Structure and dynamics of water and silicon in carbon nanotube

最近的计算研究表明，当紧密堆积在狭窄的圆柱孔中时，球形粒子具有多种手性和非手性的一维结构，一项实验研究证实，C60分子在碳纳米管中形成了一些预测的结构。这些不同体系的结果暗示了圆柱腔中球形粒子的一般相行为，但我们对可能的相的了解仍然是零碎的，其中相变的性质很少被探索。在这里，我们展示了一个碳纳米管中的Ar的模型系统，前十个固相与前十种折叠三角形晶格的方式一一对应，每种方式都由一个像单壁碳纳米管一样的卷曲矢量来表征。然后，通过固有结构计算和分子动力学模拟，得到了系统在压力-直径和温度-直径平面上的相图，从而揭示了随着温度的升高，最终消失的层状相界表面。这种相界的后果是，对于任何一维固体，都可能发生两种类型的冻结--渐进的和突然的转变；将管径改变颗粒直径的十分之一可能会改变所产生的固相以及冻结行为的类型；改变压力可能会在给定的纳米管中引起几种结构变化，但改变温度可能只会引起单一的，在大多数情况下是渐进的液-固相转变。

Recent computational studies showed that spherical particles take several chiral and achiral one-dimensional structures when closely packed in narrow cylindrical pores and an experimental study confirmed that C60 molecules form some of the predicted structures in carbon nanotubes. These results for different systems imply generic phase behaviour of spherical particles in cylindrical cavities but our knowledge on possible phases is yet fragmentary and nature of phase transformations among them is little explored. Here we show for a model system of argon in carbon nanotubes that first ten solid phases are in one-to-one correspondence with the first ten ways of folding a triangular lattice, each being characterized by a roll-up vector like the single-walled carbon nanotube. We then obtain, by inherent-structure calculation and molecular dynamics simulation, phase diagrams of the system in pressure-diameter and temperature-diameter planes, thereby being able to unveil the layered surfaces of phase boundaries that eventually disappear as temperature is increased. Consequences of such phase boundaries are that two types of freezing, gradual and abrupt transformation, are possible for any of the one-dimensional solids, that changing the tube diameter by a tenth of particle's diameter may alter the resulting solid phase as well as the type of freezing behaviour, and that changing the pressure may induce several structural changes in a given nanotube but changing the temperature may only give rise to a single, and in most cases gradual, liquid-solid phase transformation.