Crystal structure prediction and ammonia dynamics in calcium ammine

In today's world of decreasing fossil fuel supplies by contrast with growing population and requisite for energy, hydrogen as being one of the renewable energy sources, is important and alternative energy carrier for meeting this increasing energy demand. Hydrogen has a high energy efficiency and rich source. Furthermore, it is unpolluted and eco-friendly fuel which has no carbon emission. Thus, it is getting more important everyday among other energy sources. Despite having all those advantages, storage causes huge problem as compared to the other fuel types. It is possible to store, transport and use hydrogen both in gas and liquid phases. However, storing hydrogen in the gas phase requires quite large volumes. Storing in the liquid phase needs high pressures and therefore some of the produced energy should be used for liquefaction process which ultimately causes an increase in total cost. In the solid phase, hydrogen can be much more easily and safely stored as compared to previously mentioned phases, nevertheless, there is no excellent reported material capable of fast and reversible hydrogen absorption and desorption. It is also possible to store hydrogen indirectly (not as H2). Especially metal ammines, which include ammonia, have better storage characteristics than other direct and indirect materials. They have fast and reversible ammonia absorption and desorption characteristics as well as higher volumetric hydrogen densities. Desorption of ammonia from metal ammines is easy and only limited by heat transportation. When determining the most appropriate metal ammine, it is also required to take weight, price and volume parameters into consideration. One of those alkaline earth metal is magnesium and due to having low pressure and high gravimetric capacity, Mg(NH3)6Cl2 (9.19 wt% hydrogen) is the most examined metal ammine in the literature. Calcium metal ammine, on the other hand, is an other alternative medium requiring much lower temperatures for the desorption of ammonia compared to magnesium complex. Ca(NH3)8Cl2 (9.78 wt% hydrogen) can emit six ammonia molecules at 360K (for the first four ammonia molecules at around 320K). Experimentally only the crystal structures of octa- and diammine calcium complexes have been reported. In order to study the ammonia dynamics in all complexes (Ca(NH3)nCl2 with n=8,6,4,2,1), knowledge about their crystal structures is a necessity. Therefore, this study first predicts the crystal structures of all calcium ammine complexes by using an algorithm based on simulating annealing and density functional theory methods, called as CASPESA. With the help of promising stable (verified by phonon calculations) structures obtained from CASPESA, ammonia dynamics in these complexes were investigated by using minimum energy path techniques and the density functional level of theory. Global crystal structure optimizations carried out with CASPESA yielded new crystal structures lower in energy than the experimental ones for all ammonia content (n=8,6,4,2,1). The subsequent treatment of these newly found structures in the nudged elastic band calculations led to ammonia migration paths requiring close to the experimental desorption enthalpies. This indicates that ammonia mobility in these complexes might be limited to bulk diffusion.