Partial Densities Of Electronic States Research Articles

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.

Theoretical investigation of the luminescence centres in PbWO4 and CdWO4 crystals

Electronic structures of possible centres of luminescence in the perfect lead (PbWO4) and cadmium tungstate (CdWO4) crystals are calculated using cluster approach. Electronic states of Pb2+, and Cd2+ cations, WO42- and WO66- are obtained on the basis of partial densities of electronic states calculated for several clusters containing from 72 to 104 atoms of the crystals. Calculated energies of one-electron transitions in these centers are compared with experimental data on luminescence and luminescence excitation. Obtained results evidence in favor of a tungstate nature of intrinsic luminescence in the PbWO4 and CdWO4 crystals . I ntrinsic luminescence probably originates from the tungstate groups WO42- in scheelite-type, and from the WO66- groups in the raspite-type PbWO4 crystals. Tungstate groups WO66- are assumed to be the centres of excitation of blue–green and yellow luminescence bands in CdWO4 crystals.

Electronic structure and chemical bonding of phosphorus-contained sulfides InPS 4, TI 3PS 4, and Sn 2P 2S 6

The electronic structure of phosphorus-contained sulfides InPS 4, Tl 3PS 4, and Sn 2P 2S 6 was investigated experimentally with X-ray spectroscopy and theoretically by quantum mechanical calculations. The partial densities of electron states calculated with the ab initio multiple scattering FEFF8 code correspond well to their experimental analogues—the X-ray K- and L 2,3-spectra of sulfur and phosphorus. The good agreement between theory and experiment was also achieved for K-absorption spectra of S and P in the investigated sulfides. In spite of the difference in the crystallographic structure of InPS 4, TI 3PS 4, and Sn 2P 2S 6 that influence the form of K-absorption spectra, the electronic structure of their valence bands are rather similar. This is due to the strong interaction of the P and S atoms, which are the nearest neighbors in the compounds studied. The electron densities of p- and s-states of phosphorus are shifted by about 3 eV to lower energies in comparison to the analogous electron states of sulfur. This is connected with the greater electro-negativity of sulfur, and is confirmed by the calculated electron charge transfer from P to S.

Electronic structure of FeSi2

The electronic structure of volume and film iron disilicides with crystal structure of the type of α leboit and fluorite was calculated using the linearized augmented plane-wave formalism. Joint and local partial densities of electronic states, x-ray emission spectra in different series of all inequivalent atoms of these phases, and photoelectron spectra for different excitation energies were obtained. γ-FeSi2 was found to be, unlike α-FeSi2, an unstable phase in both the volume and film realizations. X-ray L2,3 emission spectra of silicon in the iron group disilicides NiSi2, CoSi2, and FeSi2 were compared. NiSi2, CoSi2, and α-FeSi2 exhibit transformation of the maximum in the near-Fermi region of the Si L2,3 spectra as one crosses over from a bulk to a film sample. This transformation is closely connected with phase stability and may serve as a criterion of thermodynamic stability of the iron-group transition-metal disilicides.

Reduction in the electron density of states in superconductingMgB2disordered by neutron irradiation:11Band25MgNMR estimates

NMR line shift and nuclear spin-lattice relaxation rate T_1^{-1} of 11B and 25Mg were measured in superconducting MgB_2 (T_c^{ons}=38K) structurally disordered by nuclear reactor neutrons up to the fluence of thermal neutrons $\Phi=1 10^{19} cm^{-2}. The temperature of superconducting transition was shifted down to T_{c,irrad}^{ons}=7K under irradiation. The change due irradiation in the partial electron density of states (DOS) at the Fermi energy of boron and magnesium were traced by taking into account that T_1^{-1} of 11B and 25Mg are determined by hyperfine magnetic interactions with carriers. It was revealed that electronic states near E_F of Mg are influenced negligibly by irradiation whereas partial DOS of the B 2p_{x,y} states reduces greatly in irradiated MgB_2. The NMR estimates for DOS using the McMillan formula show that critical temperature decreases in irradiated MgB_2 mainly due to reduction in the partial DOS of p states of boron.

ELECTRONIC ENERGY STRUCTURE OF A SERIES OF CHALCOPYRITE-LIKE COMPOUNDS AgGaS2-CdGa2S4-InPS4

The effect of pseudovacancies on the density of electronic states of valence electrons in AgGaS2, CdGa2S4, and InPS4 is studied both experimentally, by means of X-ray spectroscopy, and theoretically, by calculating the partial densities of electronic states using the local coherent potential. The compounds under study are the derivatives of the sphalerite structural type (doubled cell) with gradually increasing deficiency of metals from \(Ag_4^I Ga_4^{III} S_8 \) to \(\square _2^{II} Cd_2^{II} Ga_4^{III} S_8 \) and further to \(\square _2^{III} In_2^{III} \square _2^V P_2^V S_8 \), where \(\square \) is a vacancy. The environment of the metal atoms remains tetrahedral, while that of the sulfur atoms changes with increasing number of vacancies from four (AgGaS2) to three (CdGa2S4) and two (InPS4). For the compounds under study, SK and PK X-ray emission and absorption spectra were recorded at a resolution of about 0.2 eV, and the local partial densities of states were calculated for all components of the compounds. The theoretical curves practically coincide in shape and energy position of fine structure elements with the corresponding experimental curves. This allowed reliable conclusions about the energy positions of electronic states at the top of the valence band and about the dependence of the SK emission and absorption spectra in the series of compounds under study on variation of the crystal structure and on the chemical composition of the nearest surroundings of sulfur atoms.

Electronic States of Chemisorbed Oxygen Species and Their Mutually Related Studies on SnO2Thin Film

Not only the clarification of chemisorbed oxygen species on oxides but also the variation of electronic states of these oxygen species under reaction condition are quite important to both science and applications. We proved the presence of four kinds of chemisorbed oxygen species (Ob, O22-, O-, O2-) on the topmost layer of SnO2 thin film with X-ray photoelectron spectroscopy (XPS). Ob was assigned to be O2- ion at the bridging site on the topmost layer of SnO2 (110) surface without oxygen vacancies around it by comparison between the binding energy values of the peak in XPS spectra and that obtained from the convoluted partial electronic density of states which were calculated using the density functional theory (DFT). The intensity variation of Ob during heat treatment up to 773 K with evacuating was weakly and inversely related to that of O22- and O-. The proportion of Ob and O- decreased after CH4 exposure at 473 K. We considered that highly reactive oxygen species, O- reacted with CH4 at 473 K, then oxygen vacancies at the topmost bridging site were produced. Some of Ob species coupled with these vacancies, and the electronic states of Ob modified to O- from O2-. This newly produced O- reacted with CH4 at 473 K.

The use of the FEFF8 code to calculate the XANES and electron density of states of some sulfides.

The FEFF8 was used to calculate x-ray absorption spectra (XANES) and the local partial electron density of states (LDOS) for several sulfides: InPS4 (twice defective chalcopyrite), CdIn2S4 (normal spinel) and Tl3AsS3 (space group R3m). The calculated XANES are compared with experimental spectra. The self-consistent MT-potentials found for clusters of about 35 atoms were used to calculate XANES and LDOS for clusters of 87 atoms in the approach of full multiple scattering. In all cases the agreement of the simulated XANES and experimental ones is quite good. The theoretical LDOS agree well with the experimental x-ray emission K- and L2,3-spectra.

Electronic structure of YM 2P 2 (M=Ni, Ru, Pd) compounds

The total and partial electron densities of states of YM 2P 2 (M=Ni, Ru, Pd) compounds have been calculated using the linear muffin tin orbital (LMTO) method. The calculated X-ray emission spectra are presented and compared with measured ones. The s-states of P hybridise with the p-states of Y and transition elements and contribute to the bottom of the valence band. In the middle part of the valence band, p-states of the components are located, while near the Fermi level d-states of transition elements dominate. The YM 2P 2 compounds are characterised by the significant increase of the effective filling number of electron d-states of Y, compare with the elemental Y.

Discrete–variational Xα calculations of graphite and alkali–metal graphite intercalation compounds

First-principles molecular orbital calculations using discrete–variational (DV)- Xα method are carried out on the model clusters of graphite and alkali–metal graphite intercalation compounds MC x (M=Li, Na, K, Rb, and Cs). Results of the calculations are used so as to simulate the experimentally observed near-edge X-ray-absorption fine-structures (XANES) and ultraviolet photoelectron spectra. For the clusters of graphite and KC x , the calculated partial electronic densities of states (PDOSs) are in good agreement with the experimental C K-edge and K K-edge XANES spectra, respectively. Furthermore, the accordance between the calculated DOSs and the observed UPS spectra of graphite and RbC x is also satisfactory. It is shown that the Fermi level is pushed up into the conduction band of graphite by doping alkali metals.

Soft X-ray measurements of the electronic states of various MgIn phases

Our present paper deals with the experiments on various phases of MgIn (40%) alloy as measured by soft X-ray emission spectroscopy (SXES). Only the Mg L 2.3-line was measured due to the low intensity of the Indium lines. The composition was chosen as it could form various crystal structures without any change in overall composition. The changes in the soft X-ray spectra (which monitor the s+d partial density of electronic states) were compared with the phases present and certain features of the spectra are identified with the presence of particular crystal structures. A distinct linear dependence was observed between the average electronic energy and the bandwidth at half maximum (BWHM) which is related to the heat treatment. We concluded that the stability of these phases has a different mechanism than the electronic stabilisation we have found in the metastable states in dilute alloys. The effects of alloying on the L 1–L 2,3 radiative transition were analysed and increased intensity was recorded, especially when the alloy was close to its equilibrium structure.

Ti L-Spectrum XANES and Electron Structure of the System Ti-Al-C

The electron structure and Ti L-absorption edge in the system Ti-Al-C have been studied theoretically using the local coherent potential of the multiple scattering theory. The calculated local partial electron density of states of Ti made it possible to study the influence of Al on the electron structure of this material and its Ti Ti L-absorption spectrum. It was shown that in the ternary system comparing with the binary titanium carbide the first maximum of the titanium unoccupied d-states nearly disappeared and the intensity of the second one fell more than twice. These calculations are in a good correspondence with the experimental TI L-Spectra and indicate that the contribution of Ti s-states to XANES can be neglected.

Soft-x-ray emission and the local p-type partial density of electronic states in Y2O3: Experiment and theory.

Photon-excited yttrium ${M}_{\mathrm{I}\mathrm{V},\mathrm{V}}$, and electron-excited oxygen $K$ x-ray emission spectra for yttrium oxide are presented. It is shown that, as in the case of yttrium metal, the decay of ${M}_{\mathrm{IV}}$ vacancies does not contribute substantially to the oxide ${M}_{\mathrm{I}\mathrm{V},\mathrm{V}}$ emission. The valence emission is interpreted in a one-electron picture as a measure of the local $p$-type partial density of states. The yttrium and oxygen valence emission bands are very similar and strongly resemble published photoelectron spectra. Using local-density approximation electronic structure calculations, we show that the broadening of the Y-$4p$ signal in yttrium oxide relative to Y metal are due to two inequivalent yttrium sites in ${\mathrm{Y}}_{2}$${\mathrm{O}}_{3}$. Features present in the oxide, but not the metal spectrum, are the result of overlap (hybridization) between the Y-$4p$ wave function and states in the oxygen $2s$ subband.

Angular momentum of conduction electron states

Core level line asymmetries for liquid Al, Ga, Ge, In, Sn and Bi have been determined by numerical analysis of XPS core level spectra using Doniach-Sunjic lineshapes. For the pure liquid metals, the asymmetry parameters α have been determined as: Al2p: 0.13, Ga3d: 0.11, Ge3d: 0.09, In4d: 0.12, Sn4d: 0.13 and Bi5d: 0.12. Nozière-De Dominicis theory is used to calculate core line asymmetries from the partial electronic densities of states at the Fermi energy. Good agreement is achieved for systematic changes in both parameters. Experimental Knight shifts of liquid metals are compared with theoretical values for s-p-metals. They support the theoretical s-electron density of states (DOS) data used for calculation. Furthermore a quantitative relation between asymmetry and total DOS at the Fermi energy is proposed to explain the observed increase of asymmetry in Ga upon melting: solid: α = 0.06, liquid: α = 0.11.

Analysis of nuclear spin-lattice relaxation rates in transition metal dihydrides, TMH2 (TM = Sc, Ti, V and Nb) based on LAPW electronic band calculations

The spin-lattice relaxation rates (T1eT)−1 of transition metal nuclei 45Sc, 49Ti, 51V and 93Nb in cubic dihydrides ScH2, TiH2, VH2 and NbH2 are calculated on the basis of scalar-relativistic self-consistent LAPW band structure calculations. It is found that the s and p electron contributions to the relaxation rates are negligibly small for all the hydrides studied here due to their very low partial densities of electron states at the Fermi level. The d- orbital contributions are dominant and the core polarization and d-dipolar contributions play a minor role. The theoretical relaxation rates reproduce the experiments very well for ScH2 and TiH2 but less satisfactory for VH2 and NbH2. The experimental data were taken from our earlier studies and the results of additional measurements on 93Nb in NbH2 are reported. These have been made in the 30–292K temperature range and at a magnetic field of 7 T.

Electron Energy Structure of Boron Nitride and Diamond. Similarities and Distinctions

AbstractWith the coherent potential method of the multiple scattering theory the electronic structure of boron nitride (in sphalerite modification) and diamond is calculated. The cluster approximation with muffin‐tin crystal potential is used. The electron structure and chemical bonding of these substances are investigated for the first time with this modern theoretical method. It is shown that the chemical bonding has mainly covalent character, the ionicity being about 20%. A fine structure of the upper part of the valence band of boron nitride is revealed and its nature is discussed. The similarity of the total electron state densities of the diamond and boron nitride in sphalerite modification is proved and the distinctions are discussed. The calculated local partial electron densities of states are used to explain the experimental X‐ray emission and X‐ray photoemission spectra of diamond and boron nitride.

The metastable states of some amorphous FeB alloys

The interdependence of the electronic and thermal properties and their effect on the stability of some Fe 86.5− x TM x B 13.5 (TM is transition metal, 0≦ x ≦5) alloys is investigated. Differential thermoanalysis and soft X-ray fluorescent spectroscopy were used to measure the thermodynamic phase changes and the partial electronic density of states, respectively. A general role of the electronic effects on the thermal stability is suggested from the trend of the measured data. It is concluded that the thermal stability is effectively controlled by the actual electronic structure of the alloy.

C K-V x-ray-emission spectra of solid C70 with comparison to C60.

Carbon K-V x-ray-emission spectra of solid C 70 and C 60 are measured, and the partial electron densities of states (DOS) are calculated by the discrete-variational (DV) Hartree-Fock-Slater (Xα) molecular-orbital (MO) method. The measured x-ray line shapes are compared with the calculated 2p DOS of C 70 and C 60 . The agreement between theory and experiment is satisfactory without any adjustable parameters in the DV-Xα MO calculations

Nuclear spin-lattice relaxation in transition metal compounds with local tetragonal symmetry

We report on the various contributions to the total spin-lattice relaxation rate in metallic materials with local tetragonal symmetry. The analytical formulae are given in the tight-binding approximation. The calculations show the relation between various partial electron densities of states and corresponding contributions to the relaxation rates. The presented formulae can be used to compare theoretically calculated electron band structure parameters with those obtained from NMR spin-lattice relaxation time measurements.