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Surface science studies of Oxidized zirconium

University of British Columbia

Oxidized zirconium surfaces have been studied by X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED). The aim is to provide a fundamental understanding of the chemistry and structure of such surfaces as well as for metal-oxide interfaces; the motivation is to help establish understanding in relation to problems associated with nuclear reactor pressure tube corrosion. The chemistry of a thin film (-20 Å) of oxidized zirconium was studied by XPS. The film, prepared by depositing zirconium onto Au foil in the presence of a H₂0 atmosphere in the low 10⁻⁹ Torr range, has an outer region of Zr0₂and inner regions containing a lower oxidation state material, ZrOx, as well as Zr-Au alloy. Initially both ZrOx and the Zr-Au alloy are oxidized by either H₂0 or 0₂ at 300°C, although this process is hindered as the Zr0₂layer gets thicker. However, even with the protective oxide layer, heating in 4xl0"7 Torr D₂(with a partial pressure of H₂0 at around lxlO"9 Torr) can result in all the zirconium being converted to the +4 oxidation state; the process is apparently facilitated by migrating D atoms. The evolution of structure at the Zr(0001) surface, after exposure to 0₂and an ordering anneal at 220°C, has been studied systematically using LEED crystallography. Additionally, a new analysis was undertaken for the clean Zr(0001) surface, which is confirmed to have a regular hep-type structure, with a slight contraction (about 1.6%) in the first Zr-Zr interlayer spacing with respect to the bulk value (2.57 Å). The surface formed by a 0.5 monolayer (ML) of O manifests a (2x2)-type LEED pattern. A detailed analysis showed a novel structure with 0.25 ML of O at octahedral hole sites in (2x2) arrays, both between the first and second metal layers and between the second and third layers; these O arrays are displaced laterally from one another, apparently to minimize the 0...0 repulsions. The incorporated O atoms induce vertical and lateral relaxations in the metallic structure, which are most significant in the second metal layer. The average O-Zr bond length of 2.28 Å is close to the value (2.30 A) in bulk ZrO, which also has 6-coordinated O atoms. For the surface formed by 1 ML of O at Zr(0001), the LEED analysis indicates a structural model where 0.5 ML of O is distributed randomly over octahedral holes between the first and second metal layers, with another 0.5 ML between the second and the third layers. The structural type changes for 2 ML of O at Zr(0001); now O bonds at 1 ML coverage in overlayer hollow sites of three-fold coordination, while there is another 1 ML of O atoms in tetrahedral hole sites between the first and second metal layers. The stacking sequence, designated as (C)B(A)AB.., corresponds to the first three layers of anion-terminated cubic Zr0₂, although some lateral compression is needed for superposition on the regular hep Zr structure. The absorption of O in tetrahedral holes between the first and second metal layers results in a significant expansion to about 3.44 Å. The O-Zr bond lengths are estimated to equal 2.07 Å for the overlayer O atoms, and 2.22 Å for the O atoms in tetrahedral hole sites.

Thesis/Dissertation

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2009-04-06

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http://hdl.handle.net/2429/6818

Chemistry

University of British Columbia

UBCV

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