Topologically frustrated ionisation in a water-ammonia ice mixture

C. Liu (1, 2) , A. Mafety (1) , J. A. Queyroux (1) , C. W. Wilson (3) , H. Zhang (1) , K. Beneut (1) , G. Le Marchand (1) , B. Baptiste (1) , P. Dumas (4) , G. Garbarino (5) , F. Finocchi (6) , J. S. Loveday (3) , F. Pietrucci (1) , A. M. Saitta (1) , F. Datchi (1) , S. Ninet (1)

1 IMPMC - Institut de minéralogie, de physique des matériaux et de cosmochimie
2 Jilin Univ, State Key Lab Superhard Mat, Changchun 130012, Jilin, Peoples R China
3 SUPA, Centre for Science at Extreme Conditions and School of Physics and Astronomy
4 SSOLEIL - Synchrotron SOLEIL
5 ESRF - European Synchrotron Radiation Facility
6 INSP - Institut des Nanosciences de Paris

Water and ammonia are considered major components of the interiors of the giant icy planets and their satellites, which has motivated their exploration under high P-T conditions. Exotic forms of these pure ices have been revealed at extreme (similar to megabar) pressures, notably symmetric, ionic, and superionic phases. Here we report on an extensive experimental and computational study of the high-pressure properties of the ammonia monohydrate compound forming from an equimolar mixture of water and ammonia. Our experiments demonstrate that relatively mild pressure conditions (7.4 GPa at 300 K) are sufficient to transform ammonia monohydrate from a prototypical hydrogen-bonded crystal into a form where the standard molecular forms of water and ammonia coexist with their ionic counterparts, hydroxide (OH-) and ammonium (NH4+) ions. Using ab initio atomistic simulations, we explain this surprising coexistence of neutral/charged species as resulting from a topological frustration between local homonuclear and long-ranged heteronuclear ionisation mechanisms

Domaines: Planète et Univers [physics]; Science des matériaux [cond-mat.mtrl-sci]; Chimie-Physique [physics.chem-ph]

C. Liu, A. Mafety, J. A. Queyroux, C. W. Wilson, H. Zhang, et al.. Topologically frustrated ionisation in a water-ammonia ice mixture. Nature Communications, 2017, 8 (1), 8 p. ⟨10.1038/s41467-017-01132-z⟩. ⟨hal-01721585⟩