Nucleation of crystals that are mixed composites of all three polymorphs in the Gaussian core model

Mithen, J.; Callison, Adam; Sear, Richard P.

doi:10.1063/1.4922321

Cited by 13 publications

(20 citation statements)

References 31 publications

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“…A natural extension of the ideas presented in the previous section is their application to polymorph selection, which has attracted a lot of investigations [1,2,44,103,111,[141][142][143][144][145][146].…”

Section: Role Of Precursors In Polymorph Selectionmentioning

confidence: 99%

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Crystal nucleation as the ordering of multiple order parameters

Russo

Tanaka

2016

The Journal of Chemical Physics

107

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Nucleation is an activated process in which the system has to overcome a free energy barrier in order for a first-order phase transition between the metastable and the stable phases to take place. In the liquid-to-solid transition the process occurs between phases of different symmetry, and it is thus inherently a multi-dimensional process, in which all symmetries are broken at the transition. In this Focus Article, we consider some recent studies which highlight the multi-dimensional nature of the nucleation process. Even for a single-component system, the formation of solid crystals from the metastable melt involves fluctuations of two (or more) order parameters, often associated with the decoupling of positional and orientational symmetry breaking. In other words, we need at least two order parameters to describe the free-energy of a system including its liquid and crystalline states. This decoupling occurs naturally for asymmetric particles or directional interactions, focusing here on the case of water, but we will show that it also affects spherically symmetric interacting particles, such as the hard-sphere system. We will show how the treatment of nucleation as a multi-dimensional process has shed new light on the process of polymorph selection, on the effect of external fields on the nucleation process, and on glass-forming ability.

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Section: Role Of Precursors In Polymorph Selectionmentioning

confidence: 99%

“…Ref. [145] then found that the nuclei have a mixed nature, not consisting of a single polymorph, and that the kinetic pathway selected during nucleation persists even when the nucleus is many times above its critical size.…”

Section: Role Of Precursors In Polymorph Selectionmentioning

confidence: 99%

Crystal nucleation as the ordering of multiple order parameters

Russo

Tanaka

2016

The Journal of Chemical Physics

107

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“…Polymorph selection plays a fundamental role in many applications, and has been the subject of many investigations [31,52,64,84,85].…”

Section: Polymorph Selectionmentioning

confidence: 99%

Nonclassical pathways of crystallization in colloidal systems

Russo

Tanaka

2016

MRS Bull.

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Colloidal systems offer the ideal conditions to study the nucleation process, both from an experimental viewpoint, due to their relative large size and long time-scales, and from a modeling point of view, due to the tunability of their interactions. Here we review some recent works that study the process of colloidal crystallization from a microscopic perspective. We focus in particular on non-classical pathways to nucleation where the appearance of the solid crystals involves fluctuations of two (or more) order parameters. We interpret the non-classical behavior as a decoupling of positional and orientational symmetry breaking. We then consider how the nucleation pathway determines which polymorph is selected upon nucleation from the melt. Moreover we show how the study of nucleation pathways not only sheds new light on the microscopic mechanism of nucleation, but also provides important information regarding its avoidance, thus suggesting a deep link between crystallization and vitrification.

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“…This state forms via the conventional nucleation then growth kinetics of a first-order transition, see Mithen et al 30 for further details.…”

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confidence: 99%

“…It did not occur in direct simulations. We attempted Forward Flux Sampling (FFS) 30,34 simulations of fcc nucleation from our bcc states, but they were unsuccessful. So although we expect that the bcc phase will ultimately transform into the fcc phase, the bcc phase is clearly stable for long periods.…”

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State between Liquid and Crystal: Locally Crystalline but with the Structure Factor of a Liquid

Mithen

Sear

2016

Crystal Growth & Design

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We are familiar with large highly-ordered single crystals, such as those used in the semiconductor industry. There the crystalline ordering in the silicon crystals extends for billions and more lattice spacings. As a result, X-ray diffraction (XRD) patterns of these crystals have sharp Bragg peaks. We are also familiar with liquids, where the molecules have a local structure that is very different to that in a crystal. In addition, in liquids there is only ordering over a few molecular diameters, and so they do not have Bragg peaks. But there are materials, where the situation is much less clear. Many of these materials are referred to as amorphous.For example, the structure of amorphous silicon is controversial. There is debate 1-3 over whether it possesses some crystalline ordering of the sort variously referred to as paracrystalline order, nanocrystalline order or medium-range crystalline order, or whether it has no crystalline ordering. We do not know if locally, silicon atoms are arranged in the structure of a liquid, or whether they are in tiny crystals only a few silicon atoms across. There are other systems at the borderline between liquid and crystal. The mineral ferrihydrite can be classified as crystalline (this is called six-line ferrihydrite), or as amorphous (this is called two-line ferrihydrite). [4][5][6] The structures of Amorphous Calcium Carbonate (ACC), 7-16 Amorphous Calcium Phosphate (ACP), 17 and Amorphous Calcium Sulphate (ACS) [18][19][20] are also poorly understood. The properties of ACC clearly depend on how it is prepared, 9,21-24 and Gebauer et al. 23 refer to 'protovaterite' and 'proto-calcite' forms of ACC, which * To whom correspondence should be addressed although amorphous have NMR and EXAFS spectra that have (broadened) features in common with vaterite, and with calcite, respectively. Vaterite and calcite are two of the crystal polymorphs of calcium carbonate.Experimental data such as XRD can clearly distinguish between large single crystals and liquids, but as we have known since Scherrer's work a hundred years ago, 25 as the size of crystallites decreases, the width of the Bragg peaks in the structure factor S(k) increases. Finite size effects give a peak width ∆k ≈ k/n, for a crystallite n lattice spacings across. For n 10, the peak is so broad that it is essentially as broad as features in the S(k)'s of liquids, and so XRD is not be able to distinguish between a liquid or glass, and a state made of very small crystallites. In computer simulations we can circumvent the problem this causes in identifying a state, as there we know the exact positions of all molecules, in real space, and so have much more information than is contained in an XRD pattern.Here we use computer simulation to study crystallisation in microscopic detail, in a simple model: the Gaussian Core Model (GCM). 26 In the GCM, molecules interact via the spherically symmetric potential v(r) = ε exp[−r 2 /σ 2 ], where the parameters ε and σ have dimensions of energy and length, respectively. This is a simple mod...

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Nucleation of crystals that are mixed composites of all three polymorphs in the Gaussian core model

Cited by 13 publications

References 31 publications

Crystal nucleation as the ordering of multiple order parameters

Crystal nucleation as the ordering of multiple order parameters

Nonclassical pathways of crystallization in colloidal systems

State between Liquid and Crystal: Locally Crystalline but with the Structure Factor of a Liquid

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