Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada.
J Chem Phys. 2013 Mar 28;138(12):12A538. doi: 10.1063/1.4769267.
Despite the fundamental nature and practical importance of melting, there is still no generally accepted theory of this ubiquitous phenomenon. Even the earliest simulations of melting of hard discs by Alder and Wainwright indicated the active role of collective atomic motion in melting and here we utilize molecular dynamics simulation to determine whether these correlated motions are similar to those found in recent studies of glass-forming (GF) liquids and other condensed, strongly interacting, particle systems. We indeed find string-like collective atomic motion in our simulations of "superheated" Ni crystals, but other observations indicate significant differences from GF liquids. For example, we observe neither stretched exponential structural relaxation, nor any decoupling phenomenon, while we do find a boson peak, findings that have strong implications for understanding the physical origin of these universal properties of GF liquids. Our simulations also provide a novel view of "homogeneous" melting in which a small concentration of interstitial defects exerts a powerful effect on the crystal stability through their initiation and propagation of collective atomic motion. These relatively rare point defects are found to propagate down the strings like solitons, driving the collective motion. Crystal integrity remains preserved when the permutational atomic motions take the form of ring-like atomic exchanges, but a topological transition occurs at higher temperatures where the rings open to form linear chains similar in geometrical form and length distribution to the strings of GF liquids. The local symmetry breaking effect of the open strings apparently destabilizes the local lattice structure and precipitates crystal melting. The crystal defects are thus not static entities under dynamic conditions, such as elevated temperatures or material loading, but rather are active agents exhibiting a rich nonlinear dynamics that is not addressed in conventional "static" defect melting models.
尽管熔化具有基本性质和实际重要性，但目前仍没有普遍接受的熔化理论。甚至 Alder 和 Wainwright 对硬碟熔化的最早模拟也表明了集体原子运动在熔化中的积极作用，在这里我们利用分子动力学模拟来确定这些相关运动是否与最近对玻璃形成 (GF) 液体和其他凝聚、强相互作用的粒子系统的研究中发现的相似。我们确实在我们对“过热”Ni 晶体的模拟中发现了串状集体原子运动，但其他观察结果表明与 GF 液体有很大的不同。例如，我们既没有观察到拉伸指数结构弛豫，也没有观察到任何去耦现象，而我们确实发现了玻色峰，这些发现对理解 GF 液体这些普遍性质的物理起源具有重要意义。我们的模拟还提供了一种新的“均匀”熔化观点，其中少量的间隙缺陷通过引发和传播集体原子运动对晶体稳定性产生强大影响。这些相对罕见的点缺陷被发现像孤子一样沿着串传播，驱动集体运动。当置换原子运动采取环形原子交换的形式时，晶体完整性得以保持，但在更高的温度下会发生拓扑转变，其中环打开形成类似于 GF 液体串的线性链，在几何形状和长度分布上相似。开放弦的局部对称破坏效应显然会使局部晶格结构不稳定，并导致晶体熔化。因此，在动态条件下，例如高温或材料加载下，晶体缺陷不是静态实体，而是表现出丰富的非线性动力学的活跃剂，这在传统的“静态”缺陷熔化模型中没有涉及。
