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Workshop / Seminar

Chemistry Proposal Defense — Sohan Ahmed, Chemistry Graduate Student

Institute of Shock Physics, Room 201

About the event

Title: Effects of Rapid Compression and Heating on Phase Transitions in Nitrogen and Water at High Pressures

Abstract: Simple molecules such as nitrogen (N2) and water (H2O) are very important materials to study and understand because of their extensive use in physical, chemical and materials science. Nitrogen is a simple homonuclear diatomic molecule with strong N≡N triple bond, which solidifies into a typical molecular solid at ~2.6 GPa at ambient temperature.1 Molecular N2 solid is bound by weak van der Waals interaction, which makes in highly compressible at low pressures. However, at sufficiently high pressures (above 100 GPa), molecular N2 is known to transform into non-molecular N solid in an extended network made of N-N single bonds – known as high energy density solid.2 N-N single bond contains an energy of ~160 KJ/mole, whereas N≡N triple bond has an energy of ~950 KJ/mole.3 Therefore, a huge amount of chemical energy is stored in this single-bonded N polymer, which can be released upon depolymerization into molecular nitrogen.

Water is the major constituents in our planet Earth and other “icy” Giant planets. Upon the solidification at 0.9 GPa at ambient temperature or at low temperatures below 0º C at ambient pressure, it forms a wide range of ice phases in various hydrogen bonding networks at various pressure-temperature (PT) conditions. Some of them are orientationally ordered, some are disordered, and some are partially disordered. Some of the ordering processes have been studied under static compression but there are not many significant studies of the order-disorder process of these materials under dynamic PT conditions.

Because of the fundamental difference in bonding between ice (i.e., hydrogen bonding) and N2 (i.e., van der Waals interaction), it is very important to study kinetics governing the solidification and solid-solid phase transitions of N2 and H2O and understand the mechanism underlaying those phase transitions under high pressure. For example, when pressure is suddenly increased, water can be supercompressed to pressures well above the equilibrium solidification pressure, because of the presence of latent heat, similarly, to form undercooled liquid water upon rapid cooling. The supercompressed water can then form a metastable phase before it relaxes back to a stable ice phase.4 This phenomenon is highly influenced by the kinetics and mechanisms governing the phase transition. In case of nitrogen where molecules are weakly bound by van der Waals interaction, unlike strongly bound hydrogen bonding network in H2O, the solidification occurs very rapidly, and no metastable phase has been found to date.

In this study we will address the kinetics of phase transition in nitrogen and water and how it affects the transition of one phase to another. We will also try to address the mechanism of molecular ordering and/or disordering during phase transitions under dynamic PT-loading conditions. We will utilize time resolve Raman Spectroscopy in conjugation with dDAC to characterize the local structure to understand the long range and short-range ordering in nitrogen and water respectively. In addition, we will also use the time resolve X-ray in combination with ramp laser heating to determine the crystal structure change during melting in nitrogen and formation of highly disordered phase i.e., superionic phase in water.

Our preliminary data suggests that under dynamic pressure loading liquid nitrogen becomes supercompressed beyond the equilibrium solidification pressure and becomes solidified at higher pressures than those measured in static experiments. The measured transition pressure points seem to have some correlation with the compression rate for both nitrogen and water. Crystallization of nitrogen under dynamic pressure loading seems to follow nucleation mechanism based on the volume change. When pure water is ramp heated isobarically at 23 GPa it shows clear evidence of the gradual formation of superionic phase. When water and helium mixture is ramp heated isobarically, evidence clearly show formation of superionic phase at 26 GPa and lower temperature than pure water. We tried to explain the chemistry of the formation of superionic phase in pure water and water helium mixture.5

We plan to extend the study of nitrogen using time resolve XRD with ramp heating to study the melting of nitrogen along the melt line. Also, we want to study the pure water and water helium mixture under dynamic pressure loading and unloading.