The 8th International Conference on Matter and Radiation at Extremes (ICMRE2025)

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.

Recent advances in the development of high resolution NMR techniques under extreme conditions however lead to the potential discrimination of structural models for hydrogen at mega-bar pressures. In this talk, we will introduce our novel high resolution technique for in-situ high pressure NMR measurements and demonstrate that space group determination is indeed possible in the high pressure polymorphs of hydrogen.

On NMR time scales, our experiments suggest that high pressure hydrogen exhibits a sequence of subtle symmetry breaks from a simple hexagonal P6₃/mmc structure, to a chiral hexagonal P6₁22 atomic arrangement at pressure between 180 and 195 GPa, followed by a transition to a monoclinic C2/c crystal structure at about 200 GPa. Further compression above the phase III → IV transition evidences a further loss of crystal symmetry up to our maximal pressure of 286 GPa.