Surface analysis of transition metal oxalates: Damage aspects

The behavior of transition metal oxalates in vacuum, under X-ray irradiation and low-energy Ar+ ion bombardment was studied. A comparative mass-spectrometric analysis was carried out of gas evolution from the surface of Mn, Co, Ni and Cu oxalate hydrates in vacuum, during exposure to X-rays and after termination of X-ray irradiation. The rates of H2O and CO2 liberation from the oxalates were found to be in an inverse correlation with the temperatures of dehydration and decomposition, respectively. X-ray photoelectron spectroscopy (XPS) was employed to study the X-ray induced damage in NiC2O4 and CuC2O4 by measuring the various XP spectral characteristics and surface composition of the oxalates as a function of time of exposure to X-rays. It was shown that Cu oxalate underwent a significantly faster degradation than Ni oxalate and demonstrated a high degree of X-ray induced reduction from the Cu2+ to the Cu1+ chemical state. 500 eV Ar+ sputter cleaning of CoC2O4 for 10 min was found to cause a strong transformation of the oxalate structure which manifested itself in an appreciable alteration of the XP core-level and valence band spectra. The analysis of changes in stoichiometry and comparison of XP spectra of bombarded oxalate with respective spectra of a reference carbonate CoCO3 implied that the bombardment-induced decomposition of CoC2O4 gave rise to the formation of CoO-like and disordered CoCO3-like phases.