Proton Disorder and Superionicity in Hot Dense Ammonia Ice

We report the experimental discovery of a new phase of ammonia ice, stable at pressures above 57 GPa and temperatures above 700 K. The combination of our experimental results and ab initio molecular dynamics simulations reveal that this new phase is a superionic conductor, characterized by a large proton diffusion coefficient (1.0 x 10(-4) cm(2)/s at 70 GPa, 850 K). Proton diffusion occurs via a Grotthuss-like mechanism, at a surprisingly lower temperature than in water ice. This may have implications for the onset of superionicity in the molecular ice mixtures present in Jovian planets. Our simulations further suggest that the anisotropic proton hopping along different H bonds in the molecular solid may explain the formation of the recently predicted ionic phase at low temperatures.