X-ray photoelectron spectroscopy of sodium borosilicate glasses

X-ray photoelectron spectra of RNa2O·B2O3·KSiO2 (R=0.2 to 5.0,K=0.5 to 6.0) glasses were measured by a monochromatised Al-Kα XPS instrument. Chemical shifts of the core-level photoelectron spectra of the glass constituent ions were investigated. Peaks at the higher and lower energy sides of an O1s signal were ascribed to bridging oxide (BO) and non-bridging oxide (NBO) components, respectively. The BO component could not be separated into the oxide ions with different environment such as Si–O–Si, B–O–Si and B–O–B. Na1s signal could not be separated into two components of Na+ ions associated with [BO4/2]− tetrahedron and NBO. For the small K values (=0.5 and 1.0), the NBO fractions measured by XPS agreed very well with the structural model proposed by Dell et al. [J. Non-Cryst. Soilds 58 (1983) 1], where NBO did not appear until R=0.5 regardless of K values. With increasing K, however, the dependence of the NBO fraction upon R gradually came to disagree with the Dell model. At smaller R values than R0=0.5+K/16, the NBO component already appeared to a considerable extent and the discrepancy between the XPS results and the Dell model became more significant with increasing K. This fact meant that additional sodium oxides were shared proportionally between silicate and borate networks even at the small R when K was larger than 2. A new sharing model was proposed on the basis of an acid–base concept.