REVERSIBLE PRESSURE-INDUCED STRUCTURAL TRANSITIONS BETWEEN METASTABLEPHASES OF SILICON

We report the existence of a reversible pressure-induced first-order s tructural transition between the metastable Si-III phase (body-centere d cubic with 16 atoms per conventional cell or 8 atoms per primitive c ell 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 r ange than was previously believed. It also implies, contrary to previo us reports, that Si-II (P-Sn structure) transforms first to Si-XII bef ore converting to the BC8 structure of Si-III. The Si-XII structure ha s a unique feature among metastable phases of silicon in that it conta ins 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 diatomi c analogs of the BC8 structure are not found in compound semiconductor s.