8.2 LIU Xiaodi

Phase diagram of hydrogen、deuterium and hydrogen-deuterium mixtures at high pressures and low temperatures

Xiao-Di Liu 1*, Ross T. Howie 2, Hui-Chao Zhang 1, Philip Dalladay-Simpson 2, Xiao-Jia Chen 1,2, Eugene Gregoryanz 1,2,3

1 Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, China

2 HPSTAR, Shanghai 201203, China

3 CSEC, The University of Edinburgh, Edinburgh, United Kingdom

*Corresponding author, e-mail: xiaodi@issp.ac.cn

The elemental hydrogen under compression has been an alluring topic in the high-pressure sciences for several decades. Despite numerous theoretical and experimental studies recently 1–3，most of them are focused on the ultrahigh pressure, there are less studies at the area of middle pressures and low temperatures where phase II existed. Phase II is important to understand the dense hydrogen and other isotopes 4–9, and also there are several outstanding problems remain in this area, such as the structure of phase II, the influence of the isotopic mass difference, quantum motion and para-ortho equilibrium on the transition pressure to phases II and III, etc. In this report, we present in situ high-pressure (0-200 GPa) low-temperature (4-300K) Raman data for both isotopes. The phase transition line between phase I, II and III are revisited. The presence of a novel phase, phase II’, which is unique to deuterium and distinct from the known phase II, is demonstrated. Furthermore, we also present in situ high-pressure (0-200 GPa) low-temperature (4-300 K) Raman data of hydrogen-deuterium mixtures, showing totally different behavior in “phase II” from that of pure hydrogen and deuterium. The updated phase diagrams of hydrogen、deuterium and hydrogen-deuterium mixtures demonstrate the difference between the isotopes at low temperatures and moderate pressures, providing new information on the phase diagrams of them.

References:

1. Philip Dalladay-Simpson, Ross T. Howie, Eugene Gregoryanz, Nature 529, 63(2016).

2. Ranga P. Dias and Isaac F. Silvera, Science 355,715 (2017).

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