Ordered Si-VI-type crystal structure in BiSn alloy under high pressure

The crystal structure of a high-pressure phase of the BiSn alloy has been solved from synchrotron powder diffraction data. The alloy with a two-phase mixture Bi+Sn was found to transform into a new intermediate phase after annealing at 150^∘C under a pressure of 3 GPa. The structure is of the Si VI type: orthorhombic with 16 atoms in the unit cell, space group Cmca. Two symmetrically distinct atomic sites allow a (partial) ordering in the structure. The Cmca structure transforms to a disordered body-centered cubic structure at 12 GPa.