Molecular dynamics simulation of the structure and properties of lithium phosphate glasses

A new forcefield model was developed for the computer simulation of phosphate materials. The model provides a fundamental basis for the evaluation of phosphate glass structure and thermodynamics, and was used to perform molecular dynamics (MD) simulations of a series of lithium phosphate glass compositions (xLi2O · (1 − x)P2O5, 0⩽x⩽0.5). Microstructural features and thermodynamic properties, as well as their correlations with chemical compositions, were analyzed. An important structural feature observed for the phosphate glasses was the occurrence of ring structures that were related to the chemical composition. Relative stability of the ring structures with respect to the number of phosphate tetrahedra within the ring was investigated using molecular orbital calculations on various phosphate conformations of clusters. An increase in stability was observed as the ring size increases from two- to four-membered rings. A larger abundance of the smallest ring size, the 3-membered rings (P3O3), corresponds to the minimum of the glass transition temperature (Tg) in the lithium phosphate glass series (corresponding to the composition 0.2Li2O · 0.8P2O5). The impact of these strained 3-membered rings, along with the changes in the Li-coordination environment, on determining the spectroscopic and thermodynamic properties of the phosphate glasses is discussed.