XPS study on silica–bismuthate glasses and glass ceramics

X-ray photoelectron spectroscopy (XPS) was used to evidence the effect of the Bi2O3 to SiO2 ratio and of partial crystallisation on the electronic charge density around the atoms entering silica–bismuthate glasses of nominal composition 0.01Fe2O3⋅0.99[xSiO2⋅(100−x)Bi2O3] with 10≤x≤60mol%. The core level spectra show significant composition dependent changes in binding energy, and the full width at half maximum of photoelectron peaks both of cations and of oxygen atoms. The analysis reveals changes in electron density correlated with the ionic and covalent character of the samples. The shift in binding energy suggests charge transfer from silicon and oxygen atoms to bismuth atoms. Contrary to the expected behaviour in conventional silicate oxide systems, the results indicate an increase of ionicity for silicon and of covalency for bismuth atoms. The same evolution of ionicity/covalency is observed after partial crystallisation.