Topologically frustrated ionisation in a water-ammonia ice mixture

Abstract

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 (~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 [Formula: see text] 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.

Fig. 1

Phase diagram and structures of AMH. a Phase diagram of AMH. b Representation of the $Im\overline{3}m$ structure of the DMA phase VI. The oxygen and nitrogen atoms are represented with the blue and red spheres, respectively. The water and ammonia molecules occupy the same sites with a 50% probability. The white spheres show the 16 possible positions along the $⟨111⟩$ direction for the five hydrogen atoms in the unit cell. The other possible sites for H atoms along $⟨110⟩$ are less probable and not represented for clarity. c Representation of the predicted ionic P4/nmm structure according to ref. . The same colour code is adopted for the atoms

Fig. 2

Vibrational spectra of AMH above 7.4 GPa. a Raman and b infrared absorption spectra of AMH, at RT and respective P of 10 and 12 GPa. The experimental spectra (collected at RT) and the theoretical ones (calculated at 0 K) for the fully ionic P4/nmm structure are shown in the upper and lower panels, respectively. In the experimental Raman spectra, the frequency windows from 1300–1400 cm⁻¹ to 2200–2600 cm⁻¹ are greyed as they are dominated by, respectively, the first- and second-order Raman signal from the diamond anvils. In the experimental infrared absorption spectrum, the frequency window from 2000 to 2300 cm⁻¹ is obscured by the strong absorption band of the diamond anvils. For visibility, Raman (respectively, IR) simulated intensities have been divided by 30 (respectively, 5) for NH${}_4^{+}$ and OH⁻ stretching modes

Fig. 3

Evolution with pressure of the experimental IR absorption spectra of AMH. The spectra were collected upon decompression at RT. Pink and black curves are spectra collected above and below the ionic-molecular phase transition, respectively. The frequency window from 2000 to 2300 cm⁻¹ is obscured by the strong absorption band of the diamond anvils. Pressures are indicated on the right. The blue curve is the experimental spectrum of the molecular AMH-I phase at ambient pressure and 95 K from ref.

Fig. 4

Diffraction patterns of AMH above 7.4 GPa. a X-ray and b neutron diffraction patterns of AMH, at RT and respective P of 10.3 and 8.6 GPa. The symbols are experimental data after subtraction of background and removal of the reflections from the diamond anvils in the neutron pattern. The purple solid lines are Le Bail (a) or full profile Rietveld (b) refinements using a mixture of $Im\overline{3}m$ and P4/nmm structures. The green line shows the difference between observed and calculated profiles. Sticks show the positions of Bragg reflections

Fig. 5

Structural and vibrational properties of the disordered ionico-molecular alloy. In a, we compare the simulated neutron pattern for the 6 × 6 × 6 simulation box (red line) to the experimental neutron pattern collected at 10 GPa, 295 K (symbols). The x scale of the simulated pattern has been multiplied by 0.994 to account for the density difference, and the intensity adjusted to scale with the main (110) peak. The Bragg peaks were modelled by pseudo-Voigt profiles of constant width. “D” indicates the reflections from the diamond anvils. b shows the vibrational density of states obtained from the AIMD trajectories. The arrows indicate the OH⁻ stretching mode. c, d Represent simulation snapshots showing the nearest-neighbour environment of an ammonium ion (c) or an ammonia molecule (d). The H atom involved in the H-bond between the central ammonia and neighbour water molecule is rendered in cyan, the others are represented by white spheres. N atoms are in green and blue for NH₃ and NH${}_4^{+},$ respectively, and O atoms are in magenta and red for H₂O and OH⁻, respectively. The solid and dashed lines depict the cubic unit cell and the H bonds, respectively. For clarity, all species have been placed on their average sites