An international group of researchers from China and the United States has deduced a working atomic-level mechanism for the melting and freezing of single bismuth nanoparticles, with wider implications for phase transitions in general.

The researchers employed a method that allowed for the in-situ growth of bismuth nanoparticles to occur using a specific precursor (SrBi2Ta2O9) under an electron beam within a high-resolution transmission electron microscope (HRTEM). Under the microscope, the researchers melted and froze the nanoparticles and imaged their triggering mechanism in real-time.

Based on their findings, the researchers have now created an interaction-relaxation model that is geared towards understanding the microscopic mechanism of phase transitions and the importance of multiscale processes.

Why is it important?

Until recently, the mechanisms and processes that govern the nucleation growth of nanoparticles have been based on a theory known as classical nucleation theory, CNT for short. This theory assumes that a particle will nucleate at a site and migrate outwards. However, this theory has run into some limitations as the model is too simple to account for the nucleation processes in phase transitions.

Since then, there have been many efforts to produce alternative mechanisms and the ones that have shown the most promise are two-step nucleation and solid-solid phase transitions. The mechanisms suggest that the phase-transition proceeds through metastable phases, but there is still a lack of fundamental knowledge (both theoretical and experimental) into the metastable transition states.

Very little is also known about the atomistic mechanisms that govern both melting and freezing in real systems. This is particularly true in the early and final stages of the transition. Most reports detail the growth process as a local event, but nonlocal events also play a major part even though they are rarely studied.

There has been a need for a mechanism to identify the atomic information on the unabridged pathway, from the early nucleation stage to the final stage of the whole system, to fundamentally understand reverse-phase transitions.

How did they do it?

The researchers looked at both the melting and freezing transition phases and employed an experimental approach. Whilst many people would think that a computational modelling approach would yield the ideal mechanisms, the rare-barrier crossing events that govern the microscopic processes are too intricate and difficult to predict.

To achieve such results, the researchers had to find a material that could undergo such transition phases. Bismuth nanoparticles can undergo a controllable and reversible phase change for both melting and freezing process, so was found to be an ideal choice.

The nanoparticles were produced, confirmed and analysed using an electron-beam irradiation in an HRTEM and energy dispersive spectroscopy (EDS). The researchers used these to create a model system to investigate the metastable states and reverse phase transitions, in-situ.

What did they find?

The research provided atomic scale evidence of point defect induced melting and the crystallisation of intermediately ordered liquids during freezing. As such, the research has proven the existence of an interaction-relaxation model that governs both the melting and freezing processes.

The researchers found that the both the melting and freezing processes are similar in nature. The superheated/cooled particles during a phase transition undergo local interactions such as vacancy formation when melting and ordered liquid formation during freezing.

This was found to lead to relaxed premelting/pre-freezing states which allow for the melting and freezing states to occur collectively, in a disordered crystal state. Ultimately, this realises that melting and freezing processes are dependent upon the whole system, i.e. bulk transitions, and not just localised nucleation.

The research has provided an important contribution that will help with future research by providing a greater understanding of phase transitions, especially the role that atomistic mechanisms play.

Whilst it is not applicable to all phase transition systems, the research does provide key information into the fundamental microscopic mechanism of phase transitions, and the researchers have stated that this research should be included in future theoretical investigations.

Source:

Li Y., Zang L., Jacobs D. L., Zhao J., Yue X., Wang C., In situ study on atomic mechanism of melting and freezing of single bismuth nanoparticles, Nature Communications, 2017, 8, 14462

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