Derivation of an Interatomic Potential for Fluoride-Containing Microporous Silicates and Germanates

A new interatomic potential for simulating the properties of zeolites synthesized through the fluoride route has been derived. Hexafluoro compounds have been used as a basis for deriving a force field for the interaction of Si/Ge/Al ions with fluoride ions, as a starting point for describing the chemical behavior of the fluoride anions in the cavities of zeolites. From this, as well as ab initio data concerning the interaction of fluoride with framework, the equilibrium location of F- in the different small cavity environments found in zeolites can be calculated. It is found that the pentacoordination of Si due to formation of Si−F bonds is predicted by our model. The calculated Si−F distances for each case are found to be in excellent agreement with recent 19F NMR and single-crystal XRD results. Furthermore, our force field can be used to predict distances in any zeolite, and in particular to establish the effect of Ge−F interactions and the corresponding equilibrium distances, which in some cases are not yet available experimentally. This new force field can be used in zeolites containing any compositional range with Si/Ge/Al as framework atoms, in the presence or absence of fluoride. Aluminogermanates and silicoaluminogermanates can therefore also be studied and the results used as a guide for new synthesis experiments using the fluoride route.