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 the distributions of Si species Qn in several stoichiometric glasses were estimated by NMR. The following glasses were considered: CaO.SiO2 (CS), BaO.2SiO2 (BS2), Na2O.2CaO.3SiO2 (123), 2Na2O.CaO.3SiO2 (213), 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. Agreement was found between the predicted and experimental Qn populations for glasses 123, BS2 and 213. 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 Si in the glass is organized in SiO2-like domains of molecular size, having Q3 units at their interfaces. 31 refs.