LEED crystallographic studies of oxygen adsorbed on the (0001) surface of zirconium
Creators: name: Hui, Ka-Chung
Publisher: name: University of British Columbia
Year: 2010

description: The work in this thesis includes LEED crystallographic investigations for the (2x2) structure obtained by adsorbing oxygen on the (0001) surface of zirconium at below 250°C, a study of the stability of the intensity of fractional order beams with different coverages of adsorbed species on hcp(0001) metal surfaces, a preliminary structural investigation of the Zr(0001)-(1x1)-O structure, and the setting up of a TV camera system for faster acquisition of diffracted beam intensities. The LEED crystallographic study for oxygen adsorption on the Zr(0001) surface yields the first structural data for the initial stages of oxidation on any hcp(0001) surface. The zirconium surface was cut from a high purity single crystal, and characterized with LEED and Auger electron spectroscopy. Intensity versus energy (I(E)) curves were measured by the photographic method at normal incidence for seven and two diffracted beams respectively for the (2x2) and (1x1) structures. Theoretical I(E) curves were calculated using the 'combined space' approach to multiple scattering calculations (e.g. composite layer calculations, renormalized forward scattering and layer doubling) for a range of Zr-O interlayer spacings and many different adsorption models. Levels of correspondence between experimental and calculated 1(E) curves were assessed with the Pendry R-factor. The analyses suggest that O atoms occupy octahedral holes between successive bulk Zr layers in the (2x2) structure, and that the substrate Zr layers undergo a fee type reconstruction which spans the depth of O atoms. The Zr-O interlayer spacing is found to be 1.33 Å; this corresponds to a Zr-O bond length of 2.29 Å, and therefore is in close agreement with the value (2.31 Å) for bulk ZrO. Having only two independent beams for the (1x1) surface precludes any definitive structural conclusions, but the preliminary analysis here indicates that the first three layers resemble (111) layers of ZrO with the Zr-O interlayer spacing equal to 1.37 Å and a consequent bond length of 2.31 Å. Diffracted beam 1(E) curves were calculated at normal incidence with the renormalized forward scattering method for (2x1) and (2x2) structures involving adsorption on (0001) surfaces of titanium and zirconium, and these data were compared with the R-factors of Pendry and of Zanazzi and Jona. This study supports an observation by Yang et al. that LEED fractional order beam intensities may be closely constant with changing adsorbate coverage; a new feature found here is that this conclusion holds even for structures where neighboring adsorbed atoms are separated by just the substrate interatomic distance. This observation could be exploited in the Zr(0001)-(2x2)-O analysis by using whichever translational symmetry that gave the easier computational effort. This work provides further support for the neglect-of-beam-set approximation, which was introduced by Van Hove et al. for making more tractable the calculation of LEED intensities from surfaces with large unit meshes. Also established in this work is an intensity acquisition system, which utilizes a surveilance type TV camera and commercial video LEED analyzer (VLA). Initial examinations were given for two background intensity subtraction schemes. I(E) data collected with the video LEED system in the later stages of this work are compared satisfactorily with the independent measurements obtained by the photographic method.