The Structure Phase Transition of Iodine at High Pressure

The structure phase transitions of iodine have been studied by using X-ray diffraction with a diamond anvil cell at high pressure up to 32 GPa. As iodine is reactive with iron in air, a corrosion-resisting and high-strength gasket was put between the diamonds, thus the sample can be mounted at atmospheric pressure and room temperature. Pressure was measured in terms of the ruby fluorescence and the molybdenum diffraction as an internal pressure standard. According to the X-ray diffraction results, iodine undergoes a structural phase transition at 21 GPa from face centre orthorhombic phase to body centre orthorhombic phase. A change of volume is 2%. At 21~25 GPa, the coexistence of two phases has been observed. Above 25 GPa, the single new phase of the high pressure structure appeared. When pressure goes inversely, this transition was found to be reversible.