Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India.
J Chem Phys. 2013 Nov 21;139(19):194702. doi: 10.1063/1.4829762.
Solid-solid collapse transition in open framework structures is ubiquitous in nature. The real difficulty in understanding detailed microscopic aspects of such transitions in molecular systems arises from the interplay between different energy and length scales involved in molecular systems, often mediated through a solvent. In this work we employ Monte-Carlo simulation to study the collapse transition in a model molecular system interacting via both isotropic as well as anisotropic interactions having different length and energy scales. The model we use is known as Mercedes-Benz (MB), which, for a specific set of parameters, sustains two solid phases: honeycomb and oblique. In order to study the temperature induced collapse transition, we start with a metastable honeycomb solid and induce transition by increasing temperature. High density oblique solid so formed has two characteristic length scales corresponding to isotropic and anisotropic parts of interaction potential. Contrary to the common belief and classical nucleation theory, interestingly, we find linear strip-like nucleating clusters having significantly different order and average coordination number than the bulk stable phase. In the early stage of growth, the cluster grows as a linear strip, followed by branched and ring-like strips. The geometry of growing cluster is a consequence of the delicate balance between two types of interactions, which enables the dominance of stabilizing energy over destabilizing surface energy. The nucleus of stable oblique phase is wetted by intermediate order particles, which minimizes the surface free energy. In the case of pressure induced transition at low temperature the collapsed state is a disordered solid. The disordered solid phase has diverse local quasi-stable structures along with oblique-solid like domains.
在开放骨架结构中，固-固坍塌转变是普遍存在的。在分子体系中理解这种转变的详细微观方面的真正困难源于涉及的不同能量和长度尺度之间的相互作用，这种相互作用通常通过溶剂来介导。在这项工作中，我们采用蒙特卡罗模拟来研究通过各向同性和各向异性相互作用相互作用的模型分子体系中的坍塌转变，这些相互作用具有不同的长度和能量尺度。我们使用的模型被称为梅赛德斯-奔驰（MB），对于特定的参数集，它维持着两种固体相：蜂窝状和斜交状。为了研究温度诱导的坍塌转变，我们从亚稳态的蜂窝状固体开始，通过升高温度来诱导转变。由此形成的高密度斜交固体具有两个特征长度尺度，分别对应于相互作用势能的各向同性和各向异性部分。与普遍的观点和经典成核理论相反，有趣的是，我们发现线性带状成核团簇具有明显不同的有序性和平均配位数，与体相稳定相不同。在生长的早期阶段，团簇作为线性条带生长，随后是分支和环形条带。生长团簇的几何形状是两种相互作用之间的微妙平衡的结果，这种平衡使得稳定能超过了不稳定的表面能。稳定斜交相的核被中间有序的粒子润湿，从而最小化了表面自由能。在低温下的压力诱导转变中，坍塌状态是无序的固体。无序固体相具有多种局部准稳定结构，以及斜交固体状的畴。
