Ultrahigh-pressure isostructural electronic transitions in hydrogen

Area of Science:

• Condensed matter physics
• Materials science under extreme conditions
• Quantum materials

Background:

• High-pressure hydrogen transitions are theorized to yield metallic states with unique superconducting and superfluid properties.
• Understanding these transitions is key to condensed matter physics, but experimental data on crystal structures is scarce due to technical challenges.
• Previous studies lacked detailed crystallographic information for most high-pressure phases of solid hydrogen.

Purpose of the Study:

• To investigate the crystallographic nature of solid hydrogen transitions under extreme pressures.
• To provide crucial experimental data on the crystal structure of solid hydrogen phases.
• To elucidate the precursors to atomic metallic hydrogen.

Main Methods:

• Single-crystal X-ray diffraction was employed to study solid hydrogen.
• Experiments were conducted at pressures up to 254 gigapascals.
• Analysis focused on crystal structure, unit cell volume, and anisotropy changes.

Main Results:

• The hexagonal close-packed (hcp) crystal structure of hydrogen was observed to persist through phases I, III, and IV.
• An increase in anisotropy and a slope change in unit cell volume decrease were noted upon entering phase IV, suggesting a second-order phase transition.
• Evidence points to electronic transitions and molecular symmetry breaking as precursors to atomic metallic hydrogen.

Conclusions:

• The study reveals the detailed crystallographic evolution of solid hydrogen under extreme compression.
• A second-order isostructural phase transition into phase IV was identified.
• The findings suggest that electronic transitions within a distorted hcp structure precede the formation of atomic metallic hydrogen.