Reversible pressure-induced structural transitions between metastable phases of silicon

We report the existence of a reversible pressure-induced first-order structural transition between the metastable Si-III phase (body-centered cubic with 16 atoms per conventional cell or 8 atoms per primitive cell known as BC8) and a topologically distinct rhombohedral structure (Si-XII) which also contains 8 atoms. The observed transition pressure of 20 kbar indicates that Si-III exists over a far smaller pressure range than was previously believed. It also implies, contrary to previous reports, that Si-II (\ensuremath{\beta}-Sn structure) transforms first to Si-XII before converting to the BC8 structure of Si-III. The Si-XII structure has a unique feature among metastable phases of silicon in that it contains both five- and six-membered rings, the presence of which may have important consequences for the electronic properties of the material. The existence of odd-membered rings also serves to explain why diatomic analogs of the BC8 structure are not found in compound semiconductors.