New Insights into the Atomic Structure of 45S5 Bioglass by Means of Solid-State NMR Spectroscopy and Accurate First-Principles Simulations

An integrated computational method that couples classical molecular dynamics simulations with density functional theory calculations has been used to simulate the solid-state 17O and 23Na MQMAS, 29Si, 31P, and 23Na static and MAS NMR spectra of the 45S5 Bioglass structural models with up to 248 atoms. Comparison with the experimental spectra collected in this work (the 17O MQMAS spectrum of the 45S5 Bioglass is reported for the first time in the literature) shows an excellent agreement. The results provide deep insights into fundamental open questions regarding the atomic-scale structural details of this glass of great medical interest. In particular, the host silica network, described by the Qn distribution (a Qn species is a network-forming ion bonded to n bridging oxygens), consists of chains and rings of Q2Si (67.2%) SiO4 tetrahedra cross-linked with Q3Si (22.3%) species and terminated by a low quantity of Q1Si (10.1%) species. No Si−O−P bridges have been detected by both 31P NMR and 17O MQMAS experiments, and therefore isolated orthophosphate units are able to form nanodomains that subtract sodium and calcium cations from their network modifying role into the silicate network. Finally, both the experimental and theoretical results show a mixture of dissimilar cations (Na,Ca) around NBO, according to a nonrandom distribution of these species.