Insight into the structure of magnesium and sodium mixed phosphate glasses: A molecular dynamics study

Abstract

Three phosphate glasses from the series (50-x/2)Na2O–xMgO–(50-x/2)P2O5 (x = 20, 30 and 40) were studied using classical molecular dynamics simulation. In addition, the metaphosphate compositions of 50MgO-50P2O5 and 50Na2O-50P2O5 were also modelled. The model results showed good agreement with the experimental results considering reported experimental uncertainties. Both Na and Mg act as network modifiers. The magnesium metaphosphate glass model showed that the local environments of the magnesium consist of distorted structures with mean coordination number of 4.4, while the Na-O mean coordination number is found to be 5 in sodium metaphosphate composition. For sodium magnesium phosphate glasses, the models show that Mg-O coordination number has no change over the range of the target compositions with an average of NMgO =4.4 and the mean distance at 1.99 Å. There is also no noticeable change in Na-O coordination number ∼5 and the Na-O distance is found at 2.41 Å in sodium magnesium phosphate glasses. The depolymerization of the phosphate glass network is observed upon the addition of MgO. The relative amounts of bridging Ob and non-bridging Onb oxygen atoms change toward increasing Onb with addition of MgO content. The connectivity in the models has been studied and it is characterised by Q2 sites at metaphosphate composition, decreasing to Q1 sites upon the addition of MgO. The nearest neighbor distances of Mg-Mg and Na-Na in which modifier cations are bonded to non-bridging oxygens, Onb, are measured and show Mg-Mg correlations to be unfavorable in comparison to Na-Na correlations. In addition, Mg(Onb)N polyhedra are predominantly connected to each other by corner sharing.