Orientational Disorder Drives Site Disorder in Plastic Ammonia Hemihydrate

N Avallone and S Huppert and P Depondt and L Andriambariarijaona and F Datchi and S Ninet and T Plé and R Spezia and F Finocchi, PHYSICAL REVIEW LETTERS, 133, 106102 (2024).

DOI: 10.1103/PhysRevLett.133.106102

In the 2-10 GPa pressure range, ammonia hemihydrate H2O:(NH3)(2) (AHH) is a molecular solid in which intermolecular interactions are ruled by distinct types of hydrogen bonds. Upon heating, the lowtemperature ordered P2(1)/c crystal (AHH-II) transits to a bcc phase (AHH-pbcc) where each site is randomly occupied by water or ammonia. In addition to the site disorder, experiments suggest that AHHpbcc is a plastic solid, but the physical origin and mechanisms at play for the rotational and site disordering remain unknown. Using large-scale (similar to 10(5) atoms) and long-time (> 10 ns) simulations, we show that, as temperature rises above the transition line, orientational disorder sets in, breaking the strongest hydrogen bonds that provide the largest contribution to the cohesion of the ordered AHH-II phase and enabling the molecules to migrate from a crystal site to a neighboring one. This generates a plastic molecular alloy with site disorder while the solid state is overall maintained until melting at a higher temperature. The case of high (P; T) plastic ammonia hemihydrate can be extended to other water-ammonia alloys where a similar interplay between distinct hydrogen bonds occurs.

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