XPS and magnetization studies of cobalt sodium silicate glasses

Cobalt sodium silicate glasses with the chemical composition (0.70 - x)SiO2-(0.30)Na2O-xCoO, where 0.0 ≤ x ≤ 0.20, have been investigated by means of X-ray photoelectron spectroscopy (XPS) and magnetization techniques. The Co 2p spectra show intense satellite structures ∼ 6 eV above the photoelectron peak and the Co separation was ∼ 15.9 eV for all the samples studied. These observations indicate the presence of high spin Co2+ ions in the glasses. The Co 3p spectra have been fitted with contributions from octahedral and tetrahedral Co2+ and the ratio [Co2+(oct)]/[CoTotal2+] increases with increasing CoO content. The O Is spectra also show composition-dependent changes. The fraction of non-bridging oxygen atoms was determined from these spectra and was found to increase with increasing cobalt oxide. Co(II) ions are found to be incorporated in the glass as network modifiers but the contribution from SiOCo(II) to the non-bridging part of the O Is signal could be separated from that from SiONa by simulating the spectrum. DC magnetic susceptibility measurements were performed on the same samples but these suggest that Co2+ exists mainly in octahedral coordination. The magnetic data indicate that the exchange interaction is antiferromagnetic and increases with increasing CoO in the glass.