Structural Phase Transitions of Iodine at High Pressure

The compression and polymorphic phase transitions of iodine have been studied by using x-ray diffraction with a diamond anvil cell at high pressure up to 77GPa. Considering that 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, the iodine undergoes the following three structural phase transitions with pressure increasing: ( 1 ) At 21 GPa, from base centred orthorhombic phase (phase I) to body centred orthorhombic phase (phase II). A 2% change in volume occurs. (2) At 44 GPa, from phase II to body centred tetragonal phase (phase III). This is a second order phase transition. (3) At 71 GPa, from phase III to fcc phase (phase IV). The volume decreases by 3%. At this time, the volume is 45% of the original one at atmospheric pressure.