Ab initio LCAO study of the atomic,electronic and magnetic structures and the lattice dynamics of triclinic CuWO4

The electronic, structural and phonon properties of antiferromagnetic triclinic CuWO₄ have been studied using the first-principles spin-polarized linear combination of atomic orbital (LCAO) calculations based on the hybrid exchange–correlation density functional (DFT)/Hartree–Fock (HF) scheme. In addition, the local atomic structure around both Cu and W atoms has been probed using extended X-ray absorption fine structure (EXAFS) spectroscopy. We show that, by using the hybrid DFT–HF functional, one can accurately and simultaneously describe the atomic structure (the unit cell parameters and the atomic fractional coordinates), the band gap and the phonon frequencies. In agreement with our EXAFS results, the LCAO calculations reproduce a strong distortion of both the CuO₆ and the WO₆ octahedra, which occur due to the first-order and second-order Jahn–Teller effects, respectively. We found that the HF admixture of 13–16%, which is implemented in the PBE0–13% and WCGGA–PBE-16% functionals, produces the best result for CuWO₄. The calculated properties agree well with the available experimental data provided by diffraction, optical, X-ray photoelectron and Raman spectroscopies.     

Synthesis and hydrogen reduction of nano-sized copper tungstate powders produced by a hydrothermal method

The pure nano-sized copper tungstate (CuWO₄) powders were prepared by hydrothermal method and consequent annealing at 500 °C for 120 min. The thermogravimetric analysis was used to study dehydration processes, and the scanning electron microscopy (SEM) indicated that CuWO₄ particles were mostly spherical in the size range from 60 to 90 nm. Hydrogen reduction at 800 °C for 60 min converted the CuWO₄ to W–Cu composite powders. The hydrogen reduction results showed that nano-sized CuWO₄ particles calcining at 500 °C for 120 min indicated finer microstructure than the other calcination temperatures of 0 °C, 400 °C, 620 °C, 650 °C and 700 °C. W–Cu particles were observed finest and homogeneous in the size range from 90 to 150 nm by SEM images. Homogeneous distribution of W and Cu particles was clearly demonstrated by elemental mapping. Encapsulation of Cu phase by the W phase was observed by EDS and TEM. From FFT and HRTEM images, the orientation relationship of (01-1)_Cu ∥(01-1)_W and a semicoherent interface between W and Cu phases could be observed. A good correlation between the HRTEM image and the calculated lattice misfit (δ) was obtained.     

Energy band structure and density of states for BaBiBO4 nonlinear optical crystal

We present results of the band structure and density of states (DOS) for BaBiBO₄ nonlinear optical single crystals, using full potential linear augmented plane wave (FP-LAPW) method. We have for the first time, performed the experimental investigations of the electronic density of the states. We have found that this crystal is a semiconductor with an indirect energy band gap. Theoretical results of the DOS spectra are in good accord with the X-ray photoelectron spectroscopy measurements with respect to peak positioning. The band structure can be thought as arising from a superposition of the BO₃ and BiO₅ clusters possessing substantially different ionicities.     

Electronic structure of Pd42.5Ni7.5Cu30P20, an excellent bulk metallic glass former: Comparison to the Pd40Ni40P20 reference glass

In-house photoemission and inverse-photoemission spectra (PES and IPES) were measured on Pd_42.5Ni_7.5Cu₃₀P₂₀ and Pd₄₀Ni₄₀P₂₀ bulk metallic glasses in order to clarify the origin of excellent glass-forming ability from the viewpoint of electronic structure. Minima are observed for both the metallic glasses at a slightly higher energy than the Fermi level. Incident photon-energy dependent PES spectra were obtained using synchrotron radiation and the Pd 4d partial density of states (DOS) was estimated from the PES data. Soft X-ray emission spectra were also measured near the Ni and Cu 2p_3/2 absorption edges to evaluate, respectively, the Ni and Cu 3d partial DOS in the valence band. The Pd 4d and the Ni and Cu 3d partials in the conduction band were obtained from X-ray absorption spectra around the Pd 3p_3/2 and Ni and Cu 2p_3/2 absorption edges, respectively. It was found that the Pd 4d partial DOS near the Fermi energy largely decreases and becomes localized by replacing the Ni atoms with the Cu atoms, which may be closely related to the excellent glass-forming ability of the Pd_42.5Ni_7.5Cu₃₀P₂₀ bulk metallic glass due to a selective formation of Pd–P covalent bonds.     

Electronic structure and chemical bonding of zirconium disulfide

This paper involves the carrying out of the first-principle molecular-orbital calculations for ZrS₂ with a model cluster composed of 129 atoms. The nature and intensity of intra- and interlayer chemical bondings are also investigated by use of the bond overlap population. The valence band structure produced by this calculation agrees well with experimental results reported from X-ray photoelectron spectroscopy. Each peak in the density of states (DOS) is identified from the viewpoint of chemical bonding. There is a strong tendency toward covalent bonding between zirconium and sulfur atoms. The bonding energy between inter-layer S-S and intralayer S-S and Zr-Zr was much weaker than that between the Zr-S intra-layer bonding.     

Relaxations and bonding mechanism of arsenic in-situ impurities in MCT： first-principles study

1 Introduction Mercury cadmium telluride (Hg1?xCdxTe, MCT) is currently one of the most widely used semiconductor materials for infrared detector arrays. It is well known that the applications of the semiconductor materials, especially for Ⅱ-Ⅵ compound…     

Magnetic properties, crystal and electronic structure of GdNi5−xCux series

The effect of Ni/Cu substitution on the magnetic properties, crystal and electronic structure of the polycrystalline GdNi_5−xCu_x series has been studied. All compounds crystallize in the hexagonal CaCu5 type of crystal structure (space group P6/mmm). The temperature dependence of magnetic phase transition (T_mag) estimated from χ_AC(T) susceptibility as well as magnetization M(T) below room temperature indicates the maximum for x = 1.0 copper concentration. In the paramagnetic range (above 300 K) the magnetic susceptibilities follow a Curie-Weiss-type dependence. The effective magnetic moments are higher than theoretical value for free Gd³⁺.From X-ray photoelectron spectroscopy (XPS) data the valence band as well as the core level spectra have been analyzed. The filling of Ni3d band in the GdNi_5−xCu_x system by charge transfer of Gd conduction electrons is revealed by a reduction of the satellite intensities in the Ni2p core level spectrum. The obtained results exhibit that the valence bands at the Fermi level are dominated by hybridized Ni3d and Gd5d states, when Cu3d states are rather localized about 3 eV below the Fermi level. Quite good relation between the magnetic properties and electronic structure has been found.     

Existence, structure and valence properties of the skutterudites CeyFe4−xCoxSb12

We report on sample preparation, annealing effects, electron microprobe analysis in the series Ce_yFe_4−xCo_xSb₁₂ which shows that a phase separation occurs for substituted samples (0<x<4) annealed at 650 and 550 °C. Single phase samples are obtained for either Ce_yFe₄Sb₁₂ or Ce_yCo₄Sb₁₂ samples annealed at 650 °C and for all compositions when annealed at 700 °C. The valence state of Ce in homogeneous samples has been studied using X-ray absorption spectroscopy (XAS). Ce ions are trivalent throughout the series and the XAS spectra does not show effect of the crystal field on the 5d-final state.     

Theoretical study of structural, electronic, lattice dynamical and dielectric properties of SrAl2O4

We investigate the structural, electronic, lattice dynamical, and dielectric properties of SrAl₂O₄ within density-function theory. The crystal structure is fully relaxed, and the structural parameters are found to be well consistent with the experimental data. The first pressure derivatives of the bulk modulus are predicted to be 2.5 and 4.3 for local density approximation (LDA) and generalized gradient approximation (GGA), respectively. The electronic band structure shows that the valence band maximum is comprised of O 2p states and a small amount of Al 3s and 3p states, and the conduction band minimum is comprised of Sr 5s and a small amount of O 2p, Al 3s and Al 3p states. The phonon frequencies at the center of the Brillouin zone and the dielectric permittivity tensors are calculated using density-function perturbation theory. The electronic (?_∞) and static (?₀) dielectric permittivity tensors are theoretically predicted by the calculations with both LDA and GGA formalisms. The results show that the electronic dielectric permittivity is isotropic, while the static dielectric permittivity exhibits to be somewhat anisotropic due to the dominant ionic contributions in static dielectric permittivity.     

Oxidation of tungstate‐containing green rust (Cl–) in aqueous solution

The influence of tungstate on the oxidation of green rust [GR(Cl^–)], which contains both Fe(II) and Fe(III), was investigated by synthesizing suspensions of GR(Cl^–) containing tungstate and oxidizing them via injection of N₂ gas containing O₂. XRD and TEM analyses were used for characterizing the solid particles formed during synthesis and oxidation. The results showed that the formation of fine α‐FeOOH was enhanced by the addition of tungstate to the GR(Cl^–) suspensions, while GR(Cl^–) without tungstate was transformed primarily into γ‐FeOOH. The pH, oxidation‐reduction potential (ORP), and dissolved oxygen (DO) values of the aqueous solution were measured during oxidation of GR(Cl^–) with and without tungstate. The results showed that whereas the pH value of the solution was decreased and the ORP value was increased monotonically by oxidation of GR(Cl^–), the pH and ORP values during oxidation the GR(Cl^–) suspension containing tungstate revealed characteristic changes with time. XAS was also used for characterizing the chemical state and local structure of tungstate in the oxidized particles. The results indicated that the local structure of WO was essentially retained in the particles precipitated from GR(Cl^–) suspensions.     

An open aperture z-scan study of Sr2CeO4 blue phosphor

Sr₂CeO₄ blue phosphor has been prepared by the solid-state reaction method. The X-ray diffraction (XRD) study confirms the structure of the system to be orthorhombic. High resolution electron transmission microscopy reveals that Sr₂CeO₄ prepared by the solid state reaction method is composed of elongated spherical structures of length ∼0.2-0.6 μm and width ∼90-150 nm. The excitation spectrum shows a broad band which peaks at 275 nm. The emission spectrum shows a broad band which peaks at 467 nm when excited at 275 nm. The emission band is assigned to the energy transfer between the molecular orbital of the ligand and charge transfer (CT) state of the Ce⁴⁺ ion. The Commission International de l’Eclairage (CIE) co-ordinates are x = 0.15, and y = 0.23. The nonlinear absorption behavior of Sr₂CeO₄ has been investigated using the open aperture z-scan technique. The calculated effective two-photon absorption coefficient shows that the Sr₂CeO₄ blue phosphor is a promising optical limiting material.     

Synthesis, structure and optical properties of CoAl2O4 spinel nanocrystals

CoAl₂O₄ nanocrystals were synthesized by sol-gel method using citric acid as a chelating agent at low temperature. The as-synthesized samples were characterized by thermal analysis, X-ray powder diffraction, infrared spectroscopy and transmission electron microscopy. The results show that CoAl₂O₄ spinel is the only crystalline phase with a size of 10-30 nm in the temperature range 500-1000 °C. The temperature dependence of the distribution of Al³⁺ and Co²⁺ ions in the octahedral and tetrahedral sites in nanocrystals was investigated by X-ray photoelectron spectroscopy (XPS). It is observed that the inversion parameter decreases with increasing annealing temperature. Analysis of the absorption properties indicates that Co²⁺ ions are located in the tetrahedral sites as well as in the octahedral sites in the CoAl₂O₄ nanocrystals. The origin of the green color (300-500 nm absorption band) should be due to the octahedrally coordinated Co²⁺ ions.     

Electron spectroscopic studies on films of SnO2 and SnO2:Sb

Pure and antimony-doped SnO₂ films have been deposited by two different techniques: metal-organic chemical vapour deposition (MO-CVD) and spray pyrolysis of SnCl₄ in the presence of air with different concentrations of antimony. X-ray and UV photoelectron spectroscopic studies have been carried out on these films and the combined data from these studies provided information about the electronic properties of antimony-doped SnO₂ films and their compositions. That the binding energy of SnO is higher than that of SnO₂, contrary to expectations, is because of the fact that SnO₂ is more covalent than SnO. XPS has been used successfully to confirm the presence of Sb³⁺ and Sb⁵⁺ beyond a certain concentration of antimony (0.5 mol%) in the film, which is responsible for the decrease in conductivity and IR reflectivity. UV photoelectron spectroscopy of transparent, conducting SnO₂ films has been reported for the first time. The onset of photoemission in the valence band around 2.6 eV has been explained by invoking the idea of surface states near the valence band below the Fermi level.     

First principles study of the electronic and magnetic properties of semi-Heusler alloys NiXSb (X = Ti, V, Cr and Mn)

The electronic density of states (DOS), magnetic moments and band structure of semi-Heusler alloy NiXSb (where X = Ti, V, Cr and Mn) has been studied by using the first principles full-potential linearized augmented plane wave method (FP-LAPW) based on density functional theory (DFT). For the exchange and correlation potential generalized-gradient approximation (GGA) is used. From the observations, NiTiSb shows the possibility of half-metallic ferromagnet (HMF) behavior with a band gap of 0.53 eV and an effective moment of 0.35 μ_B. The alloys like NiVSb, NiCrSb and NiMnSb are HMF with band gap of 0.49 eV, 0.38 eV and 0.48 eV and an effective moment of 1.995 μ_B, 3.01 μ_B and 3.99 μ_B respectively. DOS and band structure result shows the 3d states of Ni overlap with 3d states of X atoms suggesting hybridization between them. The exchange-splitting of Ni-3d and X-3d state electrons lead to localized spin moment which determines the HMF behavior of NiXSb. The results obtained are compared and found to be in close agreement with the available data.     

Bismuth-containing semiconductors: Linear and nonlinear optical susceptibilities of GaAs1−xBix alloys

Using all electron full potential - linearized augmented plane wave (FP-LAPW) method the linear and nonlinear optical susceptibilities of cubic GaAs_1−xBi_x alloys with x varying between 0.25 and 0.75 with increment of 0.25 are investigated. We have applied the generalized gradient approximation (GGA) for the exchange and correlation potential. In addition the Engel-Vosko generalized gradient approximation (EVGGA) was used. The reflectivity, refractivity, absorption coefficient and the loss function of these ternary alloys were investigated. The absorption coefficient shows that GaAs_0.25Bi_0.75 possess the highest coefficient among the investigated alloys which supports our previous observation that the band gap decreases substantially with increasing Bi content and the materials with very small energy band gap possess the highest absorption coefficient. The investigation of the linear and nonlinear optical susceptibilities of GaAs_1−xBi_x shows a strong band gap reduction as commonly found experimentally.     

石墨烯/6H-SiC(0001)表面高质量USb2 薄膜的制备和表征

重费米子体系可以通过维度等调控手段来展现出丰富而有吸引力的量子基态。首次通过分子束外延技术在石墨烯/6H-SiC（0001）衬底成功制备了高质量的USb₂薄膜。结合反射式高能电子衍射、X射线衍射、电输运和X射线光电子能谱测量,证明了所制备的USb₂薄膜是高质量的单晶薄膜。此外,利用扫描隧道显微镜和角分辨光电子能谱对USb₂薄膜的表面形貌、原子结构和能带结构进行了表征。结果显示,生长的USb₂薄膜的表面原子结构、电输运性质和能带结构与块体USb₂单晶相似。最后,高质量USb₂薄膜的成功制备和表征为未来通过生长理想厚度的超薄膜在低维铀基重费米子系统中探索奇妙性能提供了宝贵的实验经验。     

Effect of N, C and B interstitial atoms on local bonding structure in mechanically activated TiH2/h-BN, TiH2/C, and TiH2/B mixtures

Mechanically activated TiH₂/h-BN, TiH₂/C and TiH₂/B mixtures was studied by temperature-programmed desorption, X-ray diffraction, transmission electron microscopy and X-ray emission spectroscopy. Ball milling in the presence of additives results in a modification of hydrogen occupation sites. Additional Ti-N, Ti-C or Ti-B bonds from chemical bonding of Ti with interstitial N, C and B atoms, are formed in TiH₂ due to contact of TiH₂ nanoparticles with the respective additive matrix materials. Mixed configurations around Ti atoms with proportional combination of local Ti-H and Ti-N, Ti-C or Ti-B bonds significantly decrease the thermal stability of TiH₂. The effect is most pronounced when boron is the additive.     

Characterisation of corrosion products formed on copper exposed at indoor and outdoor sites with high H2S concentrations

Abstract

Corrosion products formed on copper exposed indoors and outdoors at sites with high hydrogen sulphide (H₂S) concentrations were characterised using several analytical techniques. The crystalline corrosion products that formed on the copper exposed indoors were chalcocite (Cu₂S) and cuprite (Cu₂O), while those that formed on the copper exposed outdoors were chalcocite, cuprite and basic copper sulphates. Surface analysis by X-ray photoelectron spectroscopy revealed differences between the copper exposed indoors and outdoors that are explained by the composition, localisation and oxidation of the corrosion products. The surface morphologies of the corrosion products also differed. Elemental depth profiling by glow discharge optical emission spectroscopy revealed that the corrosion products that formed indoors were mainly chalcocite with cuprite only at and near the surface. In contrast, the corrosion products that formed outdoors were a mixture of chalcocite and cuprite. These differences in corrosion products are attributed to the differences in relative humidity during exposure.     

Magnetic properties and electronic structure of R3T4X4 compounds (R = Pr,Nd; T = Cu,Ag; X = Ge,Sn)

Polycrystalline samples of R₃T₄X₄ (R = Pr, Nd; T = Cu, Ag; X = Ge, Sn) intermetallics were investigated by means of magnetometric and XPS methods. All these compounds crystallize in the orthorhombic Gd₃Cu₄Ge₄-type structure. They were found to be antiferromagnets, except Pr₃Ag₄Ge₄ which is a collinear ferromagnet. Analysis of the XPS valence band data indicates strong overlaps of the Pr 4f states with the Cu 3d states and the Nd 4f states with the Ag 4d states. Analysis of the R 3d states on the basis of the Gunnarsson–Schönhammer model gave the information on the hybridization of the 4f orbital with the conduction band. The results showed that the Pr-based compounds are characterized by larger values of the hybridization energy than the Nd-based compounds.     

Electronic structure and transport properties of Fe–Al alloys

Electronic structure of iron-aluminides (Fe_1−xAl_x) has been calculated for a range of aluminum concentration (0⩽x⩽0.5) by using first principles density functional theory to explain the variation of electrical resistivity with increasing Al content. The Fe–Al intermetallics are modeled by a cluster of 15 atoms confined to their bulk geometry. The location of Al atoms as a function of concentration, x was determined by minimizing the total energy of the clusters. The electronic structure was determined by calculating the total as well as partial density of states around each of the Fe and Al atoms. With increasing Al concentration, the transfer of Al 3p electrons into the minority 3d orbital of Fe not only has a profound effect on the magnetic properties of these intermetallics, but affects their transport properties as well. For example, the observed anomaly in the electrical resistivity of Fe_1−xAl_x that peaks at x=0.33 is found to be a direct consequence of the filling of the Fe 3d orbital with Al valence electrons. The density of states is characterized by three distinct features: a narrow 3d band just below the Fermi energy originating from the Fe atoms, an Al s-band lying deeper in energy, and an Al p-band above the Fermi energy. The energy gap between Al 3p and Fe 3d density of states decreases with increasing Al concentration and for x=0.40, the density of states at the Fermi energy is a strongly hybridized p–d state giving Fe_1−xAl_x metallic-like properties. These features are consistent with the recent photoemission studies carried out at the synchrotron facility at Lawrence Livermore National Laboratory. An anomaly in the temperature dependence of electrical resistivity is also explained in terms of the unique electronic and magnetic structure of these intermetallics.