Compressed solid hydrogens are introduced with the basic property of the hydrogen molecule. Starting from a brief introduction of old calculations, first-principles calculations of compressed hydrogens and their results are reviewed with some of our results of calculations based on the local density approximation (LDA). The probable structures over 150 GPa and the effects of the zero-point motion of the nuclei on those structures are discussed. The structures are also examined from the view point of electronic band structure. The problems remaining for the future studies of compressed hydrogens are summarized.
Solid oxygen shows a rich variety of phase transitions under high pressures such as structural phase transition, magnetic transition, insulator-metal transition, superconducting transition (pressure-induced composite phase transition). In this article, the present status of a theoretical understanding about the insulator-metal transition, magnetic transition and structural phase transition of solid oxygen under high pressures is reviewed on the basis of the results of first-principles electronic structure calculations obtained by our group and other groups.
The many-atom character of interactions in fcc solid argon at high pressures is discussed. The cubic elastic constants and violation of the Cauchy relationship as a function of hydrostatic pressure, which have been measured recently by Shimizu et al., are shown to be very well reproduced by a full-potential and all-electron total energy scheme with a generalized gradient approximation for the exchange-correlation energy functional. It is shown that an essential many-atom feature can most simply be introduced through some appropriate embedding or environment-dependent functions brought into pair-wise interactions.
Applying pressure to hydrogen-bonded crystal hydrogen bromide and hydrogen sulfide significantly alters the bonding nature and induces various kinds of phenomena: order-disorder transition, hydrogen-bond symmetrization, and molecular dissociation accompanied by the solid-state reactions. This article reviews the relationship between pressure-induced structural phase transitions and solid-state reactions, revealed from the cooperative works between high pressure experiments and the ab initio molecular dynamics simulations.
Recent applications of the first-principles molecular dynamics method to the study of structural transformation of some tetrahedral molecules are reported. Contrary to the interpretation of shock-wave experiments, CH4 was found to dissociate into a mixture of hydrocarbons below 100 GPa and separate into diamond and hydrogen only above 300 GPa. The fcc-phase of SnI4 above 60 GPa was unambiguously shown to be a substitutional solid solution of Sn into fcc I. B2H6, in which BH4 tetrahedra shares an edge, will undergo more extensive polymerization leaving fewer isolated hydrogen molecules.
Iodanil (tetraiodo-p-benzoquinone) C6O2I4 and hexaiodbenzene C6I6 crystals are known as very rare aromatic monomolecular solids showing the pressure-induced metallisation. However, the metallisation mechanism and the relation with molecular decomposition have not been fully understood yet. Recently, I have tried to elucidate these problems by means of the constant-pressure first-principles calculations, and presented probable mechanisms on their metallisation and decomposition. In this article, I review the results of this study with some recent progress.
Methane hydrate, known as Burning Ice, consists of network of hydrogen-bonded water molecules containing methane molecules. It is known as one of the most important materials for solving energy and environment problems as well as for understanding the history of solar planets and satellites. The mysterious properties of methane hydrate are studied using the density functional theory.
We report constant-pressure first-principles molecular-dynamics simulations on liquid-liquid phase transition of phosphorus. By compressing a low-pressure liquid composed of tetrahedral P4 molecules, a structural transition from a molecular to polymeric liquid (a high-pressure liquid) observed in the recent experiment by Katayama et al. [Nature 403, 170 (2000)] was successfully realized. It is found that this transition is caused by a breakup of the tetrahedral molecules with large volume contraction. The same transition is also realized by heating. This indicates that only the polymeric liquid can stably exist at high temperatures.
Polycrystalline diamond was successively synthesized recently at very high pressure and temperature using a Kawai-type apparatus, which turned out to be extremely hard when compared with currently available sintered diamond products and natural single crystal diamonds. Here the author summarizes the results of experiments and reviews aspects of the synthesis of polycrystalline diamonds by the direct conversion of graphite. Some future perspectives and related unsolved problems are also discussed on the basis the results of our and other experimental studies on the synthesis of polycrystalline diamond.
Accurate characterization of the pressure and temperature environment in high-pressure apparatuses is of essential importance when we apply laboratory data to the study of the Earth's interior. Difficulties in pressure determination of multi-anvil apparatuses are summarized. The First International Pressure Calibration Workshop is briefly reported with the Preliminary International Pressure Scale (PIPS-97). Some prospects on the pressure scale at high-temperatures for the multi-anvil apparatuses are discussed.