Fluorine environment in bioactive glasses: ab initio molecular dynamics simulations.

Car-Parrinello molecular dynamics (CPMD) simulations have been performed on a 45S5Bioglass composition in which 10 mol% of the CaO has been replaced with CaF2. The veryaccurate characterization of the fluorine environment in this system has allowed us to resolvesome longstanding issues about the atomic structure of fluorinated bioglasses. F is coordinatedalmost entirely to the modifier ions Na and Ca, with a very small amount of residual Si-Fbonds, whose fraction only becomes significant in the melt precursor. There is no evidencefor preferential bonding of F to either modifier ion: almost all F atoms are coordinated toboth calcium and sodium in a “mixed state”, rather than exclusively to either, as had beenconjectured. We discuss the consequences of these findings on the properties of fluorinecontainingbioglasses.