Pressure-induced phase transition, metallization, and superconductivity in boron triiodide

Pressure evolution of structural and electrical properties of boron triiodide, a highly anisotropic molecular crystal consisting of stacked layers of planar BI₃ molecules, has been studied by x-ray diffraction and resistivity measurements. A new phase transition was observed to occur at 6.2 GPa from the molecular phase with hexagonal structure to a monatomic phase with the face-centered-cubic lattice of iodine atoms. This first-order phase transition is characterized by the discontinuous crush of stacking of molecular layers. The monatomic phase becomes metallic at ~23 GPa and exhibits superconductivity above ~27 GPa. The process of molecular dissociation and electrical properties of BI₃ are discussed in comparison with those of iodine and other simple molecular iodides.