New low temperature phase in dense hydrogen: The phase diagram to 421 GPa

Understand

In the quest to make metallic hydrogen at low temperatures a rich number of new phases have been found and the highest pressure ones have somewhat flat phase lines, around room temperature.

• We have studied hydrogen to static pressures of GPa in a diamond anvil cell and down to liquid helium temperatures, using infrared spectroscopy.
• We report a new phase at a pressure of GPa and T=5 K.
• Although we observe strong darkening of the sample in the visible, we have no evidence that this phase is metallic hydrogen.