To develop a new practical method of purifying and recycling ionic liquids, we performed direct microscopic observations and in situ crystallization of low-melting ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF(6)]), in detail by high pressure Raman spectroscopy. Compression of [BMIM][PF(6)] was measured under pressures up to about 2.0 GPa at temperatures 293-353 K by using a high pressure diamond anvil cell (DAC). At room temperature, with pressure increasing, the characteristic bands of [BMIM][PF(6)] displayed nonmonotonic pressure-induced frequency shifts, and [BMIM][PF(6)] experienced the liquid-solid phase transition at about 0.50 GPa. In separate experiments, in situ crystallization of low-melting ionic liquid [BMIM][PF(6)] were also measured at various P-T regions, in order to improve the understanding of its stability limits. Finally, the T versus P phase diagram of [BMIM][PF(6)] was constructed, and it showed that the melting point was an increase function of pressure. It was also indicated that the structure changes in the crystalline and liquid states under high pressure might also be associated with conformational changes in the butyl chain. Pressure-released Raman spectra also showed that the phase transition of [BMIM][PF(6)] was reversible.

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Behavior of the phase transition of an ionic liquid, [Cn-mim][PF(6)], has been investigated under pressures up to 1.0 GPa by using a high-pressure differential thermal analysis (DTA) apparatus. The T versus P phase diagrams of [BMIM][PF(6)] and [EMIM][PF(6)] are constructed. The DTA curve of [BMIM][PF(6)] shows one endothermal valley in heating course at each given pressure, which indicates that a simple phase transition from solid to liquid has taken place under high pressure and that the melting point is an increase function of pressure. However, the DTA curve of [EMIM] x [PF(6)] shows two endothermal valleys in the heating course within the tested pressure range, implying that there may exist another phase. After treatment of [EMIM][PF(6)] at different temperatures under high pressure, the structures of the recovered samples are also investigated by wide-angle x-ray scattering. By considering the results above, it indicates that another crystalline phase exists between the solid and liquid of [EMIM][PF(6)].

The melt's solidification behaviour of elemental sulfur was investigated by a series of experiments including natural cooling at ambient pressure, thermal quenching under high pressure of 1 GPa, slow compressing to 2 GPa for 10 min and rapid compressing to 2 GPa within 20 ms. Based on the XRD and DSC results of the recovered samples, it is clearly shown that rapid compression is an effective process in the solidification of an amorphous phase from the melt in sulfur. We have successfully recovered large bulk amorphous sulfur with a diameter of 20 mm and a thickness of 3 mm by the rapid compression method.

Amorphous sulfur (a-S) with excellent stability is obtained by rapid compression method. The prepared a-S has a single glassy phase and exhibits a wide supercooled liquid region of 112 K and much high thermal and kinetic stability at room temperature compared to that of conventional a-S fabricated by quenched method. The substantial improved thermal and kinetic stability is attributed to low energy state induced by rapid compressing process. The stable a-S is a model system for facilitating the studies of the nature of glasses and supercooled liquids.

A rapid-compression apparatus with a large pressure jump is described. The pressure jump was performed principally by using a large volume bladder-type power accumulator that contains compressed nitrogen and the connected electromagnetic valves. Maximum force induced by the press is 1.1 MN. Within 20 ms the pressure of the sample could be increased from 0.1 to 2.0 GPa in piston-cylinder modules of 26 mm diameter and to 5.8 GPa in Bridgman-type anvils of 26 mm top diameter. The apparatus has been used successfull in determining the Grűneisen parameter of NaCl at high pressures. Furthermore a metastable amorphous phase of poly (ethylene terephthalate) was solidified from its melt in a rapid compressing process; this phase could not be obtained by slow compressing to high pressure or by naturally cooling at ambient pressure.

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