Crystal chemistry of layered carbide, Ti3(Si0.43Ge0.57)C2

 The crystal structure of a layered ternary carbide, Ti₃(Si_0.43Ge_0.57)C₂, was studied with single-crystal X-ray diffraction. The compound has a hexagonal symmetry with space group P6₃/mmc and unit-cell parameters a=3.0823(1) Å, c=17.7702(6) Å, and V=146.21(1) Å³. The Si and Ge atoms in the structure occupy the same crystallographic site surrounded by six Ti atoms at an average distance of 2.7219 Å, and the C atoms are octahedrally coordinated by two types of symmetrically distinct Ti atoms, with an average C-Ti distance of 2.1429 Å. The atomic displacement parameters for C and Ti are relatively isotropic, whereas those for A (=0.43Si+0.57Ge) are appreciably anisotropic, with U₁₁ (=U₂₂) being about three times greater than U₃₃. Compared to Ti₃SiC₂, the substitution of Ge for Si results in an increase in both A-Ti and C-Ti bond distances. An electron density analysis based on the refined structure shows that each A atom is bonded to 6Ti atoms as well as to its 6 nearest neighbor A site atoms, whether the site is occupied by Si or Ge, suggesting that these bond paths may be significantly involved with electron transport properties.     

Structural and electronic properties of U2Ti: A first principles study

Using the state of the art first principles calculation, we report the structural, electronic, and thermodynamic properties of U₂Ti, U, and Ti. All calculations have been performed using a plane wave (PW) based pseudopotential method under the framework of spin polarized density functional theory. The electron-ion interaction and the exchange correlation energy are described using the projector-augmented wave (PAW) method and the generalized gradient approximation (GGA) scheme, respectively. The effect of the relativistic spin-orbit interaction on these properties has been investigated. The results are analyzed to obtain the structural parameters, lattice constants, bulk moduli, electronic specific heat, and the compound formation energy. On the basis of energetics, the formation energy (Δ_fH at 0 K) of U₂Ti compound is estimated to be −30.84 kJ/mol. A satisfactory agreement between the present investigation and available experimental data demonstrates the applicability of PW based PAW approach for such systems. Further, the nature of chemical bonding between U and Ti atoms in the U₂Ti is illustrated by comparing their electronic density of state spectrum.     

Many-body effects in core-level spectroscopy of rare-earth compounds

Many-body effects in core-level photoemission and core-level photoabsorption are discussed for rare-earth systems, especially for Ce and La compounds, both in metallic and insulating forms. Emphasis is put on effects of metallic mixed valency and insulating covalency of 4f electrons on these spectra. For the insulating compound CeO₂, detailed analyses of the 3d core photoemission (3d-XPS) and the 2p core photoabsorption (L₃-XAS) are presented by using the impurity Anderson model with a filled valence band. In order to give a consistent interpretation for 3d-XPS and L₃-XAS, it is shown to be essential to take account of the Coulomb interactions U _fd (between a 4f electron and a photoexcited 5d electron in the L₃-XAS) and -U _dc (between the 5d electron and a core hole), in addition to -U _fc (between the 4f electron and the core hole). Discussions are given on the physical information derived from the analysis, on similarities and differences in spectral features between insulating and metallic systems, and also on some related topics.     

A photoelectron and energy-loss spectroscopy study of Ti and its interaction with H2, O2, N2 and NH3

A unified electron spectroscopic study of polycrystalline Ti and its interaction with H₂, O₂, N₂, and NH₃ are described. Auger electron spectroscopy (AES), electron energy-loss spectroscopy (ELS), ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS) are combined to provide detailed information about the electronic structure of the titanium surface and its interaction with these adsorbates. X-ray and ultraviolet photoelectron spectra and electron energy-loss spectra are presented for the clean titanium surface, and following exposure to H₂, O₂, N₂ and NH₃. Spectral assignments are provided in each case. The electron spectra of oxygen exposed Ti and nitrogen sputtered Ti are quite similar, and are interpreted with reference to band structure calculations for TiO and TiN. Electron spectroscopy indicates essentially complete dissociative adsorption of NH₃ on the clean titanium surface.     

A DFT + U description of oxygen vacancies at the TiO2 rutile (1 1 0) surface

Experimental observations indicate that removing bridging oxygen atoms from the TiO₂ rutile (1 1 0) surface produces a localised state approximately 0.7 eV below the conduction band. The corresponding excess electron density is thought to localise on the pair of Ti atoms neighbouring the vacancy; formally giving two Ti³⁺ sites. We consider the electronic structure and geometry of the oxygen deficient TiO₂ rutile (1 1 0) surface using both gradient-corrected density functional theory (GGA DFT) and DFT corrected for on-site Coulomb interactions (GGA + U) to allow a direct comparison of the two methods. We show that GGA fails to predict the experimentally observed electronic structure, in agreement with previous uncorrected DFT calculations on this system. Introducing the +U term encourages localisation of the excess electronic charge, with the qualitative distribution depending on the value of U. For low values of U (?4.0 eV) the charge localises in the sub-surface layers occupied in the GGA solution at arbitrary Ti sites, whereas higher values of U (?4.2 eV) predict strong localisation with the excess electronic charge mainly on the two Ti atoms neighbouring the vacancy. The precise charge distribution for these larger U values is found to differ from that predicted by previous hybrid-DFT calculations.     

Elektronenspin-Resonanz und Photochemie des U2-Zentrums in Alkalihalogenid-Kristallen

The photochemical reactions involved in the production and in the bleaching of U₂ centers (interstitial hydrogen atoms) in KCl, KBr and NaCl crystals are investigated by measuring the optical absorption spectra of the crystals at low temperatures. The U₂ centers are prepared by UV irradiation of hydroxide- and hydride-doped crystals. The electron spin resonance of KCl, KBr and NaCl crystals with U₂ centers at low temperatures reveals a well-resolved hyperfine splitting due to interactions of the hydrogen atom with four neighbouring halogen ions. The U₂ center can be described as a tetrahedal [HX₄]^4?-molecule ion, in which one hole, bound mainly at the hydrogen atom and partially at the surrounding halogen ions, gives rise to the spin resonance.     

Electronic structure studies of Mg0.95Mn0.05Fe2−2xTi2xO4 (0x0.8)

The electronic structure of Mg_0.95Mn_0.05Fe_2−2xTi_2xO₄ (0x0.8) compound is investigated using near edge X-ray absorption fine structure, (NEXAFS) spectroscopy measurements, carried out at O K, Fe and Ti L_3,2-edges at room temperature. The O K-edge spectra indicate that the Fe 3d orbitals have been considerably modified and a new spectral feature start dominating in the pre-edge region at higher Ti doping. The Fe 2p NEXAFS spectra exhibit a mixed valent Fe²⁺/Fe³⁺ states apart from the conversion of Fe³⁺ to Fe²⁺ with the substitution of Ti ions. The Ti L_3,2-edge spectra indicate that Ti ions remain unchanged at 4+ state. These variations in the host electronic structure due to Ti substitution are consistent with the dielectric and transport properties of the material.     

Raman investigations of TiO2 nanotube substrates covered with thin Ag or Cu deposits

A tubular array of TiO₂ nanotubes on Ti matrix was used as a support for Ag or Cu sputter‐deposited layers intended for surface‐enhanced Raman scattering (SERS) investigations. The composite samples of Ag/TiO₂–nanotube/Ti and Cu/TiO₂–nanotube/Ti were studied with the aid of scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) [and scanning Auger microscopy (SAM)] to reveal their characteristic morphological and chemical features. Raman spectra of pyridine (as a probe molecule) were measured after it had been adsorbed on the TiO₂–nanotube/Ti substrates covered with thin Ag or Cu deposit as well as on the bulk electrochemically roughened Ag or Cu reference substrates. It was found that the SERS spectra measured for pyridine adsorbed on the bulk silver substrate were significantly different than the spectra measured on the TiO₂–nanotube/Ti substrates covered the Ag layer. The spectra measured for pyridine adsorbed on the Ag/TiO₂–nanotube/Ti suggest that on the surface of such a composite substrate there are many Lewis acidic sites. Spectra typical for pyridine adsorbed on acidic sites were observed even after deposition of a relatively thick silver layer (e.g. an Ag layer with an average thickness of 80 nm) on the TiO₂–nanotube/Ti support. Our findings suggest that TiO₂–nanotube/Ti support is a promising substrate for the preparation of metallic nano‐clusters on a support containing acidic active sites. Copyright © 2009 John Wiley & Sons, Ltd.     

Comparative Raman study of the Ti complex Cp2Ti(η2-C60) · C6H5CH3 and TixC60 films

Raman spectra from the first Ti fullerene complex Cp ₂Ti(η²-C₆₀) · C₆H₅CH₃ are presented. Compared to spectra of pure C₆₀, the spectra of the Ti complex exhibit a number of new peaks due to the symmetry lowering for C₆₀. The A _g(2) mode is downshifted by 12 cm⁻¹ compared to C₆₀, which corresponds to a charge transfer of one electron per Ti-C₆₀ bond. This value (6 cm⁻¹ for one transferred electron) is identical to the downshift of the A _g(2) mode in alkali metal fullerides with ionic bonding. The spectra of Cp ₂Ti(η²-C₆₀) · C₆H₅CH₃ were compared to the spectra of evaporated Ti_xC₆₀ films. The A _g(2) mode in Ti₄C₆₀ showed a downshift of about 25 cm⁻¹ compared to pure C₆₀, which corresponds to a charge transfer of one electron per Ti atom; this is similar to the ionic alkali metal fullerides and different from η²-C₆₀-type bonding. From Fizika Tverdogo Tela, Vol. 44, No. 3, 2002, pp. 483–485. Original English Text Copyright ? 2002 by Talyzin, Jansson, Usatov, Burlakov, Shur, Novikov. This article was submitted by the authors in English.     

Effect of copper and cobalt impurities on the electronic structure and optical spectra of the intermetallic compound PrNi5

The electronic structure and optical properties of the intermetallic compound PrNi₅ and their evolution during the substitution of copper or cobalt atoms for nickel atoms have been investigated. The band spectra of the studied compounds have been calculated in the local spin density approximation corrected to account for strong electron-electron interactions in the 4f shell of the rare-earth ion (LSDA + U method). The dispersion relations of the optical conductivity in the interband light absorption region have been interpreted using the results of calculations of the electron density of states.     

Effect of bias voltage and nitrogen pressure on the structure and properties of vacuum-arc (Mo + Ti6%Si)N coatings

Effect of deposition conditions in reactive nitrogen atmosphere on the growth morphology, phase composition, structure, and mechanical characteristics (microhardness) of vacuum-arc multilayer coatings obtained using evaporation of the (Ti6%Si) and Mo cathodes is studied with the aid of raster electron microscopy, energy-dispersive elemental microanalysis, and microindentation. It is demonstrated that nitrogen atoms are redistributed to the region of the strongest nitride-forming element (Ti) in relatively thin layers (about 7 nm) consisting of substances with substantially different heats of formation (−336 kJ/mol for TiN and −34 kJ/mol for MoN). Such a process leads to lamination with the formation of nitride TiN and metal Mo (weaker nitride-forming element). Nitrogen–metal bonds are saturated in the layers of strong nitrideforming elements Ti(Si) when the nitrogen pressure increases from 6 × 10^–4 to 5 × 10^–3 Torr in the condensation procedure. Thus, the compound is filled with nitrogen to the stoichiometric composition and, then, the second system of layers based on molybdenum is saturated with nitrogen with the formation of the γ-Mo₂N phase. An increase in bias potential U _SP from–100 to–200 V stimulates mixing in thin layers with the formation of the (Ti, Si, Mo)N solid solution and leads to a decrease in microhardness from 37 to 32 GPa.

     

The behavior of 3d electrons and defects in TiAl-based alloys containing V and Cu studied by positron annihilation

Information of defects and 3d electrons in transition metals (Ti, V, Cu) and TiAl-based alloys (Ti₅₀Al₅₀, Ti₅₀Al₄₈V₂, Ti₅₀Al₄₈Cu₂) can be extracted from the positron lifetime and coincidence Doppler broadening spectra. The results show that the 3d electron signals for the transition metals Ti, V and Cu increase with the number of 3d electrons. The 3d electron signal and the electron density for binary TiAl alloy are relatively low due to the (Ti)3d-(Al)3p interactions. The addition of V and Cu atoms to TiAl alloy leads to the increase in the electron densities in bulk and the defects on grain boundaries simultaneously, as well as the enhancement of the 3d electron signal. The 3d electron signal in the spectrum of Ti₅₀Al₄₈Cu₂ alloy is higher than that of Ti₅₀Al₄₈V₂ alloy.     

In situ spectroelectrochemical surface-enhanced Raman scattering (SERS) investigations on composite Ag/TiO2-nanotubes/Ti substrates

A tubular array of TiO₂-nanotubes on a Ti substrate was used as a support for an Ag sputter-deposited layer intended for surface-enhanced Raman scattering (SERS) investigations. Composite samples of Ag/TiO₂-nanotube/Ti were studied with the aid of scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) to reveal their characteristic morphological and chemical features. Raman spectra of pyridine (as a probe molecule) were measured at different cathodic potentials ranging from −0.2 down to −1.2 V after the pyridine had been adsorbed on the TiO₂-nanotube/Ti substrates covered with the Ag deposit. In addition, SERS spectra on a bulk electrochemically-roughened Ag reference substrate, were also measured.The SERS activity of the composite samples was strongly dependent on the amount of Ag deposit and, in some cases, was even higher than that for the Ag reference substrate. The SERS intensity vs. electrode potential dependences measured were interpreted in terms of the modified electronic structure of the Ag deposits due to the interaction of the Ag clusters with the TiO₂-nanotube/Ti substrate.     

Conductivity-dependent cathodoluminescence in BaTiO3, SrTiO3 and TiO2

It is reported that similar cathodoluminescence spectra are excited by an electron beam striking BaTiO₃, SrTiO₃ and TiO₂ ceramics at room temperature. The energy location of the luminescence bands does not depend on various doping or reduction treatments. The luminescence intensity increases with the electron beam current as well as with the conduction electron density. The luminescence is interpreted as a fundamental transition of local character in the TiO₆ octahedron; the conduction electrons localized at the Ti sites in polaron states recombine with the 0–2p valence electron defects. The shape and energy location of the luminescence spectra are qualitatively in accordance with an explanation in terms of a configuration coordinate model.     

Surface evolution of a gradient structured Ti in hydrogen peroxide solution

In this work, the interaction between hydrogen peroxide (H₂O₂) and a gradient structured Ti was investigated extensively. The gradient structured Ti (SMAT Ti) was produced by surface mechanical attrition treatment (SMAT), and then it was immersed in H₂O₂ solution for different time until 48 h at room temperature (25 °C). The structure and surface morphology evolution were examined by Raman spectra and scanning electron microscopy (SEM). The formation mechanism of nanoporous titania was discussed based on above results.     

Electronic structure of the valence band for perovskite-type titanium double oxides studied by XPS and DV-Xα cluster calculations

XPS spectra of the valence bands for the perovskite-type titanium double oxides BaTiO₃, SrTiO₃ and CaTiO₃ have been measured and analyzed by means of DV-Xα calculations for the TiO₆ embedded cluster model. The theoretical photoelectron spectra modulated by the photoionization cross-sections are in good agreement with experiment. The XPS results show that the O 2p valence band is constructed of two peaks whose spacing becomes larger in the order BaTiO₃ < SrTiO₃ < CaTiO₃. The DV-Xα results indicate that the greater part of the lower-energy peak is attributed to the levels which have O 2p orbitals pointing to Ti cations and are thus stabilized by the electrostatic potential concomitant with a decreased TiO bond distance. The electrostatic potential also reduces the O 2p-Ti 3d mixing and, together with the repulsion of the electron cloud between the Ti and O ions, makes the TiO bond more ionic.     

动力学对称群方法对三原子分子高激发振动态的理论研究

利用动力学对称群方法研究了弯曲三原子分子的振动高激发态能谱-该方法显示：三原子分子的动力学对称性为U₁(4)U₂(4),则三原子分子的Hamiltonian量可写成代数的各元素之和,通过李代数处理而求得分子代数Hamiltonian量的本征值,进而得到分子的振动能谱-并具体计算了O₃分子- 关键词：     

High-order above-threshold ionization in a laser field: Influence of the ionization potential on the high-energy cutoff

High-energy above-threshold ionization by a strong linearly polarized laser field is analyzed using the strong-field approximation and the quantum-orbit formalism. An analytical formula for the cutoff is derived. The maximal electron kinetic energy is 10.007U _P + 0.538I _P. The first term with the ponderomotive energy U _P is equivalent to the known classical cutoff law, while the second term is a new result and depends on the ionization potential I _P. The validity of this formula is confirmed by comparison with the numerical results for the high-energy electron spectra.     

Photoemission investigations on nanostructured TiO2 grown by cluster assembling

Nanostructured titanium dioxide (ns-TiO₂) films were grown by supersonic cluster beam deposition method. Transmission electron microscopy demonstrated that films are mainly composed by TiO₂ nanocrystals embedded in an amorphous TiO₂ phase while their electronic structure was studied by photoemission spectroscopy. The cluster assembled ns-TiO₂ films are expected to exhibit several structural and chemical defects owing to the large surface to volume ratio of the deposited clusters. Ultraviolet photoemission spectra (hv = 50 eV) from the valence band unveil the presence of a restrained amount of surface Ti 3d defect states in the band gap, whereas Ti 2p core level X-ray photoelectron (hv = 630 eV) spectra do not manifestly disclose these defects.     

EELS characterization of TiN grown by the DC sputtering technique

Titanium nitride thin films were deposited on monocrystalline silicon (mc-Si) substrates by direct current reactive magnetron sputtering. Auger electron spectra (AES) of deposited films at different nitrogen partial pressures, show the typical N KL₂₃L₂₃ and Ti L₃M₂₃M₂₃ Auger transition overlapping. Also, changes in the Ti L₃M₂₃M₄₅ Auger transition peak are observed. X-ray diffraction and high resolution electron microscopy (HRTEM) of a golden color TiN/mc-Si sample, reveal a preferential polycrystalline columnar growth in the 〈111〉 orientation. This sample was also analyzed by electron energy-loss spectroscopy (EELS). The N/Ti elemental ratio is slightly different to the value determined by AES. Atomic distribution around the N atoms is in agreement with that expected from the N atom in the fcc unit cell of TiN. This distribution was obtained via an extended energy-loss fine structure (EXELFS) analysis from EELS spectra.