Partial Densities Of Electronic States Research Articles

Electronic structure and magnetic properties of monoclinicβ-Cu2V2O7: AGGA+Ustudy

A first-principles study on monoclinic $C2/c$ copper pyrovanadate $\ensuremath{\beta}{\text{-Cu}}_{2}{\text{V}}_{2}{\text{O}}_{7}$ has been performed using the generalized gradient approximation (GGA) and $\text{GGA}+U$ method. The optimized unit-cell parameters and atomic coordinates of $\ensuremath{\beta}{\text{-Cu}}_{2}{\text{V}}_{2}{\text{O}}_{7}$ agree well with experimental data. The optimized crystal structure of $\ensuremath{\beta}{\text{-Cu}}_{2}{\text{V}}_{2}{\text{O}}_{7}$ indicates the existence of one-dimensional -Cu-Cu-Cu-Cu- chains. The electronic structure and magnetic properties were evaluated by the $\text{GGA}+U$ calculations, which indicate that the $\ensuremath{\beta}{\text{-Cu}}_{2}{\text{V}}_{2}{\text{O}}_{7}$ is a semiconducting antiferromagnetic material with an indirect band gap and local magnetic moment per Cu atom of $0.73{\ensuremath{\mu}}_{B}$. The intrachain exchanges for short and long Cu-Cu couples are estimated to be 6.4 and 4.1 meV, respectively, while the calculated interchain exchange (2.1 meV) is smaller, which indicate the one-dimensional character. The top of the valence band is composed of $\text{V}\text{ }3d$, $\text{O}\text{ }2p$, and $\text{Cu}\text{ }3d$ electrons while the bottom of the conduction band is primarily composed of $\text{Cu}\text{ }3d$ electrons. Valence electron-density distribution map indicates the V-O and Cu-O covalent bonds. Calculated partial electronic density of states strongly suggests that the V-O and Cu-O covalent bonds are mainly attributed to the overlaps of $\text{V}\text{ }3d$ and $\text{O}\text{ }2p$ atomic orbitals and of $\text{Cu}\text{ }3d$ and $\text{O}\text{ }2p$, respectively.

Large-scale ab initio simulations of binary transition metal clusters for storage mediamaterials

In the quest for ultra-high-density magnetic recording, new materials in the nanometrerange have attracted much interest over the last decade involving intense studies ofL10 phases of contemporary or future storage media materials like FePt or CoPt nanoparticles.Based on large-scale density functional theory calculations, we provide a systematic overviewof the structural and magnetic properties of various morphologies of FePt and CoPtnanoclusters with diameters up to 3 nm. In this size range, the ordered multiply twinnedmorphologies are energetically favoured over the nanoclusters with the desired layer typeL10 and high magnetocrystalline anisotropy. Other nanoparticles of interest, like FePd, alsoshow a preference for multiply twinned structures or exhibit, as in the case ofMnPt nanoclusters, a strong tendency for antiferromagnetic ordering instead offerromagnetic order. The compositional trends of the various nanoparticles can be tracedback to differences in the partial electronic density of states of the 3d element.

Cluster study of Al–Co–Ni decagonal quasicrystal

Abstract The electronic structure and atomic arrangement of a decagonal Al–Co–Ni quasicrystal have been investigated in terms of a model unit cluster by using a discrete variational X α (DV-Xα) cluster calculation. The partial electronic density of states (DOS) and overlap population analysis for local clusters around the constituent transition-metals (TM´s) show the energy reduction of both the Co and Ni 3d states but the increase in the anti-bonding character for their TM-Al bonds at the second inner ring TM site of the model cluster in comparison to the most inner ring TM site. Comparing the calculated DOS with the observed valence-band photoelectron and TM Lα X-ray emission spectra, we discuss the occupation at the transition metal sites by Co and Ni.

X-ray Raman scattering for structural investigation of silica/silicate minerals

We have performed X-ray Raman scattering (XRS) measurements on the oxygen K and silicon L absorption edges of four silica minerals: α-quartz, α-cristobalite, coesite, and stishovite. We have also calculated the partial electron densities of states (DOSs) and compared these with the XRS spectra. This study demonstrates that the short-range structure around the atom of interest strongly influences the XRS spectral features. Importantly, the oxygen K-edge XRS spectra are found to reflect the p-orbital DOS while the silicon L-edge spectra reflect the s- and d-orbital DOSs, even when a product of a momentum transfer and a mean radius of a electron orbit (1s for oxygen and 2p for silicon), Qr, is close to or larger than unity. Building on this, calculations of the partial DOSs for other silica phases are presented, including ultra-high-pressure phases, which provide a good reference for further XRS study of silica and silicate minerals. XRS measurements should be performed on not only either of oxygen or silicon but also on many kinds of constituent elements to reveal the structural change of glasses/melts of silicates under extreme conditions.

Multiply Twinned Morphologies of FePt and CoPt Nanoparticles

Based on large-scale density functional theory calculations we provide a systematic overview of the size dependence of the energetic order and magnetic properties of various morphologies of FePt and CoPt clusters with diameters of up to 2.5 nm. For FePt, ordered multiply twinned icosahedra and decahedra are more favorable than the L1_(0) phase throughout the investigated size range. For CoPt, segregated morphologies predominate with considerably increased energy differences to the L1_(0) structure. The compositional trends are traced back to differences between the morphologies in the partial electronic density of states associated with the 3d element.

Ab initio study of Cmca phase of Bi 50Sn 50 ordered alloy

The geometrical structure, energetics and electronic properties of Cmca phase of Bi 50Sn 50 ordered alloy at 3 GPa were investigated using first-principle method, where two configurations were considered. The optimized structure parameters are consistent with the available experimental results. And it is found that the structure with Bi occupying 8d Wyckoff position is more preferable energetically and mechanically. The mechanical analysis suggests that the structure where Sn occupies 8d Wyckoff position is elastically unstable. For the structures considered, the corresponding total and partial electronic density of states were also discussed.

Electronic structure and optical properties of Ti-doped phosphate crystals

Luminescence properties of NaAl 1− x Ti x P 2O 7 ( x = 0–1) double phosphates under VUV excitation are studied. Partial densities of electronic states are calculated for perfect NaAlP 2O 7, NaTiP 2O 7 and Ti-doped NaAlP 2O 7 crystals. Pure NaAlP 2O 7 crystals exhibit emission bands peaking near 300 and 770 nm. Doping of NaAlP 2O 7 crystals with Ti ions leads to appearance of emission bands in blue-green spectral region with main peak near 500 nm. NaTiP 2O 7 exhibit emission in blue-green region with similar shape as for NaAlP 2O 7:Ti. The luminescence mechanisms in Ti-doped phosphates are discussed using both experimental and theoretical results. Observed emission bands in blue-green region of spectrum are ascribed to emission centers like Ti 4+ + F-centers.

Cluster structure of the surface in the amorphous, quasicrystalline, and liquid states

The electronic structure of the Zr60Ni20Ti20, Al65Cu20Fe15, and Al86Mn14 systems has been investigated by X-ray photoelectron spectroscopy and band calculations of the total and partial densities of electronic states have been carried out by the TB-LMTO-ASA method with the aim of studying the chemical bonds and the nearest environment of atoms in the liquid, amorphous, and quasicrystalline states. A direct proof of the inheritance of the chemical structure of the liquid state in the amorphous and quasicrystalline states has been obtained for the first time. The similarity of the electronic structure and the nearest environment of atoms indicates identity of clusters in the liquid, amorphous, and quasicrystalline states. The degree of hybridization of valence electrons of atoms in clusters increases in going from the quasicrystalline to the amorphous state due to the increase the number of transition metal atoms in clusters.

NMR STUDIES OF LAYERED NITRIDE SUPERCONDUCTORS

Layered nitride superconductor, Li 0.48( THF )y HfNCl with Tc=25.5 K , was investigated by means of nuclear magnetic resonance (NMR). 7 Li -, 35 Cl -NMR studies clarified that the partial electron density of states at Fermi level at Li and Cl sites are negligibly small. From the 15 N NMR Knight shift studies, the spin susceptibility decreases toward zero below Tc, evidencing that the pairing symmetry is an even-parity spin-singlet. These results indicate that superconductivity is derived from the HfN layer. We also measured 15 N -, 7 Li -NMR spin-lattice relaxation rate in the temperature range of 4.2K - 100 K and observed the enhancement of the spin-lattice relaxation rate, 1/ T 1, by field fluctuations generated by the vortex dynamics.

Electronic structure and optical properties of [formula omitted] double phosphates

Luminescence and luminescence excitation under VUV radiation of ABP 2 O 7 ( A = Na , K, Cs; B = Al , In) double phosphates are studied. Two emission bands peaking near 330 and 420 nm are common for investigated ABP 2 O 7 crystals. The band structure and partial densities of electronic states of perfect KAlP 2 O 7 , LiInP 2 O 7 and NaTiP 2 O 7 crystals are calculated by the full-potential linear-augmented-plane-wave (FLAPW) method. It is found that the structures of the conduction bands of ABP 2 O 7 crystals, which have different B cations, are appreciably different. Experimental results are compared with results of calculations of the electronic structure. Assumptions concerning the origin of luminescence in double phosphates are made.

Cluster structure of the A1 86Mn 14 system in the liquid state

The electronic structure of the A1 86Mn 14 melt was studied with the use of the X-ray photoelectron spectroscopy, and the calculations of the total and partial densities of electron states of the A1 86Mn 14 system were carried out using the TB-LMTO-ASA method. It is shown that the calculation and the valence band X-ray photoelectron spectrum are in good agreement.

X‐ray spectroscopic investigation of forbidden direct transitions in CuGaO2 and CuInO2

AbstractWe studied experimentally the local partial density of electronic states (LPDOS) in the conduction band (CB) and in the valence band (VB) of CuGaO2 and CuInO2 by means of X‐ray absorption spectroscopy (XAS) and X‐ray emission spectroscopy (XES). The edges of both the CB and VB are found to be the copper s‐ and/or d‐states. Therefore, direct transitions between the band edges in these materials should be forbidden by symmetry selection rules. The crystal phases in the samples were identified as having delafossite structure, determined by X‐ray powder diffraction (XRD). (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

First-principles calculations of structural and electronic properties of monoclinic hafnia surfaces

We have carried out a systematic theoretical study of the surfaces of monoclinic hafnia $(\mathrm{Hf}{\mathrm{O}}_{2})$ using plane waves and density functional theory based on the generalized gradient approximation. The fully relaxed structures of the bulk phases of $\mathrm{Hf}{\mathrm{O}}_{2}$ are found to be in excellent agreement with experimental data, the monoclinic phase being the most stable. Simulations of the monoclinic phase surfaces indicate a large relaxation which reduces the total surface energy of all nine faces considered by between 23% and 36%, with a strong correlation between the unrelaxed and relaxed surface energies. Our calculations predict that the $(\overline{1}11)$ and (111) faces of the monoclinic phase have the lowest surface energies and are hence the most stable faces. An analysis of the total and partial electronic density of states of bulk monoclinic $\mathrm{Hf}{\mathrm{O}}_{2}$ reveals that the outer valence band significantly mixes the O $2p$ and Hf $5d$ atomic states indicating some covalency of the Hf-O bonds. The total density and partial density of states of the monoclinic surfaces exhibit a surface state corresponding to the surface O $2s$ states in the inner valence band region.

Atomic structure and bonding properties in amorphous Cux(As2S3)1−x by ab initio molecular-dynamics simulations

It is known that the addition of few percent of Cu into chalcogenide glasses suppress the photodarkening. However, the precise nature of the mechanism that gives rise to this phenomenon still remains elusive. In order to gain some insights about the microscopic mechanism of this behavior, understanding of the fundamental materials properties is indispensable. In the present study, we investigated the composition dependence of the atomic structure and bonding properties in amorphous Cux(As2S3)1−x (x=0,0.11) by the ab initio molecular-dynamics simulations. We have found that the electronic density of state near the top of the valence band changes appreciably by the addition of Cu. In particular, a large change in the partial electronic density of states of the chalcogen component has been observed. The pair distribution function obtained indicates that the As–As correlation is influenced appreciably by the addition of Cu, whereas the changes in S–S and As–S correlations are relatively small. Our result also indicates that the Cu introduced tends to bind with S with a well defined bond length. The presence of Cu–As bonding and a possible agglomeration of Cu atoms have been also pointed out.

Electronic energy structure of As2S3, AsSI, AgAsS2, and TiS2 semiconductors

Quantum-mechanical calculations with the FEFF8 code were used to study the electronic energy structure of 200-atomic clusters of As2S3, AsSI, AgAsS2, and TiS2 semiconductor compounds. The calculated local partial densities of electronic states are compared with the sulfur K and L X-ray emission spectra and sulfur K absorption spectra for fine powders of these compounds. Good agreement between theory and experiment has been obtained.

Local and electronic structure of FeS, CoS, NiS: Ultrasoft sulfur L2,3 X-ray absorption spectra analysis

The ultrasoft S L 2,3 X-ray absorption near-edge structure (XANES) spectra of FeS, CoS and NiS having the NiAs-type crystal structure have been measured and theoretical analysis of the experimental data has been done on the basis of full multiple scattering theory. The distribution of local partial density of electronic states has been established. A special kind of hybridization between metal 3d and sulfur p states has been found: sulfur p states are “squeezed out” of the energy region of the maximum of metal d electronic states.

Analysis of the electronic structure of human hemoglobin from soft X-ray emission

We present X-ray emission spectra (XES) of human hemoglobin excited near the iron L 2,3 threshold and the nitrogen and carbon K-edges. The experiment is compared with our calculations of the corresponding spectra and gives good agreement. We find that the Fe L 3 emission is affected by the nearest nitrogen atoms located in the heme plane around the central iron atom. The distribution of the partial electronic densities of states of hemoglobin is determined.

Electronic Structure and Spectral Properties of Si[sub 46] and Na[sub 8]Si[sub 46] Clathrates

The electronic structure of Si46 and Na8Si46 clathrates has been calculated using the linearized augmented plane wave method. The calculation yielded their band structure, total and partial electron densities of states, and SiKβ1, 3 and SiL2, 3 X-ray emission spectra. The calculated and experimental spectra for the Na8Si46 clathrate were compared.

X-ray spectra and electron densities of states of the complicated semiconductors TlAsS2, Tl3AsS3, Tl3AsS4 and Tl3PS4

The complicated as to their composition and crystal structure semiconductors TlAsS2, Tl3AsS3, Tl3AsS4, called briefly TAS, and Tl3PS4 represent a class of compounds promising in the devices of quantum electronics. The electron energy structure of these compounds is investigated both experimentally with x-ray spectroscopy and theoretically by calculations of the local partial electron densities of states (LDOS) and x-ray K-absorption spectra of S and P using FEFF8 code. All theoretical K-absorption spectra and LDOS were calculated ab initio without any adjusting parameters according to the scheme: the self-consistent mt-potentials obtained for the clusters of about 35 atoms were then used in the calculations of the absorption spectra and LDOS for clusters of 87 atoms in the full multiple scattering approximation. The comparison of the calculated K-absorption spectra of S and P of sulfides investigated with their experimental analogues shows that in all cases the theoretical curve reflects good enough the energy positions of all features of the experimental curve. The results of the calculations of LDOS and the experimental emission K- and L2,3-spectra of S and P gave the possibility to determine the energy positions of the electron occupied states of all elements in the semiconductors TAS and Tl3PS4 and to get the information about the hybridization of the electron states in the valence band. The compounds Tl3AsS3 and Tl3AsS4 have the same crystal structure, this leads to the likeness of their K-absorption spectrum of S and the electron energy structure of their valence bands.

Insights into oxygen-cation bonding in fresnoite-type structures from O K- and Ti L23-electron energy-loss spectra and ab initio calculations of the electronic structure

O K- and Ti L23-core-loss spectra of fresnoite Ba2TiSi2O8 (BTS) and Sr2TiSi2O8 (STS), which is isotypic to BTS, have been measured by electron energy-loss spectroscopy (EELS). The energy-loss near-edge structures (ELNES) of the O K edge have been identified on the basis of theoretical simulations and interpretations of the X-ray absorption near-edge structures (XANES), which have been modelled in the framework of self-consistent full multiple-scattering (FMS) theory using FEFF8. Herewith, the K-absorption spectra of oxygen μ(E) and the local partial electron density of states (DOS) of all atoms have been calculated. For BTS, the observed spectral features in the O K-edge spectra are interpreted in terms of mixing between the central O p and neighbouring Ba 5d and 4f, Si 3p and 3d, and Ti 3d orbitals. The observed differences in the O K-edge spectra for STS and BTS can mainly be attributed to three properties: (1) The lack of high local partial Sr unoccupied DOS with 4f symmetry near the Fermi level compared to the high Ba 4f unoccupied DOS results in differences of overlapping O 2p – cation orbitals. (2) The differences in the ionic radii of Sr and Ba result in a larger unit cell for BTS and, thus, in larger oxygen-cation bonding distances. (3) In comparison to STS, the strength of the incommensurate 2-D structural modulation is significantly weaker in BTS, i.e. distortions of coordination polyhedra occur to a much lesser extent. All these effects alter the oxygen-cation hybridization and, hence, result in a variation of the O 1s → p transition and consequently of the O K-edge spectral shape. The observed peak broadening in Ti L23 ELNES of STS compared to BTS is correlated with strong displacive modulations hosted in STS.