X-ray photoelectron spectroscopy and magnetic studies of transition metal silicate glasses
X-ray photoelectron spectroscopy (XPS) has proved to be a powerful technique in the investigation of the local structure of oxide glasses. In this project, XPS and magnetisation techniques were used to study sodiun silicate glasses doped with various amounts of 3d transition metals. The first glass system had the compositional formula (0.70-x)SiO2-0.30Na2O-xFe2O3 where 0.0 <= x <= 0.20. The quantitative ratio [Fe2]/[FeTotal], for each glass has been determined from the analysis of the Fe 3p spectra. For low Fe2O3 content both iron valencies are present, however, it was found that Fe3+ is the predominant species for high Fe2O3. From the analysis of the O 1s spectra, it was possible to discriminate between bridging and non-bridging oxygen atoms in each glass sample. It was found that the ratio of the non-bridging oxygen content to the total oxygen content increases with increasing iron concentration. It has also been shown that the nonbridging oxygen contribution to the O 1s spectra can be simulated by summing the contributions from SiONa, SiOFe(II) and SiOFe(III) components present in the glass. The second glass system had the compositional form (0.70-x)SiO2-0.30Na2O-xCuO where 0.0 <= x <= 0.20. Evidence of the presence of copper in the Cu+ state for x<=0.15, and both oxidation states, Cu+ and Cu2+, in the glass with x=0.20, was obtained from the "shake up" satellite structure of the Cu 2p core level spectra. A deconvolution procedure was undertaken to determine qualitatively the ratio [Cu2+]/[CuTotal]. The non-bridging oxygen content, obtained from the deconvolution of the O 1s core level spectra, increases with increasing copper oxide content showing that copper acts as a network modifier. The O 1s spectra were modelled in order to separate the contribution from SiOCu and SiONa to the non-bridging oxygen signal. The Na 1s core level spectra seem to be insensitive to change in copper content in the glass, while Si 2p showed some dependence. Magnetization measurements were also performed on the same samples. The M versus H data at different temperatures collapses nicely to a single curve in the M versus H/T representation for all the glass samples investigated, indicating that Cu2+ is behaving paramagnetically in these glasses. The M versus H curves have been fitted with a Brillouin function keeping the number of magnetic ions as the fitting parameter and deducing the number of Cu2+ ions in each glass from the best fit to the experimental data. It was found that the Cu2+ content measured by XPS was much lower that the one found from the magnetic measurements. This might indicate that there is less Cu2+ on the surface than in the bulk of these glass samples. The third glass series investigated has the composition (0.70-x)SiO2-0.30Na2O-xCoO, where 0.0 <= x <= 0.20. The Co 2p spectra showed intense satellite structures about 6 eV above the main photoelectron peaks, and the Co 2p3/2-Co2p1/2 separation was found to be ~ 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 high-spin Co2+ in both tetrahedral and octahedral coordinations and the ration [Co2+ (oct)]/[Co2+total] increases with CoO content. The O 1s spectra show significant compositionally dependent changes. The concentration ratio of bridging and non-bridging oxygen atoms was determined from these spectra. High-spin Co2+ ions are found to be incorporated in the glass as network modifiers, irrespective of their coordination. The d.c. magnetic susceptibility measurements done on the same samples also suggest that Co2+ ions exist in both tetrahedral and octahedral coordinations. The magnetization versus magnetic field data indicate that the exchange magnetic interaction increases with CoO content in the glass. Thermal expansion coefficient, differential thermal analysis, heat treatment of the parent glass and identification of the crystalline phases present as well as density measurements have been performed on each glass series and these results are discussed in connection with the XPS and magnetic findings.