High-Pressure Behavior of Hydrogen and Deuterium at Low Temperatures.

Abstract

In situ high-pressure low-temperature high-quality Raman data for hydrogen and deuterium demonstrate the presence of a novel phase, phase II^{'}, unique to deuterium and distinct from the known phase II. Phase II^{'} of D_{2} is not observed in hydrogen, making it the only phase that does not exist in both isotopes and occupies a significant part of P-T space from ∼25 to 110GPa and below 125K. For H_{2}, the data show that below 30K the transition to phase II happens at as low as 73GPa. The transformation from phase II to III commences at around ∼155 GPa and is completed by 170GPa with the average pressure of ∼160 GPa being slightly higher than previously thought. The updated phase diagrams of H_{2} and D_{2} demonstrate the difference between the isotopes at low temperatures and moderate pressures, providing new information on the phase diagrams of both elements.