A team of scientists from Institute of Solid State Physics (ISSP), CAS, Hefei and Shanghai Advanced Research in Physical Sciences (SHARPS), Shanghai has investigated the phase diagram of H2-HD-D2 mixture in a wide temperature range and up to 350 GPa. This pressure is among the very few highest pressure claims hydrogen was ever experimentally subjected to and the highest pressure experimentally reached on hydrogen in China. The study mapped out the location of phase IV and demonstrated that the appearance of the mixture’s phase diagram is similar to those of pure H2 and D2 contrary to the previous claim. The researches also optically measured the closing band-gap showing that alloying one isotope with another will not bring the pressure of metallisation any closer. The paper was chosen as the Editor’s Suggestion.
Hydrogen is the lightest element and among the elements has the largest isotopic mass ratio with its heavy brother - deuterium. As a result, the observation of the isotopic effects provides an interesting information into the behaviour of hydrogens. Among the several isotopes, hydrogen deuteride (HD) molecule is composed of hetero-nuclear atoms, which make its mass to be exactly in between hydrogen and deuterium. The HD molecule is not limited by the exchange symmetry and the direction of the nuclear spin has no effect on the energy level configuration. These physical properties and the fact that HD was very little studied under extreme conditions make its phase diagram and comparison to two other isotopes of great interest.
Currently, five high-pressure solid phases of hydrogen (phases I, II, III, IV, V) are know (N.B. phase V was not observed in deuterium and is believed to exist at pressures higher than those needed to reach it in hydrogen). Hydrogen deuteride was studied considerably less with the claims that solid HD adopts the structures which were never observed in the pure isotopes and speculation that HD might metallise earlier than H2 and D2. In this study the authors from Hefei Institute of Solid State Physics (ISSP) and SHARPS mapped out the phase diagram of a H2-HD-D2 mixture up to 350 GPa (3.5 MAtm) and between 10 and 300 K utilising the Raman and absorption/transmission spectroscopy. The team established the record pressure (in China) at which hydrogens were studied and HaiAn Xu, the first author of the study and PhD student between USTC and ISSP, said: Hydrogen and deuterium are notoriously difficult to work with at high pressures due to its reactivity and “softness” but it’s very tough to study any of them at above 300 GPa. But my goal is to reach even higher pressure in the nearest future.
They authors traced the phase line separating phases III and IV up to 350 GPa showing the negative slope (-0.95 K/GPa). They see it flattens at above 250 GPa (−0.55 K/GPa), implying the quantum nature of phase IV. Hydrogen deuteride phase IV has an interesting configuration with the 6-atom rings being made of H and D atoms randomly distributed in the ring, leading to the mass driven Anderson localisation. Moreover, In phase III located at lower temperatures than IV, they observe the same Anderson localisation, the effect which is also fundamental in shaping the appearance of the Raman spectra of the H:D mixtures in phases IV. The Anderson localisation, driven by the mass difference, is the phenomenon that could be only observed in alloys such as H:D mixture but apparently does not play any role in determining their phase diagram.
Caption: The phase diagram of H2-HD-D2.
By optically measuring the absorption/transmission of the sample they also show that the band-gap of H2-HD-D2 is closing and that its values lie almost exactly between those of pure hydrogen and deuterium. This implies that alloying one isotope with another would not shift the metallisation pressure to the lower values, as was previously suggested.
Overall, these observations imply that hydrogen deuteride has very similar properties to those exhibited by the pure species.
近日，由上海前瞻物质科学研究院和中国科学院合肥固体物理研究所等组成的研究小组通过高压低温拉曼光谱和光学透射/吸收光谱绘制了H2-HD-D2混合物的相图，研究覆盖了广泛的温度范围，并达到了350 GPa的高压。这一压力是氢气实验中曾经施加的少数几个最高压力之一，也是中国实验上氢气所能达到的最高压力。研究绘制了相IV的位置，并证明该混合物的相图呈现出与纯H2和D2相似的特征，这与先前的研究结论相反。研究人员还通过光学测量了带隙的关闭，表明将一种同位素与另一种同位素合金化并不会使金属化的压力接近。相关成果以“Phase transitions in H2-HD-D2 mixtures up to 350 GP” 为题发表于《PHYSICAL REVIEW B》，并被选为编辑推荐。文章链接：https://doi.org/10.1103/PhysRevB.111.024109
