Anomalous Bond Length Expansion in Liquid Iodine at High Pressure

We have performed an x-ray absorption spectroscopy experiment on iodine under extreme conditions up to 6 GPa and 1050 K, using a large volume pressure cell. We measured the bond length of I₂ in the room-temperature solid phase and in the liquid phase just above the melting curve, as a function of pressure. Our results show that for the solid it is nearly constant, while in the liquid it strongly increases and at 2.1 GPa becomes larger than in the solid. Indication of molecular dissociation in the liquid phase at ≈1000K and 4.5 GPa is obtained.