High-pressure phase transition of boron triiodide (BI${}_3$) is investigated using first-principles methods, with the crystal structure of the high-pressure phase predicted. The structure is consistent with the information on this phase available from experiments. Significantly, the BI${}_3$ molecules are predicted to form ${\mathrm{B}}_2$I${}_6$ dimers that are structurally analogous to the $D_{2h}$ diborane molecules. Dimerization of BI${}_3$ molecules yields a notable volume drop that triggers a first-order phase transition. Using the predicted structure, the calculated equation of state, phase transition pressure, and the metallization transition pressure are in an excellent agreement with the experimental measurements. Dimerization in crystalline BI${}_3$ provides insight for the understanding of covalently bound boron halide dimers, which were previously unknown.

• Received 3 March 2011

DOI:https://doi.org/10.1103/PhysRevB.83.214105