Moessbauer effect of iodine at pressures to 30 GPa

The authors report the first high pressure 129I Moessbauer measurements with elemental iodine at pressures to 30 GPa. A 20 mg/cm2129I2 absorber was mounted in a diamond anvil cell with an effective diameter of 0.21 mm. The source used was Mg3sup(129m)TeO6. Experiments were performed mainly at 4 K and the pressure was monitored by the ruby fluorescence method. With increasing pressure a gradual decrease in mod(e2qQ) and an increase in IS and eta values of the low pressure, molecular phase are observed; at approx. 16 GPa a new phase (HP1) is detected characterized by a change in sign of e2qQ and a smaller value of mod(e2qQ), and a substantial increase in eta. At approx. 24 GPa a new phase (HP2) is formed that is characterized by a smaller value of eta. In general the population of the molecular phase decreases from 1.0 near 15 GPa to a value of 0.4 at 30 GPa. The fraction of the high pressure phase (HP1 + HP2) increases at the expense of the molecular phase and that of the HP2 at the expense of the HP1 phase. These observations are discussed in relation to the onset of a metallic phase near 16 GPa and recent X-ray diffraction studies. (Auth.)