Mechanism of W(CO)6 sonolysis in diphenylmethane

The present work analyses the mechanism of W₂C/C nanocomposite formation during sonolysis of W(CO)₆ in diphenylmethane (DPhM) solutions. Carbon supported WC_x nanoparticles attract much interest as an alternative fuel cell electrocatalysts. Sonolysis of neat DPhM under the effect of 20 kHz power ultrasound in argon at 80 °C yields a sonopolymer as a solid product and acetylene, hydrogen, methane, diacetylene and benzene as gaseous products. Diacetylene is formed due to the secondary sonochemical dimerisation of acetylene obtained at the primary stage of DPhM sonolysis. FTIR and μ-Raman studies show that the sonopolymer consists of a mixture of some polymeric partially oxidized aromatic species, and disordered carbon. Sonolysis of W(CO)₆ in diphenylmethane solutions follows the first order kinetics. This process yields monodispersed 2-3 nm X-ray amorphous WC_x nanoparticles embedded in amorphous sonopolymer. The annealing of air sensitive as-prepared solids in an inert atmosphere at 600 °C causes formation of stable W₂C/C nanocomposite with W₂C average particle size in the range of 4-7 nm and hexagonal carbon fine particles with the average size of 30-40 nm. Kinetic study revealed that tungsten carbide is formed inside the cavitation bubble due to the reaction of tungsten nanoparticles originated from primary sonolysis of W(CO)₆ with acetylene produced as a result of diphenylmethane sonochemical degradation.     

Phase composition and the micro- and macrostructures of quasi-crystalline powders obtained by plasma-chemical synthesis

Quasi-crystalline Al₆₅Cu₂₃Fe₁₂ powders have been obtained by the method of plasma-chemical synthesis. Coarse particles have a spherical form and an inhomogeneous structure because of the laminar growth of crystallites toward the surface of the particles. The influence of heat treatment on phase formation and structuring in Al–Cu–Fe alloys has been studied. It has been found that the heat treatment of as-prepared powders changes the phase relationship in the quasi-crystalline alloy.     

Nano-designing of Mg doped phosphate tungsten bronzes and SiO2 composite obtained by ultrasonic spray pyrolysis method

In this study, the structure and substructure of SiO₂–Mg phosphate tungsten bronzes, MgPTB, (MgHPW₁₂O₄₀ · 29H₂O) obtained by ultrasonic spray pyrolysis method from a silica sol, and a MgPTB solution, obtained by the ion exchange method, as precursors were investigated.The mechanism of the formation of aerosol droplets is discussed. Phase composition, structure and substructure of SiO₂–MgPTB particles were investigated by X-ray powder diffraction (XRPD) analysis, transmission electron microscopy (TEM), and scanning electron microscopy (SEM).Good agreement between the theoretically predicted values for the mean diameters of particles and subparticles (1.27 μm and 75.4 nm, respectively) and the experimentally obtained ones (1.17 μm and 65–90 nm) was found.This agreement confirms the applicability of the model to get a satisfactory prediction of the most important data related to the nano-structural design of SiO₂–MgPTB powders.     

Surface vibrations of CO on W(100)

High resolution electron-energy-loss spectroscopy has been used to study the surface vibrations of CO on a W(100) surface at 300 K. For small exposures (β-CO) two losses at ～68 meV and ～78 meV are observed. This vibrational spectrum of β-CO is a clear indication of dissociative adsorption with the carbon and oxygen atoms in fourfold coordination sites each. With further exposure to CO two additional losses at 45 meV and 258 meV are observed, which represent the vibration of undissociated α-CO in upright position on top of a W atom. Furtheron results of coadsorption of H₂/CO and O₂/CO on W(100) are reported.     

On the possible role of magnetism on the stability of Re dimers on W(110)

We have investigated the possible occurrence of magnetism for Re monomers and dimers on W(110). We find that the Re monomer is most likely magnetic whereas the dimer is not. This could play a role in the instability, observed experimentally, of Re₂ dimers on this surface.     

Adsorption of CO on W(001) by high resolution electron spectroscopy

The vibrational modes induced by CO on W(001) at temperatures ? 350 K are detected by means of electron energy loss spectroscopy with resolution in the 6–7 meV range. Two β adsorption regimes are identified depending on coverage. Heating at various increasing temperatures reveals coverage dependant irreversible surface structure modifications. The β spectra after adsorption or desorption are discussed in terms of the usual questions of multiple β states, dissociation, and reconstruction. The α₁ and α₂ states are detected both by their WC and CO frequencies. A small signal is assigned to a new a-state, named α₃, which may explain some thermal desorption results.     

Theoretical debye-waller factors for the (100) and (110) surfaces of tungsten

The ratios of the atomic mean square displacements at the (100) and (110) surfaces to those in the bulk have been calculated as a function of temperature for the body-centered cubic crystal tungsten. The calculations have been carried out for a slab-shaped crystal in which the atoms interact through central potentials up to fourth neighbors and through a harmonic angle-bending interaction involving pairs of nearest neighbors. The required averages over the two-dimensional Brillouin zone were obtained using sets of special points which are most efficient in finding such averages. A comparison between the theoretical results and the available experimental data is given.     

Absolute coverage measurements: CO and oxygen on the (110) plane of tungsten

An effusion source, calibrated with a vibrating quartz microbalance, has been used to determine absolute coverages of CO and of oxygen adsorbed on a tungsten (110) plane by an extension of the field emitter detector method: The amount of gas reflected from the substrate is measured as a function of the absolute amount impinged per unit area; maximum coverage in the chemisorbed layer can be obtained very directly from this information. Work function increments vs. absolute coverages were measured in the same apparatus by the vibrating condenser method. Results were as follows: For virgin CO, adsorbed at 100 K, the maximum coverage obtained was CO/W = 0.71 ; this leads to a maximum coverage for beta or beta-precursor CO of CO/W = 0.28. The maximum work function increment for virgin CO was 0.8 eV. For oxygen, adsorption at 100 or 300 K a decrease in sticking coefficient by several orders of magnitude occurs when the coverage is O/W = 0.5. Adsorption at 20 K leads to the molecular precursor noted by Leung and Gomer, which converts to atomic oxygen at <50 K. By adsorbing at 20 K the O atom coverage can be increased to O/W = 0.62. Work function versus coverage data for this system are also presented.     

Adsorbate characterisation by correlation of various photoelectron spectroscopies: CO on W(110)

As a test of the value of various electron spectroscopies and their combination for the characterisation of adsorption states, UPS valence spectra, XPS core spectra (O (1s) and C (1s)) and core satellite spectra (O (1s)), and X-ray induced Auger spectra (O KLL) were measured for various adsorption layers of CO on W(110) prepared at and above room temperature and, for comparison, of oxygen on the same surface. Virgin- and β-CO can readily be distinguished in all four kinds of spectra, while α-CO shows spectra very similar to those of virgin-CO. The conversion of virgin- to β-CO and their desorption can be followed in some detail. For all four techniques, the oxygen derived spectra of β-CO are identical to those of adsorbed oxygen, at about half the intensity. This makes it very likely that CO is dissociated in the β-layer on W(110). Virgin- and α-CO show the typical features of molecularly-adsorbed CO.     

A comment on the W(001) surface stability and reconstruction

Arguments are presented in favour of an additional softening which those changes of surface force constants that lead to a pronounced “soft” surface phonon mode (e.g. M₅) can invoke in the global surface phonon spectrum. This result is illustrated for the W, Mo and Cr (001) surface. Analysis of phonons of an isolated layer is used together with a “perturbation theorem”.     

Electronic changes of dysprosium and tungsten during growth of Dy on W(100)

Dy growth on W(100) has been studied with W 4f_7/2 core-level shift (CLS) and Dy/W(100) valence band analysis. CLS measurements of the W 4f_7/2 peaks during the growth resulted in three Dy-induced features at 31.24±0.02, 30.92±0.02 and 30.74±0.02 eV binding energies. The shift in the W 4f_7/2 peak is consistent with the reconstruction seen in the Dy thin film. Valence band study of Dy/W(100) showed an interfacial hybridization between Dy and W(100) which may be responsible for the high thermal stability of Dy superstructures on W(100). The same direction of shift in Dy 4f levels and the W 4f_7/2 level during growth suggest that these shifts are mainly strain dependent. Increasing the Dy overlayer thickness to more than 2 ML resulted in Dy 5d–6s hybridization which has been attributed to the formation of islands of Dy.     

On the continuous eigenvalues of the Lippmann-Schwinger operatorK(W)

The continuous eigenvalues of the Lippmann-Schwinger operatorK(W)=G ₀ (W)·V at real negative energyW are investigated. It is shown that for energies in the stability region of the system the continuous spectrum is located inside the unit circle. As an application of this result a method for computing the groundstate energy of a threenucleon system is described.     

名义组分为Y_6Ba_(11)Cu_(16)O_x的纳米超导材料

采用柠檬酸盐热分解法制备了名义组分为Y6Ba11Cu16Ox的纳米超导材料。利用XRD对其粉体进行物相表征,发现其相是Y123和Y211的混合相,并测得两者之间的摩尔比为1∶0.14,同时把其XRD图谱与Y1Ba2Cu3O7-x的XRD图谱进行了对照分析。利用标准的四端引线法对其块体进行R-T分析,测出其起始转变温度Tonset=86.0K,转变宽度△Tc=6.7K,并表现出良好的超导电性,也相应地把其与Y1Ba2Cu3O7-x进行了对比。根据Scherrer公式计算得到样品的平均晶粒尺寸约为67.1nm。文中也分析了所制材料为混合物的原因。     

Diffusion behavior of tungsten clusters on the tungsten (110) planes

Diffusion of tungsten diatomic clusters on the W (110) exhibits comparable prefactor (1.6×10^-4 cm²/sec) as single W atoms, but with slightly higher activation energy. Clusters with three atoms are unstable with respect to diffusion.     

Surface structures of W(110) and W(100) faces by the dynamical LEED approach

By means of an efficient dynamical LEED method, the surface structures of W(110) and W(100) faces are examined. It is found that the W(110) surface maintains the bulk structure, despite the possibility for the top tungsten atoms to settle into sites of higher coordination number. The W(100) face exhibits a possible contraction of the top atomic layer by 0.10 ± 0.10 Å. These results fit into a trend correlating a top-layer contraction with the absence of close-packing in the top layer, i.e. with the roughness of the surface.     

固态六羰基钼的FT-拉曼光谱研究

固态六羰基钼的ＦＴ┐拉曼光谱研究李淑玲（地矿部岩矿技术研究所北京１０００３７）任玲（北京理工大学化学与材料学院北京１０００８１）ＦＴ┐ＲａｍａｎＳｐｅｃｔｒａｏｆＳｏｌｉｄＭｏ（ＣＯ）６ＬｉＳｈｕｌｉｎｇ（ＩｎｓｔｉｔｕｔｅｏｆＲｏｃｋａｎｄＭｉｎｅ...     

Theoretical study of the stability of 5d dimers on W(211) and Ta(211)

It is known experimentally that Re₂ dimers are stable on W(211) whereas they are unstable on W(110). In the framework of the model previously used for 5d dimers on W(110), we explain this different behaviour by the larger coordination number on the (211) face, which tends to increase the stabilizing role of the one electron contribution and to decrease the destabilizing role of electronic correlations. A maximum of stability is obtained for dimers in the middle of the 5d series.