Hexagonal to Monoclinic Phase Transition in Dense Hydrogen Phase III Detected by High-Pressure NMR

TL;DR

This study uses high-pressure NMR spectroscopy to identify a phase transition in dense hydrogen, revealing a shift from hexagonal to monoclinic structure at around 197 GPa, advancing understanding of hydrogen's high-pressure phases.

Contribution

First in-situ high-resolution NMR experiments at megabar pressures demonstrating a structural phase transition in hydrogen phase III.

Findings

Hydrogen phase III transitions from hexagonal to monoclinic structure around 197 GPa.

High-resolution NMR spectra align with theoretical predictions.

Highlights the importance of NMR and ab-initio calculations in high-pressure structural analysis.

Abstract

Conclusive crystal structure determination of the high pressure phases of hydrogen remains elusive due to lack of core electrons and vanishing wave vectors, rendering standard high-pressure experimental methods moot. Ab-initio DFT calculations have shown that structural polymorphism might be solely resolvable using high-resolution nuclear magnetic resonance (NMR) spectroscopy at mega-bar pressures, however technical challenges have precluded such experiments thus far. Here, we present in-situ high-pressure high-resolution NMR experiments in hydrogen phase III between 181 GPa and 208 GPa at room temperature. Our spectra suggest that at lower pressures phase III adopts a hexagonal P6122 crystal structure, transitioning into a monoclinic C2/c phase at about 197 GPa. The high resolution spectra are in excellent agreement with earlier structural and spectral predictions and underline the possibility of a subtle P6122 to C2/c phase transition in hydrogen phase III. These experiments show the importance of a combination of ab-initio calculations and low-Z sensitive spectral probes in high-pressure science in elucidating the structural complexity of the most abundant element in our universe.