Pressure-induced structural sequence in silicon: Diamond to β-tin to Imma

We propose a theoretical approach based on the Landau scheme for studying the phase diagram of nonhydrostatically stressed silicon. It is shown that the first two pressure-induced phase transitions, namely the diamond-cubic to the body-centered tetragonal (β-tin) phase transition and the β-tin to orthorhombic (Imma) phase transition, are elastic, i.e., the soft mode is an acoustic mode and the order parameters are deformations of unit cells. The effect of an external stress on these phase transitions is also investigated. The model is tested on the experimental structural studies available in the literature.