High-pressure phase transitions in adamantane

We report angle dispersive X-ray diffraction (ADXRD) measurements on adaman tane carried out at SPRING-8 to the pressures of 25 GPa. The tetragonal pha se observed at 0.5 GPa remains stable up to 12.5 GPa. In this pressure rang e the intermolecular hydrogen separation reduces from 2.37 to 1.87 Angstrom with relative angle of rotation of the two molecules increasing from 8.5 d egrees to 10.5 degrees in agreement with values from energy minimization. A t 16 GPa, the diffraction pattern could be indexed either by a tetragonal o r a monoclinic cell. Beyond 22 GPa only monoclinic cell indexes the pattern s. The present findings corroborate the earlier Raman results. (C) 2000 Els evier Science B.V. All rights reserved.