Bolmatov Dima, Zhernenkov Mikhail, Zav'yalov Dmitry, Tkachev Sergey N, Cunsolo Alessandro, Cai Yong Q
National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, NY 11973, USA.
Volgograd State Technical University, Volgograd, 400005 Russia.
Sci Rep. 2015 Nov 5;5:15850. doi: 10.1038/srep15850.
Supercritical fluids play a significant role in elucidating fundamental aspects of liquid matter under extreme conditions. They have been extensively studied at pressures and temperatures relevant to various industrial applications. However, much less is known about the structural behaviour of supercritical fluids and no structural crossovers have been observed in static compression experiments in any temperature and pressure ranges beyond the critical point. The structure of supercritical state is currently perceived to be uniform everywhere on the pressure-temperature phase diagram, and to change only in a monotonic way even moving around the critical point, not only along isotherms or isobars. Conversely, we observe structural crossovers for the first time in a deeply supercritical sample through diffraction measurements in a diamond anvil cell and discover a new thermodynamic boundary on the pressure-temperature diagram. We explain the existence of these crossovers in the framework of the phonon theory of liquids using molecular dynamics simulations. The obtained results are of prime importance since they imply a global reconsideration of the mere essence of the supercritical phase. Furthermore, this discovery may pave the way to new unexpected applications and to the exploration of exotic behaviour of confined fluids relevant to geo- and planetary sciences.
超临界流体在阐释极端条件下液态物质的基本特性方面发挥着重要作用。它们已在与各种工业应用相关的压力和温度条件下得到广泛研究。然而，对于超临界流体的结构行为了解得还很少，并且在临界点以外的任何温度和压力范围内的静态压缩实验中都未观察到结构转变。目前认为超临界态的结构在压力 - 温度相图上处处均匀，即使在临界点附近移动，不仅沿着等温线或等压线移动，其结构也仅以单调方式变化。相反，我们首次通过在金刚石对顶砧细胞中的衍射测量，在深度超临界样品中观察到结构转变，并在压力 - 温度图上发现了一个新的热力学边界。我们使用分子动力学模拟在液体声子理论的框架内解释了这些转变的存在。所获得的结果至关重要，因为它们意味着对超临界相本质的全面重新思考。此外，这一发现可能为新的意外应用以及探索与地球和行星科学相关的受限流体的奇异行为铺平道路。
