Record pressure squeezes stability of Ar(H2)2- Dr. Cheng Ji

MARCH 15, 2017

Hydrogen-rich materials have been predicted to be promoters for the metallization of hydrogen. A group of scientists led by HPSTAR director, Dr. Ho-kwang Mao has studied Ar(H2)2, a hydrogen-rich material formed by Argon (Ar) and Hydrogen (H2), to 358 gigapascals — almost the pressure in the inner core of the Earth, by combining experimental and theoretical methods. Contrary to the previous thought, it was observed that Ar damps the intermolecular interactions between H2 molecules, an effect as ‘negative’ chemical pressure which postpones metallization. The results were published in Proceeding of the National Academic of Sciences, USA.

Hydrogen molecule (H2), the simplest molecule of the elements and the most abundant element in the universe, is an insulating gas at normal conditions. But under compressing to above 4 million atmosphere pressure – exceeds the pressure at the center of the earth, hydrogen was predicted to be metal and even room-temperature superconductor. Metallic hydrogen thus became a holy grail for materials science because it could be used for superconductors. However either probing the exotic structure of H2 high pressure phase (H is the weakest scatter of X-ray) or transforming H2 into metallic phase presents as extremely grand challenge.

Scientists have predicted that hydrogen-rich material may help the metallization of hydrogen through chemical pressure from the foreign atoms or molecules. Ar(H2)2 belongs to a typical hydrogen-rich material in form of Van der Waals compound. In Ar(H2)2, Ar atoms and H2 molecules are ‘glued’ together by London dispersion force, so that H2 units are preserved. It provides a unique opportunity to explore its crystal structure (due to the presence of heavier atom Ar), molecular properties (Raman), and electronic properties (optical absorption) at the same time to extremely high pressure.

The X-ray beam with high brilliance and sharp focus at beam lines 16-IDB and 13-IDD, APS, ANL, enables X-ray diffraction (XRD) data of Ar(H2)2 to be measured up to 265 GPa — a record pressure for studying hydrogen-rich materials by XRD.

This new study shows that the insulating Ar(H2)2 keeps the MgZn2-type structure in the pressure range up to at least 358 GPa. A splitting of the H2 vibron recorded above 216 GPa points to a molecular orientational ordering transition.

“Now, our work have clarified previous controversies in crystal structures of Ar(H­2)2 at high pressures”, said Dr. Chen Ji, the first author on the study, leading the experimental work.

To explore the pressure affected electronic features on Ar(H2)2, the researchers measured the molecular vibrational properties and electronic properties by an advanced confocal micro-Raman system with 660 nm excitation to 358 GPa.  

“Our results suggest that Ar does not facilitate the molecular dissociation and bandgap closure of H2, moreover it works in the opposite direction,” explained Dr. Mao. “The results provide a solid basis for future searches of hydrogen-rich materials which facilitate metallization of hydrogen.”

Caption: XRD (Right bottom), Raman(left) and optical absorption spectroscopy (top right) of  Ar(H2)2 at 265 and 358 gigapascals in diamond anvil cell (Middle). Image courtesy of Cheng Ji.

Media report：

Eurekalert:  https://www.eurekalert.org/pub_releases/2017-03/cifs-ain032317.php

Carnegie science: https://carnegiescience.edu/news/argon-not-%E2%80%9Cdope%E2%80%9D-metallic-hydrogen

Phys.org: https://phys.org/news/2017-03-argon-dope-metallic-hydrogen.html

CHEMIE.DE: http://www.chemeurope.com/en/news/162528/argon-is-not-the-dope-for-metallic-hydrogen.html

ScienceDaily: https://www.sciencedaily.com/releases/2017/03/170323125511.htm

知社学术圈:  http://chuansong.me/n/1684748251028

在富氢化合物中，有一类新型的化合物是由氢分子与其他的原子或分子在高压下结合而成的范德瓦尔斯化合物，这种新的富氢材料在实验与理论上引起了人们的广泛关注与研究。一方面由于非氢元素的存在会对氢的子晶格产生化学预压作用，这些体系被认为有可能比纯氢更容易金属化。另一方面由于氢含量较多，富氢化合物可能具有像金属氢那样较高的超导转变温度，有望成为新型的超导体-氢基超导体。作为富氢材料的典型代表，Ar(H₂)₂在上世纪90年代被发现并且被认为可能促进氢的解离从而促成绝缘体-金属转变。随之引发了一系列的实验及理论工作研究其在高压下的结构及金属化现象。然而这些结果对与Ar(H₂)₂在高压的晶体及电学性能一直存在争议。本研究中，吉诚等人借助金刚石对顶砧（DAC）将Ar(H₂)₂的高压研究进一步推进到358万大气压的条件-目前研究富氢化合物的最高压力，来研究Ar的加入的氢分子作用力的影响。此团队发现在高达358万大气压的实验条件下，Ar(H₂)₂仍然保持原有的晶体结构，能带间隙的改变相对于纯氢也更小。这与之前研究发现的结构相变及高压下的金属化完全不符。