High-Pressure Behavior of Hydrogen and Deuterium at Low Temperatures - Drs. Ross Howie and Xiaojia Chen

AUGUST 17, 2017

New work by HPSTAR’s Ross Howie and Xiao-Jia Chen, in collaboration with the ISSP Chinese Academy of Sciences, presents the first high-pressure experimental study of hydrogen that was completely conducted within China. The authors present in situ high-pressure low-temperature high-quality Raman data for hydrogen and deuterium where they demonstrate the presence of a novel phase, phase II’, unique to deuterium. This work is published Physical Review Letters (DOI: 10.1103/PhysRevLett.119.065301).

Elemental hydrogen under compression has been an alluring topic in the high-pressure sciences for several decades. Despite numerous theoretical and experimental studies, several outstanding problems remain. Despite the remarkable progress in our under- standing of the behavior of both isotopes at very high compressions, there has been no attention given to the P-T regimes where phase II exists.

Phase I of H2 & D2 is known to adopt a hexagonal close-packed structure with freely rotating molecules. The application of high pressures and low temperatures forces the molecules to adopt a broken symmetry phase, phase II, whereby the quantum-mechanical rotational motion is present, though hindered by the increased density.

Using a state-of-the art low-temperature Raman spectroscopic system at the Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences based in Hefei as part of a collaborative effort, the authors systematically map out phase II of H2 and D2.

The changes in the Raman spectra of deuterium with pressure and/or temperature indicate the presence of a previously unknown solid phase existing between 20 and 110 GPa and below 130 K. The novel phase, called phase II’, occupies the larger part of the P-T area that used to be known as phase II. If pressure is further increased at above 110 GPa, phase II’ transforms into phase II.

Remarkably, phase II’ of deuterium is not observed in hydrogen, making it the only phase that does not exist in both isotopes.

“It really is remarkable that this is the first phase which is exhibited in deuterium but not in hydrogen. This is a clear demonstration of the role that quantum effects play in the isotopes” – Ross Howie

Caption: Proposed phase diagrams of hydrogen (a) and deuterium (b) in a low-temperature, medium-pressure range.

经过一个多世纪的研究，位为元素周期表中的第一个元素－氢的温度和压力相图本以为已基本确立。北京高压科学研究中心Ross Howie、陈晓嘉、Eugene Gregoryanz三位研究员与中国科学院固体物理研究所的研究人员一起，利用高压低温拉曼光谱系统，获得了氢和氘在压力（0-200 GPa）和温度（4-300 K）范围内的一系列等温变压和等压变温拉曼光谱。分析发现，氢的同位素氘的振动频率随温度变化中出现与早前熟知的第二相(II)不同的特征, 这个新相存在于25-110 GPa之间低于125 K的低温区域，但在相同温度压力条件下在氢中并没有观察到此相，使得氘II¢相成为目前唯一一个氢氘不共有的低温相。氘II¢相的出现，推测是量子效应的结果，由于氘相对与氢较低的零点能以及分子间强的相互作用而导致复杂的转动运动。另外，研究还获得了氢和氘的相I到相II转变，相II到相III转变的证据，得到了精确的氢和氘相I、II和III的相边界，更新了氢和氘的高压低温相图。此研究是三十多年来氢氘在中压低温区研究的一大突破，对于理解氢的高压相图、相II中的量子效应等具有重要意义。