Effects of Six-fold Coordinated Silicon on Structure and Properties of BaO-SiO2-P2O5 Glasses

We studied the local structure and properties of six-fold coordinated silicon (Si[6]) in BaO-SiO2-P2O5 glasses. Nuclear magnetic resonance (NMR) and Raman spectroscopy revealed the existence of six-fold coordinated silicon species and network former units (NFUs) in the BaO-SiO2-P2O5 glasses. The glass transition temperature (Tg), which was measured by differential scanning calorimetry, increased rapidly along with the increase of SiO2 from 0 to 10 mol%, then declined and finally increased again, which showed a “Z” trend along with the increase of SiO2 while the density of the glasses showed the opposite trend. When the addition of SiO2 is 16 mol%, Tg decreased to an extremely low value (807.9 K). Besides, the Vickers indentation hardness (Hv) had been significantly enhanced from 4.66 to 6.63 GPa by adding 16 mol% SiO2. Furthermore, the liquid fragility index (m) of the glasses declined slowly firstly and then increased rapidly when the amount of SiO2 is greater than 13 mol%.