Structural organization of phase-separated bioactive glasses and the clustering of Si, P, B, Na and F atoms investigated by solid-state NMR and Monte Carlo simulations

A Gaddam and G Tricot and P Golebiewski and HR Fernandes and R Buczynski and JMF Ferreira and H Eckert, ACTA MATERIALIA, 259, 119203 (2023).

DOI: 10.1016/j.actamat.2023.119203

In this paper we studied the effect of NaBO2 addition to a phase- separated alkali-free bioactive glass with a composition of 38.49 SiO2 center dot 36.07 CaO center dot 19.24 MgO center dot 5.61 P2O5 center dot 0.59 CaF2. Microscopy reveals binodal phase separation involving two Si-containing microphases with a droplet size of similar to 200 mu m, driven by the thermodynamic LLPS mechanism. The local environments and spatial distribution of silicate, phosphate, and fluoride ions in this phase-separated system were studied, using Si-29, P-31, B-11, F-19, Mg-25, and Na-23 nuclear magnetic resonance (NMR) and infrared spectroscopy. The silicate units are dominantly of the metasilicate (Si-2) type. The phosphate units exist mostly as orthophosphate (P-0) while the borate is present in the form of pyroborate (B-1). Multinuclear dipolar re-coupling experiments indicate that the minority components F, P, B and Na all occur within a common phase. Thus, atomic distribution scenarios involving the separation of these components into separate phases can be excluded. The P-31 spin echo decay (SED) method was used along with Monte Carlo simulations to characterize the spatial distribution of the phosphate component. Based on the analysis, the phosphate component forms clusters of sizes 1-4 nm, which are embedded in an environment more dilute in phosphate, having a random distribution. While F-19 SED results indicate that the fluoride ions do not form clusters and are close to randomly distributed, dipolar recoupling of P-31 suggests a local environment resembling that of fluorapatite.

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