Pressure evolution of structural and electrical properties of boron triiodide, a highly anisotropic molecular crystal consisting of stacked layers of planar ${\text{BI}}_3$ 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 $\sim 23\text{GPa}$ and exhibits superconductivity above $\sim 27\text{GPa}$. The process of molecular dissociation and electrical properties of ${\text{BI}}_3$ are discussed in comparison with those of iodine and other simple molecular iodides.

• Received 29 May 2010

DOI:https://doi.org/10.1103/PhysRevB.82.094506