Iodine molecule modifications with high pressure

We explored the iodine-iodine intramolecular evolution in iodine crystals under high pressure using X-ray absorption spectroscopy and atomistic modeling. Both methods show that with increasing pressure, the iodine diatomic molecule elongates up to a critical pressure Pc ~7 GPa and then contracts with a lower rate. Experiments show that the iodine-iodine intramolecular distance increases by 0.03 {\AA} up to Pc and then decreases to reach again the initial ambient pressure value at about 20 GPa. The observed change rate of the iodine-iodine bond length with pressure corresponds to the one already observed in bromine. Universal trends in halogens under increasing density are discussed allowing to predict for chlorine a critical pressure of 50+/-5 GPa for molecular bond-length reversal.