XPS and SAM studies of the hydrogen-absorbing ZrCr₂ compound

X-ray photoelectron spectroscopy (XPS) and scanning Auger microprobe (SAM) were utilized to investigate the surface layer of ZrCr₂ down to several hundreds angstroms. The surface depth profile shows initially a Zr-rich, Cr-defficient region. This deviation from the bulk stoichiometry is assumed to be due to a Zr segregation towards the surface boundary in the presence of atmospheric air and a formation of the appropriate oxide. Cr exists mainly in a metallic form even close to the external surface. These surface properties resemble those of other previously investigated hydrogen absorbing compounds. SAM measurements revealed local surface regions 10-20 microns wide which are rich either in ZrO₂ or metallic chromium. This surface decomposition may also contribute to the good hydrogen sorption kinetics. In addition, small and opposite shifts in binding energies of Zr and Cr in their intermetallic ZrCr₂ form were observed with respect to their pure metal state. This observation may be qualitatively explained in terms of charge transfer from the less to the more electronegative metal.