CHEMICAL AND PHYSICAL TRANSFORMATIONS OF SIMPLE MOLECULAR SYSTEMS UNDER EXTREME PRESSURES AND TEMPERATURES
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Under sufficient compression molecules behave in exotic ways leading to interesting solid state transformations such as disorder-order transitions, polymerization/ionization, or path dependent metastable phases. These transitions are largely kinetic and strain controlled processes well beyond that of thermodynamic constraints or minimum energy configurations. To understand these processes the application of both static and dynamic pressure/temperature regimes with proper in-situ¬ diagnostic techniques are essential. Solidification of hydrogen and deuterium has been studied under dynamic compression using dynamic-diamond anvil cell, time-resolved Raman spectroscopy, and fast micro-photography. Liquid H2 or D2 was found to solidify into a grain boundary free crystal grown from the outer edge of the sample chamber in 1-30 ms depending on the compression rate. The time scale of solidification agrees well with that of the discontinuous Raman shift across the liquid/solid phase boundary. Raman studies of nitrogen were performed to investigate the melting curve and solid-solid phase transitions in the pressure-temperature range of 25 to 103 GPa and 300 to 2000 K. The solid-liquid phase boundary has been probed with time-resolved Raman spectroscopy on ramp heated nitrogen in diamond anvil cell, showing a melting maximum at 73 GPa and 1690 K. The dynamic shear-induced martensitic α→ω phase transition was probed using time-resolved x-ray diffraction. Diffraction patterns were obtained with ms resolution allowing for the analysis of the structural evolution through the phase transition. Under sufficient compressive load the transition onset is delayed from 6.2 to 12.5 GPa however the kinetically inhibited reaction occurs quickly after the onset in ~ 4 GPa. The pressure-induced physical and chemical transformations of tetracyanoethylene were studied in diamond anvil cells with micro-Raman and emission spectroscopy, and synchrotron x-ray diffraction. TCNE undergoes a shear-induced phase transition at 10 GPa and then a chemical change to two-dimensional C=N polymers above 14 GPa. Laser-heating of the C=N polymer above 25 GPa further converts to a theoretically predicted 3D C-N network structure, evident from an emergence of new Raman C-N stretching frequency at 1400 cm-1, strong fluorescence centered at 640 nm, and the visual appearance of a translucent solid.