Edwin Flikkema1*, Wenlin Chen2 & Neville Greaves1
1Department of Physics, Aberystwyth University, Penglais, Aberystwyth SY23 3BZ, United Kingdom
2Department of Geoscience, Virginia Tech, United States
This presentation is concerned with the computational study of silicate materials and Metal-Organic Frameworks (MOFs). Molecular Dynamics (MD) simulation provides detailed information on both the structure and dynamics of such materials. This study focuses on ion dynamics in alkali-silicates and on pressure-induced amorphization of zeolites and MOFs.
Silicates, such as alkali-silicates form a rich class of materials, with many polymorphs, in both crystalline and glassy states. As an example of a glass-forming material, mixtures of sodium-disilicate and potassium-disilicate have been studied in super-cooled and glassy states. In this material, the network modifiers (sodium, potassium) and the excess oxygen that is included with them, lead to a partial breaking up of the silicate network, leading to what is referred to as a modified random network. Structural results from the simulations include Radial Distribution Functions and Q-species distributions. This study focuses on dynamics, especially of the alkali ions. It is known from experiments that the mobility of the alkali ions versus the Na/K mixing ratio exhibits non-linear behaviour with a minimum close to a 50-50 mixing ratio. One of the aims of this study is to see whether this so called Mixed Alkali Effect can be reproduced in MD simulations. Simulations have been performed using empirical potentials. From these, diffusion constants have been obtained for the sodium and potassium ions at a range of temperatures and at various compositions. This indeed shows a slowing down of the dynamics of the alkali ions when both species are present. The dynamics has been further characterised using intermediate scattering functions. To assess dynamic heterogeneity, Pareto plots have been produced.
A second study focuses on MOFs and in particular the material ZIF-8, which is topologically similar to sodalite, consisting of zinc atoms connected via organic linker molecules. Here, both classical and ab-initio MD simulations have been performed to study amorphization of ZIF-8 under high pressure. For comparison, similar simulations have been performed for all-silica sodalite, its zeolitic analogue. The compressibility versus pressure curve provides evidence for phase transitions. Upon applying high pressure, a High Density Amorphous phase is produced. After decompression, under certain conditions, a Low Density Amorphous phase is observed. The behaviour is similar to that of sodalite, though at a different pressure scale. Amorphization over time has been studied using high-pressure simulation runs starting from the crystalline state. Deformation of structural elements (in particular the shapes of rings) has been quantified and tracked over time, making full use of the level of detail Molecular Dynamics simulations provide.