Observation of a Plastic Crystal in Water-Ammonia Mixtures under High Pressure and Temperature.

Solid mixtures of ammonia and water, the so-called ammonia hydrates, are thought to be major components of solar and extra-solar icy planets. We present here a thorough characterization of the recently reported high pressure (P)-temperature (T) phase VII of ammonia monohydrate (AMH) using Raman spectroscopy, X-ray diffraction, and quasi-elastic neutron scattering (QENS) experiments in the ranges 4-10 GPa, 450-600 K. Our results show that AMH-VII exhibits common structural features with the disordered ionico-molecular alloy (DIMA) phase, stable above 7.5 GPa at 300 K: both present a substitutional disorder of water and ammonia over the sites of a body-centered cubic lattice and are partially ionic. The two phases however markedly differ in their hydrogen dynamics, and QENS measurements show that AMH-VII is characterized by free molecular rotations around the lattice positions which are quenched in the DIMA phase. AMH-VII is thus a peculiar crystalline solid in that it combines three types of disorder: substitutional, compositional, and rotational.

Melting curve and phase diagram of ammonia monohydrate at high pressure and temperature.

The phase diagram and melting behavior of the equimolar water-ammonia mixture have been investigated by Raman spectroscopy, x-ray diffraction, and visual observations from 295 K to 675 K and up to 9 GPa. Our results show non-congruent melting behavior of ammonia monohydrate (AMH) solid below 324 K and congruent melting at higher temperatures. The congruent melting is associated with the stability of a previously unobserved solid phase of AMH, which we named AMH-VII. Another, presumably water-rich, hydrate has also been detected in the range 4 GPa-7 GPa at 295 K on decompression of the high pressure disordered ionico-molecular alloy (DIMA) phase. Comparing our melting data to the literature suggests that non-congruent melting extends from 220 K to 324 K and that the solid phase that borders the fluid between 220 K and 270 K, called AMH-III, is not a proper phase of AMH but a solid solution of ammonia hemihydrate and ice. These results allow us to propose a revised and extended experimental phase diagram of AMH.