Orientational Disorder Drives Site Disorder in Plastic Ammonia Hemihydrate,Physical Review Letters

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Orientational Disorder Drives Site Disorder in Plastic Ammonia Hemihydrate
Physical Review Letters ( IF 8.1 ) Pub Date : 2024-09-04 , DOI: 10.1103/physrevlett.133.106102
Niccolò Avallone _{1,

2} , Simon Huppert _{1,

2} , Philippe Depondt _{1,

2} , Leon Andriambariarijaona ₃ , Frédéric Datchi ₃ , Sandra Ninet ₃ , Thomas Plé ₄ , Riccardo Spezia ₄ , Fabio Finocchi _{1,

2}

Affiliation

In the 2–10 GPa pressure range, ammonia hemihydrate

H_{2} O : ({NH}_{3})_{2}

(AHH) is a molecular solid in which intermolecular interactions are ruled by distinct types of hydrogen bonds. Upon heating, the low-temperature ordered

P 2_{1} / c

crystal (AHH-II) transits to a bcc phase (AHH-

p bcc

) where each site is randomly occupied by water or ammonia. In addition to the site disorder, experiments suggest that AHH-

p bcc

is a plastic solid, but the physical origin and mechanisms at play for the rotational and site disordering remain unknown. Using large-scale (

\sim 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