Crystal structure of a high-pressure phase in Bi-based alloys related to Si VI

The structures of high-pressure intermediate phases in the Bi-rich binary alloys with Pb and In have been determined from diffraction x-ray data as an orthorhombic oC16 cell, space group Cmca, related to Si VI. These phases are formed in Bi-based alloys at 1.5--2 GPa and are retained at ambient pressure by quenching. The same oC16-Cmca structure is proposed for Bi IV from the published data. Specific features of this structure and its stability are discussed with respect to the effective number of valence electrons and are accounted for the electronic origin.