A collaborative study between Prof. Eugene Gregoryanz, SHARPS/HPSTAR，Dr. Ross Howie, HPSTAR and Dr. Miriam Pena-Alvarez, U of Edinburgh has revised the phase diagram of methane -- a textbook example of a chemical compound that is important in fundamental and environmental sciences as well as in industry. The results were published in Physical Review Letters [links to https://journals.aps.org/prl/abstract/10.1103/7hxd-hhjf ] and featured in Physics as Editors’ suggestion.
Because of the deceptive simplicity of its molecule and being the main constituent of natural gas found on Earth methane was arguably studied more than some other iconic compounds or archetypal elements. Surprisingly, in the literature there are still has some discrepancies and controversies in the phase diagram of this common gas at high temperatures (above 300 K).
By conducting dozens of in situ high-pressure and high-temperature Raman spectroscopy experiments, the team conducted the systematic exploration of the phase diagram resolving inconsistencies of earlier studies.
The experiments yielded two distinct phase diagrams, one that demonstrates the kinetic phase transformations and the other presenting the equilibrium states usually reached with time. Raman spectroscopy demonstrated that the appearance and transitions between the higher pressure phases (VII, VIII, and IX) are strongly dependent on the pressure-temperature-time path. By combining the visual observations and optical spectroscopy the authors were able to suggest that the melting curve of methane extends to significantly higher temperatures than previously reported, e.g., ∼1000 K at 15 GPa. The first author of the study, the PhD student of Prof. Gregoryanz at U of Edinburgh and earlier Master student of Dr. Howie at HPStar, Dr Wang Mengnan said: the high pressure studies of methane require a lot patience and attention – we were waiting for days and sometimes for months to reach the equilibrium state and we found out that some of the inconsistencies in the earlier melting data could be attributed to photochemical dissociation and/or a reaction induced by high-intensity light sources, the fact which was often missed in the earlier studies.
Figure 1.The artistic impression of the kinetic and equilibrium phase diagrams of methane.
由上海前瞻物质科学研究院和北京高压科学研究中心的Eugene Gregoryanz教授和Ross Howie博士与爱丁堡大学的Miriam Pena-Alvarez博士合作，对甲烷的相图进行了修订——甲烷是基础科学、环境科学以及工业领域中一个重要的化学化合物的教科书式范例。研究成果发表在《物理评论快报》[https://journals.aps.org/prl/abstract/10.1103/7hxd-hhjf]上，并被作为编辑推荐内容刊载。
