J. Rundgren

PtxNi1−x(111) alloy surfaces: structure and composition in relation to some catalytic properties

The surface structure inside the (111) face of two substitutional alloys, Pt0.5Ni0.5 and Pt0.78Ni0.22 is investigated by LEED. In both cases we find an important compositional oscillation across the three outermost layers. The concerned Pt concentrations (at%) are 88 ± 2, 9 ± 5 and 65 ± 10 for Pt0.5Ni0.5 and 99 ± 1, 30 ± 5 and 87 ± 10 for Pt0.78Ni0.22. This oscillation is a new and important element to be accounted for in the discussion of the specific chemisorptive and catalytic properties, particularly as regards the hydrogenation of 1,3-butadiene.

Oxygen chemisorption on aluminum single crystals: Site determination by LEED studies

Abstract The interaction of oxygen with aluminum single crystal surfaces at room temperature has been studied using LEED (low-energy electron diffraction). Disordered phases of aluminum oxides are rapidly formed on the (100) and (110) crystal faces. On the (111) face an ordered overlayer of oxygen atoms exhibiting (1 × 1) symmetry is developed at monolayer coverage. LEED intensity curves from this structure have been recorded for three inequivalent beams. The experimental curves are compared with theoretical ones obtained from a dynamical (multiple scattering) calculation. Theoretical intensity curves have been calculated for different possible chemisorption sites with the observed symmetry. The occurence of oxygen atoms at the 3-fold, 3-fold chemisorption sites on the (111) surface at a distance of 1.33 A above the surface was found to give good agreement between the calculated and measured intensity curves for all the beams considered. This chemisorption site is discussed in the light of other theoretical and experimental findings for the same system.

Atomistic screening mechanism of ferroelectric surfaces: an in situ study of the polar phase in ultrathin BaTiO3 films exposed to H2O.

The polarization screening mechanism and ferroelectric phase stability of ultrathin BaTiO(3) films exposed to water molecules is determined by first principles theory and in situ experiment. Surface crystallography data from electron diffraction combined with density functional theory calculations demonstrate that small water vapor exposures do not affect surface structure or polarization. Large exposures result in surface hydroxylation and rippling, formation of surface oxygen vacancies, and reversal of the polarization direction. Understanding interplay between ferroelectric phase stability, screening, and atomistic processes at surfaces is a key to control low-dimensional ferroelectricity.

The structure of the p(2×2) tellurium overlayer on the (001) surface of copper investigated by leed

Abstract Low-energy electron diffraction (LEED) intensities have been calculated for the p(2 × 2) tellurium overlayer on the (001) surface of copper and compared with measurements. The tellurium atoms are found to sit in the hollow position (coordination number 4) 1.70 ± 0.15 A above the atomic plane of the uppermost copper layer. This distance corresponds to a copper-tellurium bond length of 2.48 ± 0.10 A which indicates a value less than 2.65–2.67 A typical of copper-tellurium bulk compounds. Five different potentials were used for the adsorbate atoms and a potential from overlap of atomic charge densities with the Kohn-Sham-Gaspar expression for the exchange clearly gave the best agreement with experiment.

Reconstruction of the Pt50Ni50(100) surface: a LEED and STM study

The structure of the Pt50Ni50(100) surface was investigated by low energy electron diffraction (LEED) and scanning tunnelling spectroscopy (STM). Superstructures corresponding to 12 A— 1 and 19 A— ...

The surface stoichiometry of VC0.80(100) studied by LEED

Abstract The surface structure of substoichiometric vanadium carbide was studied by quantitative low-energy electron diffraction on a VC 0.80 (100) sample. A simple 1 × 1 diffractogram was observed on annealing to 1000°C. Optimization in four- to seven-dimensional structural spaces gave two surface structures, close to one another with respect to the agreement between theoretical and experimental spectra but widely separated by the structure and composition of the topmost VC layer. In the primary (secondary) model the top layer is rippled with the C subplane protruding 0.88 ± 0.05 A ( 0.20 ± 0.15 A ) from the V subplane and with a C occupancy 0.4 ± 0.2 (0.2 ± 0.2). The primary model is supported by a previous photo-emission study of VC 0.80 (100), where a strong 1s core level signal from the surface C atoms was observed. Characteristic of VC 0.80 (100) is the expansion of the first interplanar spacing of the C sublattice, while for the (100) surfaces of TaC, HfC and VN 0.89 a contraction of the first interplanar spacing of the metal sublattice predominates.

General aspects of beam threshold effects in LEED

Abstract We present in this paper some general aspects of beam threshold effects in LEED concerning their observation, their extension and their detection. The observation is possible on LEED intensities but also on crystal to ground current and total reflected current. We discuss the various types of profiles due to threshold effects for LEED intensities. From universal relations coming from emergence conditions of a new beam we draw universal charts for the “speed of emergence” versus E , θ or φ. These charts are used for a discussion of the range in I ( E ) or I (φ) profiles, where threshold effects may affect the LEED intensities. The same relations are used for a discussion of the detection of threshold effects. Assuming a Gaussian distribution (in energy and angles) for the incident beam, we derive a “total equivalent experimental resolution” and we discuss the consequence on intensity profiles and on the optimal conditions of detection.

Manifestations of Broken Symmetry: The Surface Phases of Ca2-xSrxRuO4

The surface structural phases of Ca(2-x)SrxRuO4 are investigated using quantitative low energy electron diffraction. The broken symmetry at the surface enhances the structural instability against the RuO6 rotational distortion while diminishing the instability against the RuO6 tilt distortion occurring within the bulk crystal. As a result, suppressed structural and electronic surface phase transition temperatures are observed, including the appearance of an inherent Mott metal-to-insulator transition for x=0.1 and possible modifications of the surface quantum critical point near x(c) approximately 0.5.

Gas-Tight Oxides - Reality or Just a Hope

A better understanding of the transport properties of gases in oxides is certainly very important in many applications. In the case of metals, a general protection measure against corrosion implies formation of a dense metal oxide scale. The scale should act as a barrier against gas transport and consequently it needs to be gas-tight. This is often assumed but rarely, if ever, confirmed. Hence there is a need for characterization of micro- and/or meso- pores formed especially during the early oxidation stage of metallic materials. This paper presents a novel and relatively straightforward method for characterization of gas release from an oxide previously equilibrated in a controlled atmosphere. The geometry of the sample is approximated to be a plate. The plate can be self-supporting or constitute a scale on a substrate. A mathematical model for calculation of diffusivity and gas content is given for this geometry. A desorption experiment, involving a mass spectrometer placed in ultra high vacuum, can be used to determine diffusivity and amount of gas released with aid of the mathematical model. The method is validated in measurements of diffusivity and solubility of He in quartz and applied in characterization of two Zroxides and one Fe oxide. From the outgassed amounts of water and nitrogen the H2O/N2 molar ratio can be used to estimate an effective pore size in oxides.

Adsorption structure determination of a large polyaromatic trithiolate on Cu(111): Combination of LEED-I(V) and DFT-vdW

The adsorption geometry of 1,3,5-tris(4-mercaptophenyl)benzene (TMB) on Cu(111) is determined with high precision using two independent methods, experimentally by quantitative low energy electron diffraction (LEED-I(V)) and theoretically by dispersion corrected density functional theory (DFT-vdW). Structural refinement using both methods consistently results in similar adsorption sites and geometries. Thereby a level of confidence is reached that allows deduction of subtle structural details such as molecular deformations or relaxations of copper substrate atoms.