In this article, recent advances in computational materials science accelerated by K computer and high-performance computing infrastructures are reviewed. Exponentially growing computer power enables us to develop and apply new methodologies of computer ssimulation, which will realize qualitatively different approach to high-pressure science.
We review recent advances in prediction of phase behaviors of various clathrate hydrates with firm statistical mechanical ground, in which the classical theory is extended to multiple occupation of cages and substitution of a fully occupied state for an empty hydrate structure. It is demonstrated that they indeed work effectively in predicting the dissociation pressures of clathrate hydrates containing hydrogen and some hydrocarbon molecules.
Recent progress in theoretical mineral physics based on the ab initio quantum mechanical computation method has been dramatic in conjunction with the advancement of computer technologies. It is now possible to predict stability and several physical properties of complex minerals quantitatively not only at high pressures but also at high temperatures with uncertainties that are comparable to or even smaller than those attached in experimental data. Our present challenges include calculations of high-P,T elasticity to constrain the lower mantle mineralogy, transport properties such as lattice thermal conductivity, and further extensions to terapascal phase equilibria of Earth materials for studying planetary interiors.
Crystal structure determination is one of grand challenges in high-pressure materials science, and the structure prediction by computer simulations based on ab-initio calculations has played a significantly important role for it. In this article, metadynamics and genetic algorithm are focused on as the computational technique of the crystal structure searching. First, phosphorus and calcium are brought up as successful examples of the ab-initio metadynamics simulation. Then, the high-pressure phases of yttrium are shown as the results obtained by the application of the ab-initio genetic algorithm. In addition, for the three elements, the superconducting properties theoretically obtained by the use of the predicted crystal structures are compared with experimental ones.
Proton dynamics in hydrogen-bonded systems, in particular, dissociation and recombination of water molecules are basic processes in many chemical and life systems. In this article, we discuss the pressure dependence of electric conductivity of high-pressure ices VII and X, which may exist in astronomical icy bodies, from the following three viewpoints: (1) molecular dynamics simulation of the hydrogen and oxygen atoms, (2) thermodynamic transport theory of ionic and rotational defects of ice, (3) lattice model of proton hopping.
Information of a crystal structure is crucial for understanding its physical properties. Our previous powder x-ray structure analyses on proton conductors, high-energy materials, and elements under conditions of high-temperatures, low-temperatures, high-pressures have revealed their new crystal structures, however, precise positions of light elements like hydrogen or lithium remained unknown. Therefore, we employed a density functional theory (DFT) calculation and found that it could collaborate successfully with the powder x-ray experiments. In this article, we report some techniques used in our recent structure analyses.
The symposium “Development in Scientific Researches on Gas Hydrates” was held at the 53rd High Pressure Conference of Japan on November 2012. After some presentations in the symposium, the possibilities of improving or transcending the van der Waals & Platteeuw model for the phase equilibrium estimation including clathrate hydrate phase were discussed. In this article, a part of the discussion was reported.
A versatile high-pressure apparatus has been constructed to measure the electronic, magnetic and thermal properties of metals, alloys and compounds at low temperature and high magnetic field. A pressure-induced crossover from heavy fermion to intermediate valence state and pressure-induced superconductivity are reported for some intermetallic compounds including f electrons. The effect of pressure on the giant magnetoresistance (GMR) for nanoscale magnetic materials is described. The enhancement of GMR is found at high pressure. The Compton scattering at high pressure is also reported as an interesting topic.
High-pressure fluorescence study using a polarity-sensitive probe Prodan on phosphatidylcholine (PC) bilayers is reviewed. The author has constructed a three-dimensional image plot based on the second-derivative of the Prodan fluorescence spectra that change depending on the phase state of the PC bilayers, which enables us to examine the packing state of the bilayer. The effects of (1) acyl chain length, (2) vesicle size, and (3) chain asymmetry on the PC bilayers are discussed by using the technique of bilayer imaging. Further the formation of the lamellar crystal phase of the PC bilayer is also revealed by the technique.
High-pressure synthesis method can generate novel materials containing unusual high valence ions. CaCu3Fe4O12 and SrCu3Fe4O12 perovskites, with unusual high valence Fe4+ ions, demonstrate anomalous properties such as charge disproportionation, and intersite charge transfer. CaCu3Fe4O12 shows a charge disproportionation of 2Fe4+→Fe3++Fe5+ type whereas SrCu3Fe4O12 shows a crossover-like intersite charge transfer between Cu and Fe between 170 and 270 K. This gradual transition results in a giant negative thermal expansion with a linear thermal expansion coefficient of −2.26×10−5 K−1, followed by the charge disproportionation at a ratio of Fe3+:Fe5+≒4:1. The differences in the structural and electronic properties between CaCu3Fe4O12 and SrCu3Fe4O12 are attributed to the bond strains induced by A-site cations.