Pressure induced phase transitions and equation of state of adamantane

dResults of angle dispersive x-ray powder diffraction measurements under pr essure up to 25 GPa on adamantane carried out at the synchrotron source SPR ING-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 (P (4) over bar2(1)c, z = 2) phase, the shortest intermo lecular hydrogen separation (H...H) reduces from 2.37 Angstrom at 0.7 GPa t o 1.87 Angstrom at 12.5 GPa and the relative angle of rotation (phi) of the two molecules within the cell about the c-axis increases from 8.5 degrees to 10.5 degrees, This is consistent with values determined from energy mini mization in this pressure region. The anomaly in the pressure variation of cia, and the failure of the constrained Rietveld refinement, occurring clos e 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 Angstrom. Ab ove 16 GPa there is a subtle change in the diffraction pattern that indicat es a transition to a closely related phase. Beyond 22 GPa, a monoclinic pha se with one molecule per cell could only index the patterns. The ambient cu bic phase is recovered on unloading indicating the absence of decomposition or polymerization. The results of high pressure x-ray diffraction experime nts confirm the changes in the Raman spectra observed in the earlier measur ements.