Pressure induced phase transitions and equation of state of adamantane

Results of angle dispersive x-ray powder diffraction measurements under pressure up to 25 GPa on adamantane carried out at the synchrotron source SPRING-8 are reported. The disorder-order transition at 0.5 GPa in adamantane known earlier is reproduced with detailed structural information. For this ordered tetragonal (P2₁c, z = 2) phase, the shortest intermolecular hydrogen separation (H...H) reduces from 2.37 Å at 0.7 GPa to 1.87 Å at 12.5 GPa and the relative angle of rotation (ϕ) of the two molecules within the cell about the c-axis increases from 8.5° to 10.5°. This is consistent with values determined from energy minimization in this pressure region. The anomaly in the pressure variation of c/a, and the failure of the constrained Rietveld refinement, occurring close to 9 GPa, are interpreted as due to distortion of the adamantane molecule when H...H distance decreases below the critical value of 1.9 Å. Above 16 GPa there is a subtle change in the diffraction pattern that indicates a transition to a closely related phase. Beyond 22 GPa, a monoclinic phase with one molecule per cell could only index the patterns. The ambient cubic phase is recovered on unloading indicating the absence of decomposition or polymerization. The results of high pressure x-ray diffraction experiments confirm the changes in the Raman spectra observed in the earlier measurements.