Structure and dissolution behavior of orthophosphate MgO-CaO-P2O5-Nb2O5 glass and glass-ceramic

The aim of this work is to investigate the effect of lowering the phosphate content, from 27 to 20 mol%, in calcium-phosphate invert glass/glass-ceramic containing magnesia and niobia (MgO-CaO-P2O5-Nb2O5) on the structure and dissolution properties. According to 31P solid-state nuclear magnetic resonance (NMR) and Raman spectroscopies, glass containing 20 mol% of P2O5 was exclusively composed of orthophosphate (QP0), whereas orthophosphate and pyrophosphate (QP1) coexisted in 27 mol% P2O5 glass. Tetrahedral niobate was detected by Raman spectroscopy with 20 mol% phosphate, suggesting that niobate acts as a network former and therefore cross-links orthophosphate groups together (P-O-Nb bonds). The equivalent glass-ceramic consisted of crystalline phases (β-Ca3(PO4)2, Mg3(PO4)2 and Mg3Ca3(PO4)4), which did not contain niobate, and a residual glassy phase. The glass with 20 mol% P2O5 showed a lower chemical durability than the glass with 27 mol% P2O5, which was likely due to the formation of P-O-Mg bonds, which favor hydrolysis. The glass-ceramic chemical durability was improved after crystallization, probably because of an increase in the niobate concentration of the residual glassy phase.