DISCRETE BOND MODEL (DBM) OF SODIUM-SILICATE GLASSES DERIVED FROM XPS, RAMAN AND NMR MEASUREMENTS

In sodium silicate glasses, the fraction of differently bound Si species Q[i] (i = 0-4), depending on the number i of bridging oxygens bound to the quarternary silicon, is a function of stoichiometry and the Na/Si ratio. Sodium silicate glasses were investigated by high resolution X-ray photoelectron spectroscopy. To explain the differences in chemical shifts and linewidths of the 0 Is signal of the bridging and the non-bridging oxygen as a function of alkali concentration, an extended glass model was developed. This new model takes into account the influence of the alkali concentration on the Q[i] distribution and on the appearance and concentrations of differently bound bridging oxygens as well as non-bridging oxygens. In principle, four different bonds for the non-bridging oxygens (Na-O-Q[i], i = 0-3) and seven different bonds for the bridging oxygens (Q[i]-O-Q[j], i, j = 1-4, \i - j\ < 2), linking the different Q[i] species could exist. According to this 'discrete bond model', for each glass composition the number of these possible bridging and non-bridging oxygen species is limited to a maximum of two for each of them. An approximation of the measured X-ray photoelectron 0 Is signal by a superposition of 0 Is signals as calculated on the basis of this model allows an excellent reproduction of the experimental results if the glasses are non-phase-separated. In the case of phase separation, the measured spectrum shows only Q[i]-O-Q[i] bonds (with i = 1-4) identical to the bonding behaviour in the corresponding well-known crystalline phases. The chemical shifts of the non-bridging oxygens relative to the bridging oxygens (as well as the absolute binding energies), as measured by X-ray photoelectron spectroscopy, can be explained by changes in the relative concentrations and intensities of the different oxygen bonds as a function of glass composition. The energy differences between two energetically neighbouring bridging oxygens (i.e., Q[i]-O-O[i] and Q[i]-O-Q[j] with \i - j\ = 1) are equal (within the experimental error) for all bridging oxygen bonds in sodium silicate glasses and can be correlated with the cation field strengths of the modifying cations.