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 latter 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 diffraction results. Typical coordination numbers of Na and Ca are 5 and 6, respectively. The modifier 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 modifier cation size, and peak height on modifier cation concentration. The numbers of M–M nearest neighbours are in agreement with a statistical model, in which modifier cations are bonded to nonbridging oxygens, Onb, and M(Onb)N polyhedra are predominantly connected to each other by corner-sharing.