Structural investigations of borosilicate glasses containing MoO3 by MAS NMR and Raman spectroscopies

High molybdenum concentration in glass compositions may lead to alkali and alkaline-earth molybdates crystallization during melt cooling that must be controlled particularly during the preparation of highly radioactive nuclear glassy waste forms. To understand the effect of molybdenum addition on the structure of a simplified nuclear glass and to know how composition changes can affect molybdates crystallization tendency, the structure of two glass series belonging to the SiO2-B2O3-Na2O-CaO-MoO3 system was studied by 29Si, 11B, 23Na MAS NMR and Raman spectroscopies by increasing MoO3 or B2O3 concentrations. Increasing MoO3 amount induced an increase of the silicate network reticulation but no significant effect was observed on the proportion of BO4- units and on the distribution of Na+ cations in glass structure. By increasing B2O3 concentration, a strong evolution of the distribution of Na+ cations was observed that could explain the evolution of the nature of molybdate crystals (CaMoO4 or Na2MoO4) formed during melt cooling.