Simple Molecular Systems at Ultrahigh Pressure

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*** NON-TECHNICAL ABSTRACT ***This project explores the nature of materials at extreme conditions -- pressures up to and beyond 3 million atmospheres (3 megabars), and temperatures from cryogenic conditions to thousands of degrees. Under these extreme conditions, simple liquids and gases can be converted to novel materials, including unusual metals and even superconductors. This research will examine the nature of these and related high-pressure transformations and will search for altogether new phenomena when simple materials are compressed to very high densities. As such, the work will expand our knowledge of fundamental interactions in condensed matter as well as lead to the creation of new materials. The experiments will make use of major national facilities such as new synchrotron radiation and neutron scattering sources. A wide cross-section of participants will benefit from the work, including high school students, undergraduates, graduate students, postdoctoral associates, and visiting investigators. In terms of its importance to fundamental science, potential applications to technology, and the training of scientists, the work is at the forefront of a rapidly growing area of the physical sciences.*** TECHNICAL ABSTRACT *** This individual investigator award will support continued studies of the behavior of simple molecular materials at pressures up to above 300 GPa (3 megabars) over a broad range of temperatures using recently developed diamond-anvil cell, synchrotron x-ray, neutron scattering, optical spectroscopy, and transport probes. The experiments will focus on transformations in representative solids formed from low-Z diatomic systems, rare gases, and dense compounds and alloys containing these species. Synchrotron x-ray and neutron diffraction techniques will be used to identify crystal structures of novel high-pressure phases of oxygen, nitrogen, and hydrogen, as well as of selected polyatomics and newly discovered high-pressure compounds. Raman, infrared, and optical spectroscopy will probe bonding, phase transformations, and electronic properties in these systems over a broader range of conditions, including measurements in the dense conducting fluid phase of hydrogen. The high-pressure behavior of stoichiometic hydrogen compounds, doped hydrogen systems, selected clathrates, and other molecular materials will also be investigated. High-pressure x-ray inelastic scattering will be used to measure pressure effects on dynamic structure factors and electronic structure, extending direct measurements of excitons and band gaps on compression. Highly sensitive magnetic susceptibility and new resistivity methods will be used to study superconductivity in these materials to the 300-GPa range. Each of these studies will involve the training of students, post-doctoral fellows, and other researchers in state-of-the-art condensed matter techniques.