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 $\mathrm{oC}16\mathrm{}$ 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 $\mathrm{oC}16-Cmca$ 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.

• Received 12 November 1999

DOI:https://doi.org/10.1103/PhysRevB.62.9