO scatter plots. One such change in gradient occurs at the bond length corresponding to CtO in its unbound state. There are also significant differences between the second and third transition series. The analysis provides a structural means of probing the distribution of electrons in a metal-carbonyl fragment and provides important insights into the periodicity of back-bonding, σ donation, and the ability of the carbonyl to stabilize different metal oxidation states.

We present results of the band structure and density of states for the chalcopyrite compounds CuAlX 2 (X = S, Se, Te) using the state-of-the-art full potential linear augmented plane wave (FP-LAPW) method. Our calculations show that these compounds are direct band gap semiconductors. The energy gap decreases when S is replaced by Se and Se replaced by Te in agreement with the experimental data. The values of our calculated energy gaps are closer to the experimental data than the previous calculations. The electronic structure of the upper valence band is dominated by the Cu-d and X-p interactions. The existence of Cu-d states in the upper valence band has significant effect on the optical band gap.

PACS 71. 71.15.Mb, 71.20.Nr, This paper reviews the present status of recently developed ab-initio as well as semiempirical electronic structure methods that are particularly suited for semiconductors and mesoscopic semiconductor structures. In assessing each method, we provide some additional and hitherto unpublished details about its implementation. In the first part of the paper, we discuss two important extensions of the local density functional theory, the screened-exchange and the exact exchange method namely. We proceed with a discussion of the relativistic local density method that is essential for a prediction of spin-related phenomena in semiconductors. Finally, we review a rather complete and extensve implementation of the envelope function approach for arbitrary three-dimensional nanometer device structures.