Crystalline clusters in mW water: Stability, growth, and grain boundaries

Abstract

With numerical simulations of the mW model of water, we investigate the energetic stability of crystalline clusters for both Ice I (cubic and hexagonal ice) and for the metastable Ice 0 phase as a function of the cluster size. Under a large variety of forming conditions, we find that the most stable cluster changes as a function of size: at small sizes the Ice 0 phase produces the most stable clusters, while at large sizes there is a crossover to Ice I clusters. We further investigate the growth of crystalline clusters with the seeding technique and study the growth patterns of different crystalline clusters. While energetically stable at small sizes, the growth of metastable phases (cubic and Ice 0) is hindered by the formation of coherent grain boundaries. A five-fold symmetric twin boundary for cubic ice, and a newly discovered coherent grain boundary in Ice 0, that promotes cross nucleation of cubic ice. Our work reveals that different local structures can compete with the stable phase in mW water, and that the low energy cost of particular grain boundaries might play an important role in polymorph selection. arXiv:1907.05465v1 [cond-mat.soft]

Non-classical nucleation pathways in stacking-disordered crystals

“…Importantly, in both cases the difference in all thermodynamic relevant quantities (such as free-energy difference, nucleation barrier, and solid/melt surface tension) between the competing polytypes are negligibly small (within 10 −3 k B T per particle for all cases) [1,5,[17][18][19][20][21]. For example, in the mW water model [16] the stacking fault between the ice I c and I h has been estimated as low as 0.16 ± 0.05 mJ m −2 at T = 218 K [4]. In this way, the nucleation mechanisms for both systems are determined not by bulk free energy properties, or by details of their interactions, but by general principles, which we aim at elucidating in the present work.…”

“…We focus here on the stable ice I polytypes (the cubic form I c , and the hexagonal form I h ), and on the metastable ice 0 structure [81,82]. We choose this polymorph as it is currently the only known structure to satisfy all the following criteria: it has the lowest free energy outside the stable cubic and hexagonal (ice I) structures [83]; it is the simplest structure that can be built by deformation of the diamond crystal while preserving to a large degree a highly regular fourfold coordination for the sites [84]; it can stack coherently (without breaking of bonds between grains) with the diamond crystal [4]. These structures have never been observed as fully formed crystals, and instead we focus on clusters of molecules whose nearest-neighbor environment is close to those found in the bulk ice 0 crystal.…”

“…These structures have never been observed as fully formed crystals, and instead we focus on clusters of molecules whose nearest-neighbor environment is close to those found in the bulk ice 0 crystal. It has been recently shown that these clusters have a lower energy than their stable ice I counterparts up to cluster sizes of around 40 water molecules [4]. Particle structures from the same configuration have been identified using the following methods: CNA (a), BOO (b), and LID (c).…”

“…During the nucleation process of many materials, including several metals, minerals and polymers, different crystalline phases, called polymorphs, can nucleate. The structure of the growing nucleus in such materials can depend on many, eventually size-dependent [4,5], effects, such as energy and entropy competition, or frustration. Understanding the selection mechanism of polymorphs is fundamental to predict the structure of the growing nucleus, with applications ranging from Earth's weather and climate forecast, especially in relation to the formation of nanometer-sized ice crystallites in clouds [6][7][8][9][10][11], to the pharmaceutic industry, where the physical and chemical properties of the drug molecules can change with the eventual crystallization of unwanted polymorph forms [12].…”

The physics of Empty Liquids: from Patchy particles to Water

“…The Mercedes-Benz models [61,62] and the mW model [63,64] also belong to the class of coarse grained potentials. The appeal of coarse grained potentials is motivated by their high computational efficiency compared to atomistic models, allowing to considerably extend length and time-scales of simulations making it possible, for example, to study rare events like homogeneous nucleation in direct simulations [65,66]. Recently coarse grained models have also been used to interpolate between water-like and simple-liquid behaviours, studying how these properties change with varying one of the potential parameters.…”