Shear Melting of Silicon and Diamond and the Disappearance of the Polyamorphic Transition under Shear

Molecular dynamics simulations of diamond-cubic silicon and carbon under combined shear and compression show the formation of an amorphous solid with liquidlike structure at room temperature. Consistent with the opposite density changes of the two crystals upon melting, the amorphous material is denser than the crystal in silicon and less dense than the crystal in carbon. As a result, its rate of formation is enhanced by pressure in silicon but suppressed in carbon. These results are particularly unexpected for silicon, whose amorphous structure is supposed to be liquidlike only when hydrostatically compressed above the polyamorphic transition pressure (similar to 14GPa). Below this pressure, amorphous silicon is expected to have a low-density structure with density close to that of the diamond-cubic crystal. Our simulations show that this polyamorphic transition disappears under shear and high-density, liquidlike amorphous silicon with metallic ductility forms even at low pressure. These results are potentially transferable to other diamond-cubic crystals, like germanium and ice I-h, and provide insights into nonequilibrium materials transformations that govern friction and wear in tribological systems.