Application of high pressure significantly alters the interatomic distance and, thus, the nature of intermolecular interaction, chemical bonding, molecular configuration, crystal structure, and stability of solid. With modern advances in high-pressure technologies, it is feasible to achieve a large (often up to a several-fold) compression of lattice, at which condition material can be easily forced into a new physical and chemical configuration. The high-pressure thus offers enhanced opportunities to discover new phases, both stable and metastable ones, and to tune novel properties in a wide-range of atomistic length scale, substantially greater than (often being several orders of) those achieved by other thermal (varying temperatures) and chemical (varying composition or making alloys) means. The goal of this proposed work is to investigate new states of matter and novel phenomena occurring in simple low-Z molecules like carbon dioxide and nitrogen at Mbar pressures. Commonly observed at these conditions are metallic and nonmetallic extended solids that can store a large sum of energy in their three-dimensional network structures. Yet, a large cohesive energy of low Z solids gives rise to an extremely stiff lattice and novel electronic and optical properties. Broadly speaking, these molecular-to-nonmolecular transitions occur due to electron delocalization manifested as a rapid increase in electron kinetic energy at high density, but there are many outstanding questions regarding the exact nature of chemical bonding, phase stability, and chemical mechanisms. Many of the questions are fundamental problems in solid state chemistries and condensed matter physics, which will be addressed in this project.
NON-TECHNICAL SUMMARY: The research outlined in this proposal will impact fundamental solid-state chemistries and condensed materials sciences, which will eventually establish new Periodic orders of elements and solids at Mbars. The project will also have significant impacts on training graduate and undergraduate students by providing hands-on research experiences in cutting-edge experimental technologies at Washington State Univerisity (WSU) and at synchrotron facilities and national laboratories. The present study will bring the excitement of high-pressure materials research to the scientists and students in Spokane and nearby areas well beyond the WSU and, thus, enhance public appreciation of the relevance of fundamental materials research to the future and society. Hence, the benefits of the project reach well beyond the immediate scientific scope of this proposal.
Intellectual Merits and Broader Impacts of the Project This project is to understand collective behaviors of molecular solids and mixtures at high pressures via discoveries of new states and structures, fundamental properties, and novel phenomena under extreme conditions of pressures and temperatures. Specifically, our research in this project has been focused on simple molecular solids such as CO2, H2O, H2, XeF2, and C and simple binary mixtures of hydrogen with both hydrogen-bonded planetary ices such as H2O, NH3 and CH4 and non-hydrogen bonded CO2 and N2. Both static and dynamic properties were examined, associated with liquid-solid and solid-solid phase transitions, molecular-to-non-molecular phase transitions to both extended polymeric and metallic states, and path-dependent phase/chemical changes. Major findings accomplished in this NSF supported project are listed as the following: First, we have discovered several important phases of carbon dioxide at high pressures and temperatures, including coesite-carbon dioxide, a missing linkage between the four-fold quartz-like CO2 and six-fold stishovite-like CO2, and ionic carbonate carbon dioxide in the pressure-temperature conditions of Earth’s deep mantle. These findings have significant implications to Deep Carbon Cycles and are central to understanding chemistry perspectives of the pressure-induced molecular-to-non-molecular transformations that can be applied to other low Z molecular solids at Mbar pressures where the compression energy rivals the chemical bond energies. Second, we have discovered the superconductivity in highly compressed carbon disulfide. This is the first observed high Tc superconductor arising from highly disordered 3D network structure of simple diamagnetic organic molecules. The results have led to our current systematic search of highly conducting states of dense low-Z extended solids. Third, we have found the evidences for high density amorphous ice formed above the crystallization temperature, in the region known as no man’s land, under dynamic pressure loadings, demonstrating the kinetic origin of pressure-induced amorphization. This study has further extended to the investigation of crystal growth, proton ordering, amorphization, and chemistry at dense interfaces under dynamic conditions on a wide range of molecular systems such as dense hydrogen, carbon dioxide, methane, and methane hydrates. The results on methane hydrates, for example, have been found important for understanding the stability of host-guest molecules and have strong implication to the application for energy use of methane hydrates. Fourth, we have discovered novel semiconducting 2D graphite-like and metallic 3D fluorite-like extended solids of compressed XeF2. The results show intriguing ways of making chemical bonds on dense noble gas solid Xe and of storing a large sum of compression energy into chemical bonds in these 2D and 3D network structures. Finally, we have investigated dense binary mixtures including N2-D2, CO2-H2, H2-H2O, H2-NH4, and H2-CH4. The major results found include the discovery of novel N2-D2 inclusion compounds with unusually high repulsive interactions; the great level of proton exchange reactions in D2-H2O and D2-NH3 mixtures but D2-CH4; and the great level of miscibility at dense solid-solid interfaces found in all mixtures of planetary "ices". The most significant products of the present project have been for fundamental science via over thirty publications in peer-reviewed journals in disciplines of solid-state chemistry, condensed matter physics, materials science, and earth and planetary sciences. It has resulted new and unexpected discoveries and helped establish new periodic systematics of low Z elemental solids at extreme pressures. The present findings addressed several key scientific challenges of simple molecular solids and their mixtures under extreme conditions: (i) their transformation to non-molecular solids, (ii) relationships of structural orders (or disorders), electronic structures, and novel properties, (iii) the concept of low Z extended solids as novel elemental alloys, (iv) the nature of internal pressures in low-dimensional and/or incommensurate structures, and (v) structural miscibilities (or solubilities) of simple mixtures under high pressures. Addressing these challenges is not only important in understanding the relationships of crystalline structures, electron correlation, and functional properties, but will also have strong implications for novel materials and technology developments at ambient conditions. This project has also contributed to a number of key technology developments such as time-resolved Raman spectroscopy and time-resolved synchrotron x-ray diffraction applied to single event dynamic processes, pulse-laser heated diamond anvil cells (DAC), and dynamic-DAC. These technology developments are timely and synergistic to an emerging future emphasis on high-pressure materials research for understanding the dynamic response of materials under extreme conditions. This NSF project has provided pre- and post- graduate students (total of eight) major research opportunities with hands-on research experiences in cutting-edge experimental technologies at our institute and at synchrotron facilities and has produced one Ph.D. and two Master students and two additional Ph.D. students expected to graduate within a year in the 2013-2014 academic year. To date, the project has been resulted in thirty peer-reviewed papers published or in print in major journals, and several more papers are in preparation for publication in this and coming years.
