Effect of SiO2 on amorphous phase separation of CaO–P2O5–SiO2–MgO glasses

Scanning and transmission electron microscopy, differential thermal analysis and Raman spectroscopy were used to investigate glasses of the 52.75(1+x/70)3CaO · P2O5−17.25(1+x/70)MgO−(30−x)SiO2 (wt%) series (with x=0–10). In glasses for which the silica content is 24 wt% or larger a dispersed amorphous silica phase was present. Differential thermal analysis showed that the transition temperature and the second exothermic peak temperature are independent of silica content between 30 and 23 wt%. The first crystallization peak is independent of silica content in all the range investigated. Raman spectroscopy showed that the structure of the 20 wt% SiO2 glass differs from the others. We conclude that for a silica content <23 wt% the single-phase glass becomes more de-polymerised as the silica content decreases and for a silica content >23 wt% the glasses are phase separated and the degree of polymerisation of the matrix is constant; the monophasic glass with 23 wt% SiO2 has the same composition and structure as the matrices of all other phase-separated glasses.