Molecular dynamics modelling of sodium and calcium metaphosphate glasses for biomaterial applications

Many phosphate-based glasses (PBGs) for biomaterial applications have been developed from the Na₂O-CaO-P₂O₅ system. The common base compositions for PBGs are (55-x)Na₂O-xCaO-45P₂O₅ and 20Na₂O.30CaO.50P₂O₅, where the later corresponds to a metaphosphate, and there is a wealth of experimental data for 50Na₂O.50P₂O₅, 50CaO.50P₂O₅ and 20Na₂O.30CaO.50P₂O₅ metaphosphate glasses. A classical molecular dynamics (MD) method has been used to model Na₂O-CaO-P₂O₅ glass structures, and the results have been closely compared with experimental data for the same glasses. The MD models show the phosphate network to be dominated by Q₂ units, as expected, and to have short range order parameters that are in good agreement with neutron and X-ray di?raction results. Typical coordination numbers of Na and Ca are 5 and 6, respectively. The modifer cation distributions have been examined in detail through the correlation functions TMM(r), with M=Na and/or Ca. The TMM(r) functions show the expected dependence of peak position on modifer cation size, and peak height on modifer cation concentration. The numbers of M-M nearest neighbours are in agreement with a statistical model, in which modifer cations are bonded to nonbridging oxygens, Onb, and M(Onb)N polyhedra are predominantly connected to each other by corner-sharing.