Experimental and theoretical evidence for an ionic crystal of ammonia at high pressure

International audienceWe report experimental and theoretical evidence that solid molecular ammonia becomes unstable at room temperature and high pressures and transforms into an ionic crystalline form. Thismaterial has been characterized in both hydrogenated (NH3) and deuterated (ND3) ammonia samples up to about 180 and 200 GPa, respectively, by infrared absorption, Raman spectroscopy, and x-ray diffraction. The presence of a new strong infrared absorption band centered at 2500 cm−1 in NH3 (1900 cm−1 in ND3) is in line with previous theoretical predictions regarding the ionization of ammonia molecules into NH2 − and NH4 + ions. The experimental data suggest the coexistence of two crystalline ionic forms, which our ab initio structure searches predict to be the most stable at the relevant pressures. The ionic crystalline form of ammonia appears stable at low temperatures, which contrasts with the behavior of water in which no equivalent crystalline ionic phase has been found

Disorder-order phase transition at high pressure in ammonium fluoride

Solid NH4F displays intriguing parallels with ice despite its apparently ionic character. Here we investigate its phase diagram in low-temperature and high-pressure regions using Raman spectroscopy, X-ray diffraction and ab initio structure search calculations. We focus on the high-pressure cubic phase which resembles that found in ice under pressure and is also the ambient pressure phase of other ammonium halides. We detect a disorder-order transition above 10 GPa, recalling those found both in other ammonium halides and in ice. The transition reveals itself in the pressure dependence of several Raman modes as well as through the progressive splitting of lattice and bending modes of the cubic phase at low temperatures. An in-depth analysis of the Raman modes and their evolution is made

Topologically frustrated ionisation in a water-ammonia ice mixture

Water and ammonia are major constituents of icy planet interiors, however their phase behaviour under extreme conditions remain relatively unknown. Here, the authors show that ammonia monohydrate transforms under pressure into an alloy composed of molecules as well as ions, owing to a topological frustration