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) in BaOSiO-POglasses. Nuclear magnetic resonance(NMR) and Raman spectroscopy revealed the existence of sixfold coordinated silicon species and network former units(NFUs) in the BaO-SiO-POglasses. The glass transition temperature(Tg), which was measured by differential scanning calorimetry, increased rapidly along with the increase of SiOfrom 0 to 10 mol%, then declined and finally increased again, which showed a "Z" trend along with the increase of SiOwhile the density of the glasses showed the opposite trend. When the addition of SiOis 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% SiO. Furthermore, the liquid fragility index(m) of the glasses declined slowly firstly and then increased rapidly when the amount of SiOis greater than 13 mol%.