Silicon

The complex mechanisms of mechanical deformation in crystalline silicon have been a topic of extensive research over the past decade.  High pressure diamond-anvil studies have shown that Si undergoes a series of phase transformations under isostatic loading. At a pressure of 11.3-12.5 GPa, diamond-cubic Si-I undergoes a 22% increase in density to form a beta-Sn phase, Si-II.   Si-II is a metallic phase of Si and hence has the electronic and mechanical properties of a metal and not a semiconductor.  This phase is not stable at ambient pressure and, hence, will transform to another form of Si on pressure release.  Diamond-anvil studies have reported the formation a rhombohedral (r8) Si-XII phase and a body-centered-cubic (bc8) Si-III structure after quenching from Si-II.  The final structure of the remnant phase has been found to depend on the rate of pressure release.   During indentation, it  has been shown that Si undergoes a similar series of transformations on loading, however on unloading the formation of amorphous Si has also been reported (see below).

Transmission electron microscopy images of Si after indentation clearly show the region of transformation located directly under the indenter. The fast unload samples shows a region of a-Si while the slower unload sample reveals a region of Si-XII/Si-III.