High-Pressure Behavior of C 2 I 2 and Polymerization to a Conductive Polymer
Crystalline diiodoacetylene (C 2 I 2 ) was synthesized and then examined under high-pressure conditions using synchrotron X-ray diffraction, Raman/infrared spectroscopies, and first-principles calculations. At ~0.3 GPa, the starting tetragonal (P$_{4_{2}}$/n) phase, which is stabilized by donor-acceptor interactions, changes into a new orthorhombic structure (Cmca) that is more densely packed and analogous to the low-temperature phase of acetylene. Above approximately 4 GPa, compressed C 2 I 2 molecules in the Cmca structure begin to polymerize to form a predominantly sp 2 amorphous carbon network that maintains a significant fraction of C-I bonds. Transport measurements demonstrate that the polymeric material is electrically conducting. The magnitude of the electrical conductivity is similar to Br-doped polyacetylene and undoped trans-polyacetylene at 8 GPa and 1 atm, respectively. Elemental analyses performed on recovered samples show that the iodine concentration varies with specific processing conditions. Optimization of the pressure-induced polymerization pathway could allow for enhanced electrical properties to be realized, in addition to postpolymerization functionalization using the weak C-I bonds.