Effect of Ca/K replacement on the atomic structure of P₂O₅-K₂O-CaO glasses: a molecular dynamics study

A classical molecular dynamics method was used to model potassium and calcium metaphosphate glasses to apply an environmentally conscious glass fertilizer. By controlling the K₂O-CaO-P₂O₅ glass composition, the phosphate glasses are predicted to provide the slow-releasing nutrient fertilizers which are environmentally safe and widely applicable. Five models of a composition of 50P₂O₅-xCaO-(50-x)K₂O (x = 0, 10, 20, 30 and 40 mol %) were developed. The model's short-range order parameters results agree with experimental results. The two P-O bond distances are well resolved on average at 1.48 Å and 1.55 Å. The O-O mean bond distance peaked at 2.46 Å with an average coordination number of 4.1. The Ca-O bond distance is well defined at 2.40 Å with the coordination number of 6 on average. Ca acts as a network modifier; the addition of CaO is associated with the disturbance of the bridges P-O-P and therefore depolymerizes the phosphate network. The K-O coordination number changed insignificantly on composition range, with average values between 5.0-5.6. The bond angle distributions for the five models found that the average O-P-O bond angle is tetrahedral, defined to peak at 110°, while the P-O-P bond angles peaked at around 150°. As expected in metaphosphate composition, the models show the phosphate network dominated by Q2 units. The network connectivity is also examined through P-P coordination number with an average value of 2 and the bond distance is well resolved at 3 Å. (author)