Ammonia Clathrate Hydrate As Seen from Grand Canonical Monte Carlo Simulations

In this paper, the trapping of ammonia molecules into a clathrate structure has been investigated by means of Grand Canonical Monte Carlo simulations, performed at three different temperatures (100, 150, and 180 K) relevant for the astrophysics environments. The results show that ammonia clathrate of structure I is stable at partial filling, irrespective of the temperature investigated here. It could also be metastable in a chemical potential (pressure) range that corresponds to a maximum of eight ammonia molecules per unit cell, i.e., to the full occupancy of the clathrate structure at very low temperature. However, at higher chemical potential values, partial dissolution of the clathrate is evidenced, concomitant with its transformation to low-density amorphous ice at 150 and 180 K. In the clathrate stability regime, it is shown that ammonia molecules can also displace water molecules and become incorporated into the water lattice, which results in the progressive destabilization of the clathrate lattice with increasing number of trapped ammonia molecules. Our results point out the subtle interplay between the various environmental conditions (temperature, partial pressure of ammonia) on the stability of the clathrate phase in various planetary environments.