Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, USA.
Institute for High Pressure Physics, RAS 142190, Troitsk, Moscow Region, Russia.
J Chem Phys. 2013 Dec 21;139(23):234501. doi: 10.1063/1.4844135.
The state of matter above the critical point is terra incognita, and is loosely discussed as a physically homogeneous flowing state where no differences can be made between a liquid and a gas and where properties undergo no marked or distinct changes with pressure and temperature. In particular, the structure of supercritical state is currently viewed to be the same everywhere on the phase diagram, and to change only gradually and in a featureless way while moving along any temperature and pressure path above the critical point. Here, we demonstrate that this is not the case, but that there is a well-defined structural crossover instead. Evidenced by the qualitative changes of distribution functions of interatomic distances and angles, the crossover demarcates liquid-like and gas-like configurations and the presence of medium-range structural correlations. Importantly, the discovered structural crossover is closely related to both dynamic and thermodynamic crossovers operating in the supercritical state, providing new unexpected fundamental interlinks between the supercritical structure, dynamics, and thermodynamics.
物质在临界点以上的状态是未知领域，通常被松散地描述为物理均匀的流动状态，在这种状态下，液体和气体之间没有区别，并且性质随着压力和温度的变化没有明显或明显的变化。特别是，超临界状态的结构目前被认为在相图上的任何地方都是相同的，并且在沿着临界点以上的任何温度和压力路径移动时，仅以无特征的方式逐渐变化。在这里，我们证明事实并非如此，而是存在明确的结构交叉。通过原子间距离和角度分布函数的定性变化来证明，这种交叉区分了液态和气态构型以及存在中等范围的结构相关性。重要的是，发现的结构交叉与超临界状态下的动力学和热力学交叉密切相关，为超临界结构、动力学和热力学之间提供了新的意想不到的基本联系。
