Surface X-ray Diffraction Analysis Research Articles

Structure of theAl13Co4(100) surface: Combination of surface x-ray diffraction andab initiocalculations

The structure of the quasicrystalline approximant ${\mathrm{Al}}_{13}{\mathrm{Co}}_{4}$(100) has been determined by surface x-ray diffraction (SXRD) and complementary density-functional-theory (DFT) calculations. Thanks to the use of a two-dimensional pixel detector, which speeds up the data acquisition enormously, an exceptionally large set of experimental data, consisting of 124 crystal truncation rods, has been collected and used to refine this complex structure of large unit cell and low symmetry. Various models were considered for the SXRD analysis. The best fit is consistent with a surface termination at the puckered type of planes but with a depletion of the protruding Co atoms. The surface energy of the determined surface model was calculated using DFT, and it takes a rather low value of $1.09\phantom{\rule{4pt}{0ex}}{\mathrm{J}/\mathrm{m}}^{2}$. The results for the atomic relaxation of surface planes found by SXRD or DFT were in excellent agreement. This work opens up additional perspectives for the comprehension of related quasicrystalline surfaces.

Research on Humidity Resistance of Sodium Silicate Sand Hardened by Twice Microwave Heating Process

To study the high moisture absorbability of the sodium silicate boned sand hardened by microwave heating, a new special water-based coat sprayed on the surface of the sand samples hardened by the first microwave heating and then sintered by the second microwave heating to form a compact coating layer was studied. The results showed that humidity resistance of the sand samples with the coating layer had great improvement compared to the sand samples without the coating layer, the compression strength (σ 4h ) of the sand samples with the coating layer increased 2.62 times than that without the coating layer. Scanning electron microscope (SEM) and X-ray diffraction (XRD) have been used to investigate phase and composition of the compact coating layer. The SEM analysis showed that the coated sand samples formed the compact layer which could greatly improve humidity resistance of the sand samples. The line scanning analysis confirmed that the coat composition appeared on the surface of the coated sand samples, and the surface XRD analysis demonstrated that the surface of the coated sand samples consisted of four new phases, including PbTiO3, NaAlO2, Pb6Al2Si6O21 and Al2SiO5, which are the main materials for increasing humidity resistance.

Surface X-ray diffraction analysis of Fe nanostructured films grown on c(2 × 2)-N/Cu(100)

We report the results of a Surface X-Ray Diffraction (SXRD) study of Fe nanostructured films deposited on c(2×2)-N/Cu(100) at room temperature (RT), with Fe coverage ΘFe=0.5ML and ΘFe=1ML. The c(2×2)-N/Cu(100) surface is an example of self-organised system, that can be used for growth of arrays of metal nano-islands and organic molecules assemblies. We chose two different values of N coverage, ΘN=0.3ML and ΘN=0.5ML, the second value corresponding to N saturation. We monitored the presence of surface diffraction peaks in hk scans and we performed Crystal Truncation Rods (CTR) analysis with ROD fitting programme. In the case of ΘN=0.5ML, i.e. at saturation coverage, the CTR could be fitted with one surface domain with p4gm(2×2) symmetry. In the surface cell adopted, N atoms occupy four-fold hollow sites, with Fe (intermixed with Cu) giving rise to a “clock” reconstruction previously observed on iron nitride films obtained by co-deposition and annealing. This result is an indirect confirmation of N surface segregation on top of the Fe films, occurring during the growth at RT. When subsaturation N coverage (ΘN=0.3ML) is used as a substrate for Fe deposition, the best results could be obtained with a model where two surface domains are present: the first one corresponds to a surface cell with Fe sitting in four-fold hollow sites on bare Cu areas, with possible interdiffusion in the second lattice. The second domain is assigned to growth of Fe on the N-covered square islands occurring once the bare Cu areas are fully covered. The SXRD analysis on N-covered surface domains shows that the mechanism of reconstruction and of N segregation on top layer is already active at RT for all N-coverage values.

Catalytically Active Structure of Bi Deposited on a Au(111) Electrode for the Hydrogen Peroxide Reduction Reaction

The surface structure of underpotentially deposited Bi has been determined on Au(111) in perchloric acid solution using surface X-ray diffraction (SXD), scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. SXD analysis and STM images reveal that the catalytically active structure for the hydrogen peroxide reduction reaction (HPRR) is the (2 x 2)-Bi honeycomb structure with theta(Bi) = 0.50. The stability is supported by DFT calculations. The hydrated perchlorate anion is located in the center of the honeycomb structure without hydrogen peroxide. DFT calculations predict that the Bi honeycomb structure promotes the dissociation of the O-O bond of hydrogen peroxide. Hydrogen peroxide expels the hydrated perchlorate anion, and then HPRR takes place at the honeycomb center.

Strained Pd films, by epitaxial growth on Au( [formula omitted]), to control catalytic properties

Pd ultra thin films on Au(1 1 0)-(1×2) have been prepared at 300 K by vapour deposition and studied using medium energy electron diffraction (MEED), surface X-ray diffraction (SXRD), low energy electron diffraction, low energy ion scattering (LEIS) and Auger electron spectroscopy (AES). The (1×2) superstructure intensities are strongly attenuated during the deposition of the first 0.5 ML. Simultaneously, LEIS indicates that gold atoms cover a large fraction of the deposited Pd atoms. A quantitative SXRD analysis of the 0.5 ML Pd film reveals a significantly ordered interface alloy covered by two incomplete Au layers. When more Pd is deposited up to about three layers, the growth proceeds in a quasi layer by layer mode, while the surfactant effect of gold persists. In this range of thickness, the film, mainly composed of Pd atoms, is under tensile strain in coherent epitaxy with the Au(1 1 0) substrate.

Periodic lattice distortion accompanying the charge-density-wave transition for Sn/Ge(111)

The periodic lattice distortion (PLD) accompanying the charge-density-wave (CDW) transition (at -60 \ifmmode^\circ\else\textdegree\fi{}C) in the \ensuremath{\alpha} phase of Sn on Ge(111) has been determined by combining the distinct sensitivities of low-energy electron diffraction (LEED) and surface x-ray diffraction (SXRD). New LEED $I\ensuremath{-}V$ data combined with a SXRD analysis yield a significant lattice distortion. The PLD accompanying the CDW transition is a 0.37-\AA{} vertical rippling of the Sn atoms accompanied by a perpendicular (0.17 \AA{}) and parallel (0.12 \AA{}) distortion of the first-layer Ge atoms, consistent with a band Jahn-Teller-like distortion.