Recently, scientists from the Institute of Solid State Physics (ISSP), Hefei Institutes of Physical Science published new results on the characteristic lattice dynamical properties of highly fluxional hydrogen phase IV.

By applying the new established synthetical Raman-Infrared-Absorption system, scientists conducted a series of researches on the phase IV of hydrogen up to 280 GPa with designed toroidal diamond anvil cells.

The results showed that the unique enhanced fluxional hydrogen in graphene like layer consisted with three hydrogen molecules in phase IV approaching to hydrogen metallization.

The understanding of movement and arrangement of hydrogen molecules under pressure is a key problem to realize metallic hydrogen, one of the issues is, how the specific configuration of hydrogen introduced in phase IV thus brings an abrupt closure of bandgap during phase III-IV transition.

It is theoretically predicted that three hydrogen molecules constitute graphene like layer in phase IV, promoting it transforms to a molecular-atomic mixed state. This novel evidence prior to atomic metallic hydrogen is in urgent need of verification by experiments.

Herein, scientists performed Raman and infrared (IR) spectroscopy measurements of hydrogen at 295 K up to 280 GPa with new established synthetical Raman-Infrared-Absorption system.

To reach the highest pressure, hydrogen was loaded into toroidal diamond anvil cells. The intermolecular coupling has been determined by concomitant measurements of the IR and Raman vibron modes.

In phase IV, scientists find that the intermolecular coupling is much stronger in the graphene like layer (G-layer) of elongated molecules compared to the Br₂ like layer (B-layer) of shortened molecules and it increases with pressure much faster in G-layer compared to B-layer. These heterogeneous lattice dynamical properties are unique features of highly fluxional hydrogen phase IV.

The characteristic graphene like layer (G-layer) and Br2 like layer (B-layer) in phase IV. (Image by JIANG Shuqing)