High Pressure Raman and X-ray Diffraction Study of [121] Tetramantane

We investigated the effect of pressure on the structure and bonding of [121] tetramantane up to 20 GPa via in situ angle-dispersive synchrotron powder X-ray diffraction (XRD) and Raman spectroscopy in diamond anvil cells (DACs). A phase transition from the starting monoclinic P2(1)/n structure to a triclinic P1 structure was observed beginning at 13 GPa. Upon decompression, the transition pressure showed large hysteresis. After fully releasing pressure, [121] tetramantane was found to recover into a different polymorph from the starting phase. Continuous changes of the vibration modes associated with the CCC bending and CC stretching regions suggest that this phase transition was induced by the intramolecular level distortions and modifications in the molecular packing. These results provide guidance for understanding the inter- and intramolecular interactions in diamondoids under pressure and shed light on the possibility of using pressure as a tuning parameter for synthesizing higher diamondoids.