Modelling the structural evolution of ternary phosphate glasses from melts to solid amorphous materials

Abstract

The local and medium-range structural properties of phosphate-based melts and glasses have been characterized by means of first principles (density functional theory) and classical (shell-model) molecular dynamics simulations. The structure of glasses with biomedically active molecular compositions, (P₂O₅)_0.45(CaO)_x(Na₂O)_0.55−x (x = 0.30, 0.35 and 0.40), have been generated using first principles molecular dynamics simulations for the full melt-and-quench procedure and the changes in the structural properties as the 3000 K melt is cooled down to room temperature have been compared extensively with those of the final glasses. The melts are characterized by a significant fraction of threefold (P_3c) and fivefold (P_5c) phosphorus atoms, but structural defects rapidly decrease during the cooling phase and for temperatures lower than 1800 K the system is free of under- and over-coordinated species. The analysis of the structures of the glasses at 300 K shows a prevalence of the metaphosphate Q² and pyrophosphate Q¹ species, whereas the number of Q³ units, which constitute the three-dimensional phosphate network, significantly decreases with the increase of calcium content in the glass. The radial and angular distribution functions indicate that higher calcium concentration in the glass leads to an increase of the rigidity of the phosphate tetrahedral network, which has been explained in terms of the calcium's higher field strength compared to that of sodium. The structural characterization of the melts and glasses obtained from first principles simulations was used to assess and validate a recently developed interatomic shell-model forcefield for phosphate-based materials. For all three compositions, our potential model is in good agreement with the first principles data. In the glass network, the forcefield provides a very good description of the split between the shorter distances of phosphorus to non-bonded oxygen and the longer distances of the phosphorus to bonded oxygen; the phosphorus–phosphorus medium-range distribution; and the coordination environment around the Na and Ca glass modifiers. Moreover, the distribution of the Qⁿ species in the melts and glasses is in excellent agreement with the values extracted from the first principles simulations. In contrast, simulations using standard rigid ion potentials do not provide a satisfactory description of the local short-range structure of phosphate-based glasses and are therefore less suitable to model this class of multicomponent amorphous system.