12 research outputs found
Controlling crystallization: What liquid structure and dynamics reveal about crystal nucleation mechanisms
Over recent years, molecular simulations have provided invaluable insights
into the microscopic processes governing the initial stages of crystal
nucleation and growth. A key aspect that has been observed in many different
systems is the formation of precursors in the supercooled liquid that precedes
the emergence of crystalline nuclei. The structural and dynamical properties of
these precursors determine to a large extend the nucleation probability as well
as the formation of specific polymorphs. This novel microscopic view on
nucleation mechanisms has further implications for our understanding of the
nucleating ability and polymorph selectivity of nucleating agents, as these
appear to be strongly linked to their ability in modifying structural and
dynamical characteristics of the supercooled liquid, namely liquid
heterogeneity. In this perspective, we highlight recent progress in exploring
the connection between liquid heterogeneity and crystallization, including the
effects of templates, and the potential impact for controlling crystallization
processes
Interplay of structural and dynamical heterogeneity in the nucleation mechanism in Nickel
Gaining fundamental understanding of crystal nucleation processes in metal alloys is crucial for the development and design of high-performance materials with targeted properties. Yet, crystallizationis a complex non-equilibrium process and, despite having been studied for decades, the microscopic aspects that govern the crystallization mechanism of a material remain to date elusive. Recent evidence shows that spatial heterogeneity in the supercooled liquid, characterised by extended regions with distinctive mobility and order, may be a key microscopic factor that determines the mechanism of crystal nucleation. These findings have revolutionised our view of the fundamental nature of crystallization, as most research has assumed that crystal clusters nucleate from random fluctuations in a âhomogeneousâ liquid. Here, by analysing transition path sampling trajectories, we show that dynamical heterogeneity plays a key role in the mechanism of crystal nucleation in an elemental metal, nickel. Our results demonstrate that crystallization occurs preferentially in regions of low mobility in the supercooled liquid, evidencing the collective dynamical nature of crystal nucleation in Ni. In addition, our results show that low mobility regions form before and spatially overlap with pre-ordered domains that act as precursors to the crystal phase that subsequently emerges. Our results show a clear link between dynamical and structural heterogeneity in the supercooled liquid and its impact on the nucleation mechanism, revealing microscopic descriptors that could pave a novel way to control crystallization processes in metals
Interplay of structural and dynamical heterogeneity in the nucleation mechanism in Nickel
Gaining fundamental understanding of crystal nucleation processes in metal alloys is crucial for the development and design of high-performance materials with targeted properties. Yet, crystallizationis a complex non-equilibrium process and, despite having been studied for decades, the microscopic aspects that govern the crystallization mechanism of a material remain to date elusive. Recent evidence shows that spatial heterogeneity in the supercooled liquid, characterised by extended regions with distinctive mobility and order, may be a key microscopic factor that determines the mechanism of crystal nucleation. These findings have revolutionised our view of the fundamental nature of crystallization, as most research has assumed that crystal clusters nucleate from random fluctuations in a âhomogeneousâ liquid. Here, by analysing transition path sampling trajectories, we show that dynamical heterogeneity plays a key role in the mechanism of crystal nucleation in an elemental metal, nickel. Our results demonstrate that crystallization occurs preferentially in regions of low mobility in the supercooled liquid, evidencing the collective dynamical nature of crystal nucleation in Ni. In addition, our results show that low mobility regions form before and spatially overlap with pre-ordered domains that act as precursors to the crystal phase that subsequently emerges. Our results show a clear link between dynamical and structural heterogeneity in the supercooled liquid and its impact on the nucleation mechanism, revealing microscopic descriptors that could pave a novel way to control crystallization processes in metals
Identification of a multi-dimensional reaction coordinate for crystal nucleation in Ni3Al
Nucleation during solidification in multi-component alloys is a complex process that comprises competition between different crystalline phases as well as chemical composition and ordering. Here, we combine transition interface sampling with an extensive committor analysis to investigate the atomistic mechanisms during the initial stages of nucleation in Ni3Al. The formation and growth of crystalline clusters from the melt are strongly influenced by the interplay between three descriptors: the size, crystallinity, and chemical short-range order of the emerging nuclei. We demonstrate that it is essential to include all three features in a multi-dimensional reaction coordinate to correctly describe the nucleation mechanism, where, in particular, the chemical short-range order plays a crucial role in the stability of small clusters. The necessity of identifying multi-dimensional reaction coordinates is expected to be of key importance for the atomistic characterization of nucleation processes in complex, multi-component systems
Structural transformations driven by local disorder at interfaces
Despite the fundamental importance of solid-solid transformations in many
technologies, the microscopic mechanisms remain poorly understood. Here, we
explore the atomistic mechanisms at the migrating interface during solid-solid
phase transformations between the topologically closed-packed A15 and
body-centred cubic phase in tungsten. The high energy barriers and slow
dynamics associated with this transformation require the application of
enhanced molecular sampling approaches. To this end, we performed metadynamics
simulations in combination with a path collective variable derived from a
machine learning classification of local structural environments, which allows
the system to freely sample the complex interface structure. A disordered
region of varying width forming at the migrating interface is identified as a
key physical descriptor of the transformation mechanisms, facilitating the
atomic shuffling and rearrangement necessary for structural transformations.
Furthermore, this can directly be linked to the differences in interface
mobility for distinct orientation relationships as well as the formation of
interfacial ledges during the migration along low-mobility directions
Role of pre-ordered liquid in the selection mechanism of crystal polymorphs during nucleation
We investigate the atomistic mechanism of homogeneous nucleation during solidification in molybdenum employing transition path sampling. The mechanism is characterized by the formation of a pre-structured region of high bond-orientational order in the supercooled liquid followed by the emergence of the crystalline bulk phase within the center of the growing solid cluster. This precursor plays a crucial role in the process as it provides a diffusive interface between the liquid and crystalline core, which lowers the interfacial free energy and facilitates the formation of the bulk phase. Furthermore, the structural features of the pre-ordered regions are distinct from the liquid and solid phases and preselect the specific polymorph that nucleates. The similarity in the nucleation mechanism of Mo with that of metals that exhibit different crystalline bulk phases indicates that the formation of a precursor is a general feature observed in these materials. The strong influence of the structural characteristics of the precursors on the final crystalline bulk phase demonstrates that for the investigated system, polymorph selection takes place in the very early stages of nucleation
Template induced precursor formation in heterogeneous nucleation -- Controlling polymorph selection and nucleation efficiency
We present an atomistic study of heterogeneous nucleation in Ni employing
transition path sampling, which reveals a template precursor-mediated mechanism
of crystallization. Most notably, we find that the ability of tiny templates to
modify the structural features of the liquid and promote the formation of
precursor regions with enhanced bond-orientational order is key to determine
their nucleation efficiency and the polymorphs that crystallize. Our results
reveal an intrinsic link between liquid heterogeneity and the nucleating
ability of templates, which significantly advances our understanding towards
the control of nucleation efficiency and polymorph selection.Comment: Manuscript 6 pages, 3 Figures. Supplemental : 10 pages, 8 figure
Origin of Spectral Band Patterns in the Cosmic Unidentified Infrared Emission
The cosmic unidentified infrared emission (UIE) band phenomenon is generally considered as indicative of free-flying polycyclic aromatic hydrocarbon molecules in space. However, a coherent explanation of emission spectral band patterns depending on astrophysical source is yet to be resolved under this attribution. Meanwhile astronomers have restored the alternative origin as due to amorphous carbon particles, but assigning spectral patterns to specific structural elements of particles is equally challenging. Here we report a physical principle in which inclusion of nonplanar structural defects in aromatic core molecular structures (Ï domains) induces spectral patterns typical of the phenomenon. We show that defects in model Ï domains modulate the electronic-vibration coupling that activates the delocalized Ï-electron contribution to aromatic vibrational modes. The modulation naturally disperses C=C stretch modes in band patterns that readily resemble the UIE bands in the elusive 6-9 ÎŒm range. The electron-vibration interaction mechanics governing the defect-induced band patterns underscores the importance of Ï delocalization in the emergence of UIE bands. We discuss the global UIE band regularity of this range as compatible with an emission from the delocalized sp2 phase, as Ï domains, confined in disordered carbon mixed-phase aggregates
Practical guide to replica exchange transition interface sampling and forward flux sampling
Path sampling approaches have become invaluable tools to explore the
mechanisms and dynamics of so-called rare events that are characterized by
transitions between metastable states separated by sizeable free energy
barriers. Their practical application, in particular to ever more complex
molecular systems, is, however, not entirely trivial. Focusing on replica
exchange transition interface sampling (RETIS) and forward flux sampling (FFS),
we discuss a range of analysis tools that can be used to assess the quality and
convergence of such simulations which is crucial to obtain reliable results.
The basic ideas of a step-wise evaluation are exemplified for the study of
nucleation in several systems with different complexity, providing a general
guide for the critical assessment of RETIS and FFS simulations.Comment: 25 pages, 30 figures. The following article has been accepted by The
Journal of Chemical Physics. After it is published, it will be found at
https://aip.scitation.org/journal/jc