Experimental Observation of New Phase, Determination of Loading-Path of Amorphous State and Loading–Path Dependent Phase Transitions for Pyridine around Freezing Pressure via Raman Modes in Low Wavenumber

Pyridine is a key element of the aromatic compound family, with complicated phases under high pressure. In this paper, Raman modes at low frequencies are employed to investigate the phase evolution of pyridine using a self-built loading and unloading setup with a 0.1 GPa increment. Raman specifically determines the solidification pressure of pyridine at room temperature, which is 2.1 GPa. The pyridine phase evolution is determined by the number of loading and unloading cycles. After 2 days of maintaining the solid–liquid equilibrium at 0.7 GPa, phase I is obtained at room temperature and then transforms into phase II with rising pressure up to 2.1 GPa. Loading a weak pressure to the equilibrium at 0.7 GPa generates the low density phase II, which transforms back into phase III after 2 days at 2.0 GPa. The high density phase II is obtained by unloading phase III at 1.3 GPa. Several phase transitions and an amorphous state are obtained by compressing the high density phase II at 3.8, 4.3, 5.4, and 9.8 GPa, respectively.