  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.
