Stable and metastable crystal structures and ammonia dynamics in strontium chloride ammines

Metal halide ammines are promising ammonia storage materials due to their high ammonia densities and suitable decomposition properties. Here, we studied the polymorphism of ammines with a general formula of Sr(NH₃)_nCl₂ (n = 1, 2, 4, 6, and 8) by combining the Fast and Flexible CrystAl Structure Predictor (FFCASP) with density functional theory (DFT) calculations. Furthermore, the lattice stability and the minimum energy paths for bulk and surface diffusion of NH₃ were investigated by performing phonon and nudged elastic band (NEB) calculations. In addition to the successful reproduction of the reported experimental crystal structures of octammine (Pnma (IT number 62)), diammine (Aem2 (IT number 39)) and monoammine (Cmcm (IT number 63)), several isoenergetic polymorphs for each phase were also found. Not only the experimentally determined octammine and monoammine structures, but also the proposed structures for the hexammine and tetrammine phases were found to be metastable. While phonon calculations show instability for the experimental diammine structure, some of the proposed structures for the diammine phase showed thermodynamical stability. Moreover, NEB paths examining the bulk and surface diffusion of NH₃ are in accordance with the experimental desorption enthalpies.