Microscopic mechanism of structural and volume relaxation below glass transition temperature in a soda-lime silicate glass revealed by Raman spectroscopy and its first principle calculations

TL;DR

This study combines Raman spectroscopy and first-principles calculations to reveal that ring deformation and sodium displacement drive volume relaxation in soda-lime silicate glass below its glass transition temperature.

Contribution

It provides the first detailed atomistic mechanism linking ring deformation and Na movement to volume relaxation in soda-lime silicate glass.

Findings

Ring deformation causes volume relaxation by expanding internal ring space.

Na atoms are inserted into ring centers during relaxation.

Structural homogenization occurs via specific Q2+Q4 to 2Q3 reactions.

Abstract

To elucidate the atomistic origin of volume relaxation in soda-lime silicate glass annealed below the glass transition temperature (Tg), the experimental and calculated Raman spectra were compared. By decomposing the calculated Raman spectra into a specific group of atoms, we found that the Raman peak at 1050 cm-1 corresponds to bridging oxygen with a small Si-O-Si bond angle. The experimental Raman spectra indicated that, during annealing below Tg, a homogenization reaction Q2+Q4->2Q3 proceeds in the early stage of structural relaxation. Then, the Si-O-Si units with relatively small angles decrease even in the later stages, which is first evidence of ring deformation causing volume relaxation of soda-lime silicate glass because decreasing small Si-O-Si angles corresponds to the reduce of acute O-O-O angle in a ring and can expand the space inside the rings, and Na can be inserted into the ring center. In conclusion the ring deformation and Na displacement is the origin of the volume relaxation of soda-lime silicate glass below Tg.