Discrete bond model (DBM) of sodium silicate glasses derived from XPS, Raman and NMR measurements

Abstract

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 O 1s signal of the bridging and the non-bridging oxygen as a function of alkali concentration, and 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 O 1s signal by a superposition of O 1s 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 brodging 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Q [ 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.