Experimental phase transition mapping for hydrogen above 300 K up to 300 GPa

TL;DR

This study maps hydrogen's phase transitions at high pressures and temperatures using Raman spectroscopy, revealing 13 phase boundaries and 15 potential phases in previously unexplored conditions.

Contribution

It provides the first comprehensive phase transition map of hydrogen above 300 K up to 300 GPa based on extensive Raman spectroscopic data.

Findings

Identified 13 phase transition boundaries.

Mapped 15 possible phases of hydrogen.

Discovered complex and rich phase behavior.

Abstract

A vast amount of Raman spectroscopic data, obtained using diamond anvil cell technique under in situ high pressure-temperature conditions, has been used for mapping the phase transitions of hydrogen in the temperature range of 300 K to 900 K and pressure up to 300 gigapascal, a pressure-temperature region in which most phase information was unknown previously. The isothermal and isobaric dependencies in frequency and peak width for intramolecular vibration Raman mode of hydrogen are established based on thousands of data files which are measured during hundreds of independent high pressure-temperature experimental runs. Discontinuities of pressure and temperature dependencies in frequency and peak-width, together with appearance or disappearance of observed Raman modes were obtained. Those transition data show self-consistency and have been used for outlining 13 phase transition boundaries and 15 possible phases. This large, diversified behavior of spectra demonstrates that surprisingly rich and complicated phase transitions may exist in this pressure-temperature region.