High-pressure phase transitions in adamantane

We report angle dispersive X-ray diffraction (ADXRD) measurements on adamantane carried out at SPRING-8 to the pressures of 25 GPa. The tetragonal phase observed at 0.5 GPa remains stable up to 12.5 GPa. In this pressure range the intermolecular hydrogen separation reduces from 2.37 to1.87 إ with relative angle of rotation of the two molecules increasing from 8.5° to 10.5° in agreement with values from energy minimization. At 16 GPa, the diffraction pattern could be indexed either by a tetragonal or a monoclinic cell. Beyond 22 GPa only monoclinic cell indexes the patterns. The present findings corroborate the earlier Raman results.