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 (T_g), 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%, T_g decreased to an extremely low value (807.9 K). Besides, the Vickers indentation hardness (H_v) 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%. [ABSTRACT FROM AUTHOR]