X-ray photoemission studies of PtSn and PtPb bimetallic systems

The Pt(5d) and Pt(4f) in PtSn and PtPb composites produced by sequential vapor deposition have been examined by X-ray photoelectron spectroscopy. At low concentrations of Pt, the Pt(5d) becomes a single, narrow resonance at about 4 eV below the Fermi level. Its density of states at the Fermi level is small. This leads to a large suppression of the Pt(4f) lineshape asymmetry. As the Pt concentration increases, d-d interactions split the Pt(5d) resonance and increase the 5d density of states at the Fermi level by pushing one of the split resonances to the Fermi level. Consequently, the Pt(4f) lineshape asymmetry increases. The evolution of the Pt(4f) and Pt(5d) in PtSn and PtPb are very similar but different from that of small Pt particles supported on carbon. The close similarity of the electronic properties of Pt in PtSn and PtPb suggests that the poisoning of Pt by Pb is not entirely due to electronic effects although there are indications of stronger interaction between Pt and Pb than between Pt and Sn atoms. The strong surface enrichment of Pb on PtPb points to the importance of the physical blockage of Pt atoms by surface Pb in determining the poisoning effect of Pb.     

Studies on PtNi alloys: AES and photoemission determination of the surface composition and interaction with oxygen and hydrogen

Three PtNi alloys with 20, 40 and 70 at% Ni were studied by Auger and photoelectron spectroscopies. The clean surfaces were found to be enriched in platinum in an amount increasing with increasing platinum concentration in the bulk. On the contrary oxidation treatments resulted in a nickel surface segregation which was only slightly offset after reduction. Chemical shifts characteristic of oxidation were observed on XPS peaks of both platinum and nickel under experimental conditions where pure platinum could not be oxidized.     

X-ray photoelectron diffraction study of an oxidized PtCo(001) alloy surface: evidence for atomic scale rippling of the CoO overlayer

The overlayer of cobalt monoxide that forms upon low pressure oxidation of the (001) plane of a PtCo alloy was studied by X-ray photoelectron diffraction and other surface techniques. The analysis of the XPD data carried out using model calculations in the single scattering-plane wave approximation shows that the overlayer structure is similar to that of the (001) plane of fcc CoO, with the Co atoms displaced at different heights from the substrate in correspondence to the type of site occupied.     

Surface tension and density of liquid Bi–Pb,Bi–Sn and Bi–Pb–Sn eutectic alloys

Surface tension and density measurement of liquid Bi₅₆Pb₄₄, Bi₄₃Sn₅₇ and Bi₄₆Pb₂₉Sn₂₅ eutectic alloys was carried out by using the large drop method over the temperature range of 380–750 K. The regular solution model has been used in conjunction with Butler's equation to calculate the surface tension of binary and ternary alloys of the Bi–Pb–Sn system, while the surface tension of ternary alloys has also been predicted by using geometric models. The new experimental results were compared with the calculated values of the surface tension as well as with the data available in the literature.     

The interaction of water with SiMo interface. A core and valence photoemission investigation

Core level photoemission with MgKα radiation (hv = 1253.6 eV) and valence photoemission (hv = 21.2 eV) are presented for Si(1 1 1)-Mo interfaces at various Mo coverages (up to 3.2·10¹⁵ at cm⁻²) exposed to 6000 L of H₂O. The metal deposition strongly enhances the oxidation of Si with the complete H₂O dissociation and the formation of a SiO₂-like compound. The results are consistent with an interface growth with the formation of silicide islands.     

A study of core and valence levels in β-PbO2 by hard X-ray photoemission

The core and valence levels of β-PbO₂ have been studied using hard X-ray photoemission spectroscopy (hν = 6000 eV and 7700 eV). The Pb 4f core levels display an asymmetric lineshape which may be fitted with components associated with screened and unscreened final states. It is found that intrinsic final state screening is suppressed in the near-surface region. A shift in the O 1s binding energy due to recoil effects is observed under excitation at 7700 eV. It is shown that conduction band states have substantial 6s character and are selectively enhanced in hard X-ray photoemission spectra. However, the maximum amplitude in the Pb 6s partial density of states is found at the bottom of the valence band and the associated photoemission peak shows the most pronounced enhancement in intensity at high photon energy.     

Thermodynamics and phase diagrams of MoPdRu and related systems

During the irradiation of nuclear fuels up to 10% of the original fissile material (uranium and plutonium) is converted into a wide range of fission products. One particular group of fission-product elements, the transition metals Mo, Tc, Ru, Rh and Pd, are not oxidised at the quite low oxygen potentials existing in the fuels, and are precipitated in the fuel in the form of metallic inclusions. The thermodynamic properties and phase diagram of these alloys are of importance, since the precise composition of these alloys, as a function of radial position can provide important information concerning operating conditions inside a fuel pin. For this reason a thorough reassessment of these systems is under way. This paper reports values of the excess-Gibbs energies of mixing in the MoPd, PdRu and MoRu binary systems, and in the MoPdRu ternary. Calculated sections of the MoPdRu system at selected temperatures are presented.     

Enhanced electrical transport in LiBrLiI mixed crystals and LiBrAl2O3 composites

LiBrLiI mixed crystals and LiBrAl₂O₃ composites have been studied by means of complex impedance analysis an conductivity, X-ray diffraction, DTA and SEM techniques. The substitution of wrong size I⁻ ions in LiBr increases the conductivity and decreases the migration energy of Li-ion vacancies. These results are consistent with those of the KBr-KI system and earlier predictions. LiBrAl₂O₃ composites exhibit a sharp increase in the conductivity. The highest conductivity obtained was ≈10⁻³ Ω⁻¹ cm⁻¹ at 302°C for LiBr + 10 m/o Al₂O₃.     

Temperature dependence of the Mössbauer effect on the semiconductors Pb0.78Sn0.22Te and Pb0.80Sn0.20Te:In

Crystals of the semiconductors Pb_0.78Sn_0.22Te and Pb_0.80Sn_0.20Te:In were grown by Bridgman method, and investigated by Mössbauer spectroscopy and X-ray diffraction. Mössbauer spectra were taken at temperatures between 80 and 300 K. The absence of the quadrupole splitting shows a cubic symmetry of the environment for the tin atom, which is in accordance with X-ray data. The temperature dependence of the spectral area is discussed and the Debye temperature is estimated.     

Absolute composition depth-profiles in surface segregation of PtRh alloys

The absolute composition depth profiles of the first 12 atomic layers of the (001) surface and the first 9 layers of the (111) surface of a PtRh alloy have been measured. As found in earlier studies, the surface is enriched with Pt, the sub-surface layer is depleted of Pt, and the Pt concentration oscillates toward the bulk value. The detailed oscillations are, however, very different for the two surfaces. Using a simple model for the metallic bonds, Monte Carlo simulations are performed to extract a set of bond energy differences of the constituent atom pairs from the experimental data.     

Calculated local density X-ray and photoemission spectra for superconducting La2−xMxCuO4: Localization of Cu-3d

Comparison of UPS (XPS) and XES spectra for the high T_c superconductor La_2−xM_xCuO₄ - calculated using highly precise local density results - with available experimental data reveals a large shift of about 2 eV to lower energies for the Cu-d related features. This shift is interpreted qualitatively in terms of a simple two-level model taking into account the hole-hole repulsion at the Cu sites in the photoemission final state. This simple model explains nicely the magnitude of the shift, the occurence of a Cu related satellite below the valence-bands, and the charge-transfer screening of the Cu-d photo-holes by O-p electrons. The O-states, especially the bonding states at the bottom of the valence band, are less affected and agree reasonably well with the experimental findings.     

Mαβ X-ray production cross sections of Pb and Bi by 9-40 keV electron impactMαβ X-ray production cross sections of Pb and Bi by 9-40 keV electron impact

The experimental data of Ma/3 X-ray production cross sections for Pb and Bi by 9-40 keV electron impact have been given. Thin films with thick carbon substrates are used in the experiment. The effects of target structure on the Ma/3 X-ray production cross sections are corrected by using the Monte Carlo method. The corrected experimental data are compared with calculated cross sections in terms of the distorted-wave Born approximation （DWBA） theory. The measured Ma/3 X-ray production cross sections for Pb and Bi are lower than the DWBA calculations. The atomic relaxation parameters used in comparing the DWBA values with experimental results affect the degree of difference.     

The superconductivity,structure and microstructure of BaYScCu oxides

We added small amount of Sc (0.05–0.35at.%) to BaYCu oxide and found that the superconductivity deteriorated drastically when the Sc content reached 0.325at.%. X-ray and electron diffraction showed that the orthorhombic perovskite structure changed to multi-phase structure with unidentified phases. The microstructures observed by TEM are of needle like morphology. Their phases are not yet identified.     

Energy dependence of the 16O208Pb effective interaction

The Coulomb-nuclear interference in the excitation probability of the 2.615 MeV (3⁻) state of ²⁰⁸Pb by ¹⁶O at θ_CM=172° for bombarding energies 57 MeV ⩽E_lab⩽79 MeV has been studied. The data are described by an effective interaction which has a behaviour similar to that expected from the dispersion relation connecting the real and imaginary parts of the generalized optical potential.     

Lattice dynamics and debye temperature of AlCu,AlSi and AlGe alloy systems

The lattice dynamics and specific heat at constant volume for AlCu alloy system is studied using our previous treatment based on the microscopic electronic theory. Considering the volume and electron density effect on the dynamical matrix of the pure constituent, we obtain the band and local mode frequencies at the temperature-dependent specific heat in the Al_1−x Cu_x solid solution. Then, using the data about the mean elastic wave and the specific heat extrapolated to the absolute zero temperature, the Debye temperature is presented for the Al_1−xCu_x, Al_1−xSi_x and Al_1−xGe_x alloy systems. The Debye temperature decreases as a function of the Cu atomic fraction x for Al_1−xCu_x alloy, and remarkably for Al_1−xSi_x and Al_1−xGe_x solid solutions.     

Local density theory of X-ray and photoemission from YBa2Cu3O7−δ: The high Tc superconductor

Photoemission, inverse photoemission and X-ray emission spectra have been calculated using results of a highly precise local density energy band study. Compared to experiment, we find a shift of ∼2 eV to higher binding energy for the Cu-d and the O-pπ bonding states. This shift is attributed to the lack of metallic screening for the Cu-d holes which leaves only charge transfer screening by O ligand electrons. The larger shift of the O-pπ states close to E_F compared to those lying lower in energy is attributed to different degrees of localization. By contrast, the lower lying O-pσ bonding states, which show greater itinerant character than the higher lying pπ states, are less affected by relaxation effects.     

Electron diffraction and electron microscopic study of BaYCuO superconducting materials

Electron microscopy and electron diffraction have been applied to show that the orthorhombic phase in the compound Ba₂YCu₃O_7−δ is responsible for the high superconducting transition temperature. A positive correlation is found between the volume fration of the orthorhombic phase and the superconducting transition temperature. By means of an “in-situ” heating experiment it is found that the orthorhombic phase is formed on cooling from a high temperature tetragonal phase with disordered vacancies. It is suggested that the low temperature tetragonal phase that occurs in the same specimens as the orthorhombic phase also contains an ordered arrangement of vacancies different from that present in the orthorhombic phase. The order-disorder transition associated with the structural vacancies is shown to be reversible, provided there has been no oxygen loss.     

Adsorption of CO and H2 on Pt(110)(1 × 2) studied by photoemission of adsorbed xenon (PAX)

The adsorption of hydrogen and carbon monoxide on Pt(110)(1 × 2) at 97 K has been studied by photoemission of adsorbed xenon (PAX). For both species, it appears that adsorption takes place first in the valleys between the microfacets of this reconstructed surface. Only when these sites are saturated, does adsorption take place on other sites (microfacets or ridges) of the surface.