PNNL

Abstract

The factors that control the distribution of Al3+ and La3+ cations in silica and soda silicate glasses is examined by using molecular dynamics (MD) simulations. In particular, the response of the glass network to the presence of metal oxide is probed using liquid state theory that treats the glass network as a solvent and the metal cation as a solute. MD simulations are used to obtain the mean force of the solvent-solute and solute-solute interactions along a trajectory that is then used to obtain the potential of mean force. The resulting free energy is analyzed along the trajectory to determine the stable configurations of the cation pair. Details of determining the PMF for an Al cation pair in silica and silicate glass is presented. A comparison of these results with those previously calculated for a La cation pair in the same glass systems is given. The results reveal that there are distinct differences on how the network accommodates the two different size cations. It is found that the network wraps itself around the larger La cation very much like a solvent shell, whereas, the smaller Al cation is incorporated into the network backbone. In silica and soda silicate glasses clustering of the La ion pair to form La-O-La linkages is favored, in contrast, the glasses favor a separated state of the Al ion pair.