Evidence and Stability Field of fcc Superionic Water Ice Using Static Compression

Imaging the Meissner effect in hydride superconductors using quantum sensors

By directly altering microscopic interactions, pressure provides a powerful tuning knob for the exploration of condensed phases and geophysical phenomena1. The megabar regime represents an interesting frontier, in which recent discoveries include high-temperature superconductors, as well as structural and valence phase transitions2-6. However, at such high pressures, many conventional measurement techniques fail. Here we demonstrate the ability to perform local magnetometry inside a diamond anvil cell with sub-micron spatial resolution at megabar pressures. Our approach uses a shallow layer of nitrogen-vacancy colour centres implanted directly within the anvil7-9; crucially, we choose a crystal cut compatible with the intrinsic symmetries of the nitrogen-vacancy centre to enable functionality at megabar pressures. We apply our technique to characterize a recently discovered hydride superconductor, CeH9 (ref. 10). By performing simultaneous magnetometry and electrical transport measurements, we observe the dual signatures of superconductivity: diamagnetism characteristic of the Meissner effect and a sharp drop of the resistance to near zero. By locally mapping both the diamagnetic response and flux trapping, we directly image the geometry of superconducting regions, showing marked inhomogeneities at the micron scale. Our work brings quantum sensing to the megabar frontier and enables the closed-loop optimization of superhydride materials synthesis.

Double superionicity in icy compounds at planetary interior conditions

The elements hydrogen, carbon, nitrogen and oxygen are assumed to comprise the bulk of the interiors of the ice giant planets Uranus, Neptune, and sub-Neptune exoplanets. The details of their interior structures have remained largely unknown because it is not understood how the compounds H2O, NH3 and CH4 behave and react once they have been accreted and exposed to high pressures and temperatures. Here we study thirteen H-C-N-O compounds with ab initio computer simulations and demonstrate that they assume a superionic state at elevated temperatures, in which the hydrogen ions diffuse through a stable sublattice that is provided by the larger nuclei. At yet higher temperatures, four of the thirteen compounds undergo a second transition to a novel doubly superionic state, in which the smallest of the heavy nuclei diffuse simultaneously with hydrogen ions through the remaining sublattice. Since this transition and the melting transition at yet higher temperatures are both of first order, this may introduce additional layers in the mantle of ice giant planets and alter their convective patterns.

Temperature-pressure phase diagram of confined monolayer water/ice at first-principles accuracy with a machine-learning force field

Understanding the phase behaviour of nanoconfined water films is of fundamental importance in broad fields of science and engineering. However, the phase behaviour of the thinnest water film - monolayer water - is still incompletely known. Here, we developed a machine-learning force field (MLFF) at first-principles accuracy to determine the phase diagram of monolayer water/ice in nanoconfinement with hydrophobic walls. We observed the spontaneous formation of two previously unreported high-density ices, namely, zigzag quasi-bilayer ice (ZZ-qBI) and branched-zigzag quasi-bilayer ice (bZZ-qBI). Unlike conventional bilayer ices, few inter-layer hydrogen bonds were observed in both quasi-bilayer ices. Notably, the bZZ-qBI entails a unique hydrogen-bonding network that consists of two distinctive types of hydrogen bonds. Moreover, we identified, for the first time, the stable region for the lowest-density [Formula: see text] monolayer ice (LD-48MI) at negative pressures (<-0.3 GPa). Overall, the MLFF enables large-scale first-principle-level molecular dynamics (MD) simulations of the spontaneous transition from the liquid water to a plethora of monolayer ices, including hexagonal, pentagonal, square, zigzag (ZZMI), and hexatic monolayer ices. These findings will enrich our understanding of the phase behaviour of the nanoconfined water/ices and provide a guide for future experimental realization of the 2D ices.

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.

Ultralow Melting Temperature of High-Pressure Face-Centered Cubic Superionic Ice

Superionic ice with oxygen in a face-centered cubic (fcc) sublattice is ascribed to the origin of magnetic fields of Uranus and Neptune, since the melting temperature (T_m) of fcc-superionic ice is believed to be higher than the isentropes of ice giants. However, precisely measuring the fcc-superionic phase experimentally remains a difficult task. The majority of the systematic investigations of its T_m were performed using perfect oxygen fcc-sublattice computations, which could result in superheating and overestimation of T_m. On the basis of the ab initio molecular dynamics method and the model with H₂O vacancy, we avoid superheating and obtain a much lower T_m than previous reports, indicating that fcc-superionic ice cannot exist in the interiors of Uranus and Neptune. Further simulations with the two-phase method justify the conclusion. The results suggest that superheating should be seriously treated when simulating the phase diagram of other hydrogen-related superionic states, which are widely used to understand the properties of ice giants, Earth, and Venus.