Crystallographic-orientation dependence of the early-stage oxidation of Zr single crystals: an XPS study of suboxide formation and in-depth distribution
The influence of crystallographic orientation on the early stages of the oxidation of pure Zr single crystals was studied by X ray photoelectron spectroscopy (XPS). Two samples, cut from the same a-Zr single crystal, were oxidized simultaneously at room temperature and O2 pressures of 1 X 10-8, 1 X 10-7 and 1 X 10-6 torr: one with its surface normal parallel to the c-axis (Z1, basal (0001) plane) and the other with its normal 12deg from that of a prismatic plane (Z2, near-prismatic orientation). At all three pressures the oxidation kinetics of both samples followed a three stage, logarithmic type behaviour, but the near prismatic sample (Z2) incorporated oxygen faster and to a greater extent than the basal sample (Z1) in every case. Deconvolution of the Zr 3d core level spectra resolved, in addition to metallic Zr and ZrO2, two sub-stoichiometric Zr O compounds, denoted (ZrO)a and (ZrO)b, with binding-energy shifts of 1.3 and 2.3 eV with respect to metallic Zr, consistent with sub oxides previously reported for polycrystalline Zr. Angle resolved XPS showed that ZrO2 is the outermost compound in both orientations, while the two sub-oxides are distributed nearly homogeneously through the film. Oxide thicknesses, calculated from the attenuation of the metallic Zr 3d signal, ranged from 10 to 13 A for Z1 and from 14 to 19 A for Z2 depending on the oxidation pressure, in close agreement with earlier measurements on cold worked polycrystalline Zr, whose surface texture is dominated by prismatic-oriented grains.