# Yielding under compression and the polyamorphic transition in silicon

## TL;DR

This study uses molecular simulations to reveal that amorphous silicon undergoes a discontinuous polyamorphic transition under high pressure, characterized by elastic instabilities and plastic events similar to shear yielding.

## Contribution

It demonstrates that the polyamorphic transition in amorphous silicon is a nonequilibrium yield-like transition with discrete plastic events and elastic instabilities during compression.

## Key findings

- Discontinuous density and elastic constant changes at 13-16 GPa.
- Plastic events accompanied by shear modulus vanishing.
- Transition differs from gradual structural change in quench simulations.

## Abstract

We investigate the behavior of amorphous silicon under hydrostatic compression using molecular simulations. During compression, amorphous silicon undergoes a discontinuous nonequilibrium transition from a low-density to a high-density structure at a pressure of around $13$-$16$~GPa. Ensemble-averaged density and elastic constants change discontinuously across the transition. Densification of individual glassy samples occurs through a series of discrete plastic events, each of which is accompanied by a vanishing shear modulus. This is the signature of a series of elastic instabilities, similar to shear transformation zones observed during shear yielding of glasses. We compare the structure obtained during compression with a near-equilibrium form of amorphous silicon obtained by quenching a melt at constant pressure. This gives structures identical to nonequilibrium compression at low and high pressure, but the transition between them occurs gradually rather than discontinuously. Our observations indicate that the polyamorphic transition is of a nonequilibrium nature, and it has the characteristics of a yield transition that occurs under compression instead of shear.