Ammonia dynamics in magnesium ammine from DFT and neutron scattering

Abstract

Energy storage in the form of ammonia bound in metal salts, so-called metal ammines, combines high energy density with the possibility of fast and reversible NH₃ ab- and desorption kinetics. The mechanisms and processes involved in the NH₃ kinetics are investigated by density functional theory (DFT) and quasielastic neutron scattering (QENS). The crystal structures of Mg(NH₃)_nCl₂ with n = 6, 2, 1, which contains up to 9.19 wt % hydrogen and 0.115 kg hydrogen L⁻¹, are first analyzed using an algorithm based on simulated annealing (SA), finding all the experimentally known structures and predicting the C2/m structure for the uncharacterized low temperature phase of Mg(NH₃)₆Cl₂. It is found from DFT that the rotation of ammonia in the hexammine complex (n = 6) requires an activation energy of 0.09 eV in the low temperature phase of Mg(NH₃)₆Cl₂ and 0.002–0.12 eV in the high temperature phases; effectively having free rotors as observed experimentally. The findings are supported by the QENS data, which identify C3 rotations of NH₃ in the low temperature phase with an activation energy of 0.09 eV. The calculated diffusion rates were found to be 10⁶–10⁷ Hz at the desorption temperatures for all n = 6, 2, 1 systems. DFT calculations involving bulk diffusion of NH₃ correctly reproduces the trends observed in the experimental desorption enthalpies. In particular, for n = 6, 2, 1, there is a good agreement between activation barriers and experimental enthalpies. These results indicate that the desorption of NH₃ is likely to be diffusion limited.