Lattice dynamical analysis of β ↔ γ phase transformation in silicon under high pressure

β-tin and the one-atom primitive hexagonal structure (γ phase), the two high-pressure polymorphs of Si, are related to each other by correlated displacement of atoms. This is interpreted in terms of a zone boundary mode softening $\text{1}\text{2}\text{(101)}{}_{\mathrm{\gamma }}$. It is confirmed by detailed lattice dynamical calculations.