Complex Phase Behaviour of Dense Chlorine- Philip Dalladay-Simpson & Ross Howie

MARCH 14, 2019 

Due to the realization of potentially exotic states, the behaviour of elemental molecular systems at high densities has long been of particular interest in the condensed matter sciences. Of the 7 elemental diatomic systems, only O2 and the heavier halogens Br2 and I2 have conclusively demonstrated electrically conductive properties in the condensed state, whilst the latter two are the only examples of diatomic molecular dissociation. In a new investigation, an international collaboration between scientists from HPSTAR, the University of Edinburgh and GeoSoilEnviroCARS (GSECARS, APS), report the behaviour of chlorine to pressures in excess of several million atmospheres. The study, which is published in Nature communications, reveals the peculiarity associated high-pressure chlorine, such as metallic modification, structural complexity and ultimately molecular dissociation.

Chlorine is a familiar element in its ambient state, consisting of diatomic units, and is used in a wide range of applications from water chlorination to the production of plastics. It has previously been shown in the heavier halogens, I2 and Br2, that their molecular state is tested when brought to high densities, demonstrating a series of exotic properties en route to dissociating and adopting a close-packed structure. Similar behavior has long been anticipated in elemental chlorine, but its inherent hazardous nature and the extreme pressures required has resulted in the answers to these questions to theoretical examination.

“Again pressure has shown to be an effective tool for tuning matter into highly exotic and contrasting states when compared with their ambient form,” said Dr. Philip Dalladay-Simpson, the lead author of the study.

In this study, an international effort, led by HPSTAR scientists, constrained the room temperature phase behaviour of chlorine in excess of a few million atmospheres. Using a combination of synchrotron X-ray diffraction with Raman and transmission spectroscopy, the team report the existence of several previously unobserved phases in elemental chlorine. Most notably, they observe the continual band gap closure of pure chlorine indicative of a transition to a molecular metallic state. Further, at higher pressures, a precursory incommensurate structure is characterized before the observation of dissociation resulting in the the first monatomic phase of elemental chlorine.

“The observations reported in this study give us additional insight into the pressure induced metallisation and dissociation of simple molecules at high pressures, and glimpse potential properties in the ultimate simple molecule, hydrogen,” said Dr. Philip Dalladay-Simpson.

Caption: The dissociation of chlorine at ultrahigh pressures. 

虽然在卤素的溴和碘中发现了绝缘体到金属的转变及分子解离的现象，作为同族的氯，因为具有更强的化学键，理论预言氯气需要在高达250GPa的条件下才可能发生类似的分子解离。在实验上获得单原子金属氯的愿望迟迟没有实现。超高压技术的重重困难以及氯本身的毒性和化学活性使得氯压致金属化的实验研究面临着巨大的挑战。北京高压科学研究中心的Philip Dalladay-Simpson 研究员带领的研究团队多年致力于超高压力下双原子分子体系的实验研究，积累了雄厚的超高压技术，微米级样品表征手段及高纯的气体样品封装技术等等。该研究团队成功挑战了氯气在超高压下的研究的难题。他们将氯压缩至300GPa以上的压力，采用了高压原位同步辐射X射线衍射技术结合拉曼光谱和可见光吸收光谱对氯在超高压下的晶体结构和光学特性进行了全方位的表征。他们成功验证到了理论所预言的金属化及分子的解离现象。相关研究最近发表在《自然通讯》上。