Qn distribution in stoichiometric silicate glasses: thermodynamic calculations and 29Si high resolution NMR measurements

To test the validity of a thermodynamic model (TM) for glass structure (model of associated solutions previously proposed by some of us) the distributions of silicon species Qn in several stoichiometric glasses were estimated by nuclear magnetic resonance. The following glasses were considered: CaO · SiO2 (CS), BaO · 2SiO2 (BS2), Na2O · 2CaO · 3SiO2 (1 · 2 · 3), 2Na2O · CaO · 3SiO2 (2 · 1 · 3), CaO · MgO · 2SiO2 (CMS2) and PbO · SiO2 (PS). In contrast to the usual assumption that all kinds of Qn species can be present in the glass structure, the basic hypothesis of the TM limits the possible set of species in the glass to those effectively observed in the crystalline phases of the equilibrium phase diagram. The Qn distributions for the set of glasses were calculated at temperatures close to the respective glass transition temperatures. A satisfactory agreement was found between the predicted and experimental Qn populations for glasses 1 · 2 · 3, BS2 and 2 · 1 · 3. On the other hand, for glasses CS, CSM2 and PS, agreement between experiment and model results is obtained assuming that the expected amount of Q4 silicon in the glass is organized in silica-like domains of molecular size, having Q3 units at their ‘interfaces’.