High Pressure Materials Research: Novel Extended Phases of Molecular Triatomics
- LLNL
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 exotic properties in a wide-range of atomistic length scale, substantially greater than (often being several orders of) those achieved by other thermal (varing temperatures) and chemical (varying composition or making alloys) means. Simple molecular solids like H{sub 2}, C, CO{sub 2}, N{sub 2}, O{sub 2}, H{sub 2}O, CO, NH{sub 3}, and CH{sub 4} are bounded by strong covalent intramolecular bonds, yet relatively weak intermolecular bonds of van der Waals and/or hydrogen bonds. The weak intermolecular bonds make these solids highly compressible (i.e., low bulk moduli typically less than 10 GPa), while the strong covalent bonds make them chemically inert at least initially at low pressures. Carboncarbon single bonds, carbon-oxygen double bonds and nitrogen-nitrogen triple bonds, for example, are among the strongest. These molecular forms are, thus, often considered to remain stable in an extended region of high pressures and high temperatures. High stabilities of these covalent molecules are also the basis of which their mixtures are often presumed to be the major detonation products of energetic materials as well as the major constituents of giant planets. However, their physical/chemical stabilities are not truly understood at those extreme pressure-temperature conditions. In fact, an increasing amount of experimental evidences contradict the assumed stability of these materials at high pressures and temperatures. Figure 1 illustrates the principle Hugoniots of simple molecules like O{sub 2}, CO, N{sub 2}, and CO{sub 2}. Clearly, all these materials exhibit the cusps on their Hugoniots at the pressure range between 20 and 40 GPa. At these pressures, these materials could heat up to several thousand degrees because of their high compressibilities. The calculated shock temperature of carbon dioxide, for instance, is about 4500 K at 40 GPa. The presence of such a distinctive cusp on the Hugoniot is surely an indication for chemical reaction or phase change. In fact, many previous statistical mechanical calculations have shown that these materials undergo strong chemical changes such as the decomposition of CO{sub 2} and CO to the elementary products like carbon and oxygen and the dissociation of N{sub 2} and O{sub 2} to diatomic/monatomic ionic products. Note that the Hugoniots of unreacted CO and N{sub 2} are nearly identical, attributing to their isoelectronic characteristics resulting in same initial density and similar nonbonded atom-atom potential. The previous diamond-anvil cell studies of these materials also found very similar phase diagrams with many isostructural polymorphs. In the later chapter, we shall also see a similar parallelism existing in the phase diagrams of isoelectronic triatomics CO{sub 2} and N{sub 2}O. There are numerous examples, also indicating the increase of chemical instability of unsaturated molecular bonds at high static pressures. The examples include many recent discoveries: covalently bonded nonmolecular phases of nitrogen, carbon dioxide, cyanogen, and carbon monoxide, charge transferred ionic solids of nitrous dioxide, oxygen, and hydrogen, metallic phases of oxygen, iodine, and xenon, hydrogen bonded extended solids of symmetric ice and hydrogen cyanide, and dissociative products of methane and aromatic compounds. These fundamental changes in chemical bonding of simple molecular solids may or may not occur reversibly upon the reversal of pressure and temperature, offering the opportunity to understand the materials metastability. These transformations and the associated changes in thermodynamic, mechanical, electronic and magnetic properties are also fundamental to understand the state of matters in the deep interiors of Earth and other planets and the chemistry behind high energetic detonation and combustion. Modern advances in theoretical and computational methodologies now make possible to explain or even predict novel structures and properties in a relatively wide range of length scales on the basis of thermodynamic stability. These theoretical calculations have been successful, not only to explain the details of materials discovered in experiments such as crystal structures, stabilities, properties, and transition dynamics, but also to predict new often highly unusual phases that might exist at the extreme conditions.
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 1008400
- Resource Relation:
- Related Information: Chemistry at Extreme Conditions
- Country of Publication:
- United States
- Language:
- ENGLISH
Similar Records
Understanding the Role of Inter- and Intramolecular Promoters in Electro- and Photochemical CO 2 Reduction Using Mn, Re, and Ru Catalysts
Six-fold coordinated carbon dioxide VI