Crystal-Liquid Phase Relations in Silicon at Negative Pressure

The stable and metastable melting relations for silicon in the diamond and ${\mathrm{S}\mathrm{i}}_{136}$ clathrate-II structures at positive and negative pressures are calculated by molecular dynamics computer simulation. The simulated liquid and crystalline clathrates undergo cavitation at approximately $-3$ and $-12\mathrm{G}\mathrm{P}\mathrm{a}$. Between these limits a stretched crystal would transform directly to gas in response to a mechanical instability. Most importantly, the clathrate-II crystal becomes thermodynamically stable over the diamond at negative pressure below $-1\mathrm{G}\mathrm{P}\mathrm{a}$ at the melting point. ${\mathrm{S}\mathrm{i}}_{136}$ should then crystallize from a slightly stretched liquid, which would have the same volume as a diamond-structure crystal.