Sharma Abhishek Kumar, Escobedo Fernando A
R.F. Smith School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York 14853, USA.
J Chem Phys. 2024 Jul 21;161(3). doi: 10.1063/5.0216886.
Upon osmotic compression, rotationally symmetric faceted colloidal particles can form translationally ordered, orientationally disordered rotator mesophases. This study explores the mechanism of rotator-to-crystal phase transitions where orientational order is gained in a translationally ordered phase, using rotator-phase forming truncated cubes as a testbed. Monte Carlo simulations were conducted for two selected truncations (s), one for s = 0.527 where the rotator and crystal lattices are dissimilar and one for s = 0.572 where the two phases have identical lattices. These differences set the stage for a qualitative difference in their rotator-crystal transitions, highlighting the effect of lattice distortion on phase transition kinetics. Our simulations reveal that significant lattice deviatoric effects could hinder the rotator-to-crystal transition and favor arrangements of lower packing fraction instead. Indeed, upon compression, it is found that for s = 0.527, the rotator phase does not spontaneously transition into the stable, densely packed crystal due to the high lattice strains involved but instead transitions into a metastable solid phase to be colloquially referred to as "orientational salt" for short, which has a similar lattice as the rotator phase and exhibits two distinct particle orientations having substitutional order, alternating regularly throughout the system. This study paves the way for further analysis of diffusionless transformations in nanoparticle systems and how lattice-distortion could influence crystallization kinetics.
在渗透压作用下，具有旋转对称性的多面胶体颗粒可形成平移有序、取向无序的旋转体中间相。本研究以形成旋转体相的截顶立方体为试验平台，探索在平移有序相中获得取向有序的旋转体-晶体相变机制。对两个选定的截断值（s）进行了蒙特卡罗模拟，一个是s = 0.527，此时旋转体晶格和晶体晶格不同；另一个是s = 0.572，此时两相具有相同的晶格。这些差异为它们的旋转体-晶体转变的定性差异奠定了基础，突出了晶格畸变对相变动力学的影响。我们的模拟表明，显著的晶格偏斜效应可能会阻碍旋转体向晶体的转变，而有利于较低堆积分数的排列。事实上，在压缩过程中，发现对于s = 0.527，由于涉及的高晶格应变，旋转体相不会自发转变为稳定的、紧密堆积的晶体，而是转变为一种亚稳固相，通俗地简称为“取向盐”，它具有与旋转体相相似的晶格，并表现出两种不同的粒子取向，具有替代有序性，在整个系统中规则交替。本研究为进一步分析纳米颗粒系统中的无扩散转变以及晶格畸变如何影响结晶动力学铺平了道路。
