Structure and Stability of High CaO- and P2O5-Containing Silicate and Borosilicate Bioactive Glasses

The present work elucidates about the structure of bioactive glasses having chemical compositions expressed as (mol %) (50.0 – x)SiO2–xB2O3–9.3Na2O–37CaO–3.7P2O5, where x = 0.0, 12.5, 25, and 37.5, and establishes a correlation between the structure and thermal stability. The structural modifications in the parent boron-free glass (B0) with the gradual substitutions of B2O3 for SiO2 are assessed by Raman and 29Si, 31P, 11B, and 23Na magic angle spinning (MAS)-nuclear magnetic resonance (NMR) spectroscopies. The structural studies reveal the presence of QSi2 and QSi3 structural units in both silicate and borosilicate glasses. However, QSi4(3B) units additionally form upon incorporating B2O3 in B0 glass. B-containing silicate glasses exhibit both three-coordinated boron (BIII) and four-coordinated boron (BIV) units. The 31P MAS-NMR studies reveal that the majority of phosphate species exist as isolated orthophosphate (QP0) units. The incorporation of B2O3 in B0 glass increases the cross-linking between the ...