New Insights into the Atomic Structure of 45S5 Bioglass byMeans of Solid-State NMR Spectroscopy and AccurateFirst-Principles Simulations

An integrated computational method that couples classical molecular dynamics simulations withdensity functional theory calculations has been used to simulate the solid-state 17O and 23NaMQMAS, 29Si, 31P, and 23Na static and MAS NMR spectra of the 45S5 Bioglass structural modelswith up to 248 atoms. Comparison with the experimental spectra collected in this work (the 17OMQMAS spectrum of the 45S5 Bioglass is reported for the first time in the literature) shows anexcellent agreement. The results provide deep insights into fundamental open questions regarding theatomic-scale structural details of this glass of great medical interest. In particular, the host silicanetwork, described by theQn distribution (aQn species is a network-forming ion bonded to n bridgingoxygens), consists of chains and rings ofQ2Si (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 beendetected by both 31P NMR and 17O MQMAS experiments, and therefore isolated orthophosphateunits are able to form nanodomains that subtract sodium and calcium cations from their networkmodifying role into the silicate network. Finally, both the experimental and theoretical results show amixture of dissimilar cations (Na,Ca) around NBO, according to a nonrandom distribution of thesespecies.