Transition from β-tin to simple hexagonal silicon under pressure

Recently a simple hexagonal phase of Si has been discovered at ∼130-160 kbar of pressure. We have performed self-consistent density-functional calculations on the β-tin and simple hexagonal structures of Si, using the calculated stress tensor to determine the c/a ratios as a function of volume. We find the simple hexagonal phase to be stable at pressures above 143 kbar. As the two structures are simply related by small strains and displacements of the atoms, we calculate the total energy along a path between the transition structures showing that any barrier between them is lower than 0.01 eV per atom.