Yielding under compression and the polyamorphic transition in silicon

We investigate the behavior of amorphous silicon under hydrostatic compression using molecular simulations. During compression, amorphous silicon undergoes a transformation from a low-density to a high-density structure. Depending on the temperature and the compression rate, the transformation occurs in a pressure range between 6 and 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 quasiequilibrium form of amorphous silicon obtained by quenching a melt at constant pressure. This gives structures identical to compression at low and high pressure, but the transition between low- and high-pressure structures occurs gradually rather than discontinuously. Our observations indicate that the polyamorphic transition has the characteristics of a yield transition that occurs under compression instead of shear.