Pressure-induced amorphization, crystal-crystal transformations and the memory glass effect in interacting particles in two dimensions

TL;DR

This study models pressure-induced transformations in 2D interacting particles, revealing how different growth barriers lead to either crystalline or amorphous structures with memory effects, explaining experimental observations of the memory glass effect.

Contribution

It demonstrates how defect density and growth barriers influence the formation of amorphous or crystalline structures during pressure-induced transformations, linking microscopic mechanisms to the memory glass effect.

Findings

Large crystals of martensite variants can form with easy growth.

Inhibited growth leads to small-crystal polycrystals that appear amorphous.

Recovered structures retain orientation, explaining the memory glass effect.

Abstract

We study a model of interacting particles in two dimensions to address the relation between crystal-crystal transformations and pressure-induced amorphization. On increasing pressure at very low temperature, our model undergoes a martensitic crystal-crystal transformation. The characteristics of the resulting polycrystalline structure depend on defect density, compression rate, and nucleation and growth barriers. We find two different limiting cases. In one of them the martensite crystals, once nucleated, grow easily perpendicularly to the invariant interface, and the final structure contains large crystals of the different martensite variants. Upon decompression almost every atom returns to its original position, and the original crystal is fully recovered. In the second limiting case, after nucleation the growth of martensite crystals is inhibited by energetic barriers. The final morphology in this case is that of a polycrystal with a very small crystal size. This may be taken to be amorphous if we have only access (as experimentally may be the case) to the angularly averaged structure factor. However, this `X-ray amorphous' material is anisotropic, and this shows up upon decompression, when it recovers the original crystalline structure with an orientation correlated with the one it had prior to compression. The memory effect of this X-ray amorphous material is a natural consequence of the memory effect associated to the underlying martensitic transformation. We suggest that this kind of mechanism is present in many of the experimental observations of the memory glass effect, in which a crystal with the original orientation is recovered from an apparently amorphous sample when pressure is released.