Structures of Solids, Fluids, and Glasses at High Pressures and Modest Temperatures Produced by Dynamic Compression
W. J. Nellis
Harvard University
The capability to perform x-ray scattering experiments with sufficiently high-intensity and short pulse duration means that structures of materials compressed dynamically could be determined in situ at extreme conditions. Extremely little is known about such structures because it has not been generally possible to perform such experiments. Thus, there are great opportunities in this virtually untouched area, particularly because pressure, density and temperature can be tuned by a combination of shock and isentropic compression. Because x-ray beam size is relatively small, a relatively small two-stage gun (~3 m long) is sufficient for coupling with the accelerator.
Because of the short time scales, phases are generally defected fine-grained solids (including nanocrystalline materials), glasses, and fluids. The prime goals are radial distribution functions of glasses and fluids, crystal and defect structures of solids, and phase transitions and their dynamics, all in low-Z as well as higher-Z materials over a substantial range of pressure, density and temperature. The greatest opportunities are in condensed matter at conditions from near ambient up to pressures of a few 100 GPa (1 Mbar), densities up to ten times liquid in the case of hydrogen, and temperatures up to a few 1000 K.
The physics of hydrogen at very high pressures is the last frontier of the elements. Structures of dense hydrogen at modest temperatures should be measured in the regime in which zero-point energy of the proton is comparable to the binding energy of the molecule and thermal energy is small compared to both, which effects whether the system is solid or liquid and monatomic or diatomic. The prototypical issue is measuring melting temperature versus pressure, which is predicted to have a maximum near 100 GPa and 1000 K related to quantum effects. Melting would be determined by the transition from a defected lattice to the radial distribution of a fluid. Such structural determinations are expected to provide information on how molecular dissociation effects melting. If the line of metallization intersects the melting curve, a key issue is the nature of the many-body system in which melting, metallization, and possibly dissociation occur simultaneously in a system in which both electrons and protons might be quantum in nature? Another question is how metallization continues from the fluid into the solid.
More than 150 extrasolar giant planets have now been observed with masses mostly in the range 0.5 to 5 MJ, where MJ is the mass of Jupiter. These Jovian planets are composed primarily of dense fluid hydrogen. The only experimental technique used thus far to measure properties of hydrogen at both pressures and temperatures in deep planetary interiors is dynamic compression. To understand the natures of these 150 planetary interiors, the dependence of the dissociative phase transition on pressure and temperature should be determined by looking for fluid-fluid phase transitions in measured radial distribution functions. For “hot” and “cold” Jupiters, pressures of interest are 50 to 500 GPa and temperatures of 1,000 to 20,000 K. The Hugoniot of liquid D2 goes right through this regime. The melting curve of hydrogen near 100 GPa, discussed above, is important for understanding interiors of cold Jupiters.
Pressures and temperatures in deuterium-tritium fuel pellets in inertial confinement fusion (ICF) traverse this same regime of pressure and temperature in giant planets. Thus, what is learned for purely scientific reasons about giant planets would provide assistance to an R&D fusion-energy project of ever growing national importance.
Several other long-standing issues in planetary science would be resolved. For example, radial distribution functions of water, ammonia, and synthetic Uranus could be measured at densities and temperatures in the interiors of Uranus and Neptune and compared with theoretical predictions. The high-pressure phase diagram of Fe has been unresolved for years and could determined by structural determinations of solids and of the molten fluid at pressures and temperatures up to those in the Earth's deep core (300 GPa and 6000 K).
Interesting new materials have been synthesized at extreme dynamic conditions and their structures are unknown. For example, Gd3Ga5O12 (GGG) is less compressible than diamond above 170 GPa shock pressure. GGG was found using a simple theoretical predictor of where to look systematically for such materials. In this case, for example, the structure of the high-pressure phase above 170 GPa and the radial distribution function of the amorphous phase between 70 and 120 GPa should be determined. For shock pressures up to 70 GPa, it should be determined on a microscopic scale how this high-strength oxide transitions from uniaxial compression at lower pressures to isotropic compression at higher pressures. It is quite possible that other materials might be found that are less compressible and harder than diamond.
One “Holy Grail” of high-pressure research is to retain materials metastably at ambient that are synthesized at high pressure. X-ray scattering might be used to learn the nature of interatomic bonding required to bind materials into a high-pressure phase so that they are recovered from high pressures. Metastable solid metallic hydrogen is the paradigm in this regard because of its numerous scientific and technological applications, depending on the degree to which it is metastable.
