Raman Spectroscopy of Pentaerythritol Single Crystals under High Pressures

The effect of pressure on the Raman spectra of pentaerythritol (PE) single crystals was examined up to 10 GPa. Several abrupt changes in the spectra were found at 4.6 ± 0.2 GPa and 6.8 ± 0.3 GPa. These changes occur both in the internal and external modes and indicate the onset of phase transitions. Both transitions demonstrate considerable pressure hysteresis characteristic of a first-order phase change. It is concluded that PE crystals can exist in three different phases in the pressure range up to 10 GPa. Phase I, which has the bct structure, extends up to 4.6 GPa. In phase II, the number of translationally nonequivalent molecules in a PE unit cell is likely doubled. Phase III, above 6.8 GPa, has features of a disordered structure and is associated with a large softening of hydrogen bonding.