Breaking the secrets of icy protons

T. Meier, S. Petitgirard, S. Khandarkhaeva, and L. Dubrovinsky

Observation of nuclear quantum effects and hydrogen bond symmetrisation in high pressure ice,
Nat. Commun. 9, 2766 (2018)

Water in its liquid and solid forms is ubiquitous in nature, being one of the most abundant molecule in the universe, and is thought to be a prerequisite to sustain life in our solar system and beyond. Understanding of the physical properties of its high pressure ice phases is in the focus of numerous research groups.

One of the most enigmatic and elusive phenomenon in ice research is the transition from the high pressure ice phase VII towards the ominous phase X which was proposed more than 40 years ago but never unequivocally confirmed experimentally. It is believed that this transition is driven by very exotic quantum phenomena, so called nuclear quantum effects, which are rarely observed at ambient conditions.

Using our recently developed high pressure NMR method, first experiments probing protons trapped in the hydrogen-bond network of ice VII have been used to shed light on this decade long mystery. It could be shown, that the protons undergo quantum mechanical tunneling through the energy barrier of the hydrogen bond with increasing compression, leading to a quantum phase transition from the regular, commonly known, high barrier hydrogen bonds towards so-called strong hydrogen bonds. At even higher pressures of about 75 GPa, the transition towards the hydrogen symmetrized phase of ice X could be observed. This study demonstrates the importance of nuclear quantum effects in hydrogen-bonded materials and might be the governing mechanism for structural, electronic or magnetic instabilities for hydrous minerals.

Schematic explosion diagram

Sequence of hydrogen bond symmetrisation with pressure. Lowermost picture: High barrier hydrogen bonded phase: The hydrogen bond exhibits two distinct minima separated by a pronounced energy barrier. Under sufficient compression, hydrogen atoms can tunnel through the barrier from one minimum to the other. Middle picture: Under sufficient pressure, most hydrogen atoms are de-localized within the hydrogen bonds. Top picture: At the transition to ice X, the protons are forced in the symmetric mid-point between both oxygen atoms, leading to a break-down of the protonic nuclear quantum effects.

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