Understanding the structural origin of the anomalous properties of SiO₂ glass at high pressure conditions is of great interest in a wide range of scientific fields. The translational order of the silicon’s second shell is considered as the key to understand the anomalous properties. However, it has not been well identified in experiment. Here we show in situ high-pressure pair distribution function measurement of SiO₂ glass to investigate the translational order of the silicon’s second shell and the tetrahedral symmetry structure. The results show that SiO₂ glass consists of tetrahedral symmetry structure with separation between the first and second shells of silicon at low pressure conditions. On the other hand, at high pressures, the silicon’s second shell collapses onto the first shell, which indicates breaking of local tetrahedral symmetry in SiO₂ glass under pressure.

In this article, static and dynamics properties of silicate liquids at high pressure are briefly reviewed as introduction. The main theme of this review is analysis method of network topology of SiO₄ corner shared network in silicate liquids and glasses, especially in molecular dynamics simulations. Furthermore, I will introduce the results by applying newly developed analysis method to sodium tetrasilicate liquids at pressure.

In this article, recent studies regarding structures of ice polymorphs and amorphous ices are briefly reviewed from the viewpoint of why ice polymorphs and amorphous ices have remarkable structural diversity. Characteristic structure in ice would be tetrahedral structural unit and inter- or intra-penetrating structure of hydrogen bonding network. The tetrahedrality and inter-/intra- penetrating structure in ice crystals could also characterize the structure of liquid water or amorphous ices. The concept of hydrogen-bond symmetrization, structure variation owing to plastic and super-ionic ices, and stacking structures hidden in ice polymorphs are also briefly discussed.

Ice VII is one of numerous ice polymorphs and is stable at room temperature between 2 and 60 GPa. Its structure can be described using a simple cubic model, so-called a single-site model, sufficient to reproduce its diffraction behaviours for most purposes. At the same time, its disordered nature of molecular orientations associated with slight displacements has been of interest. This article reviews historical challenges to elucidate its disordered structure beyond the simple model and recent achievements from in-situ neutron diffraction under high pressure with “model-free” analyses.

Pernitrides and polynitrides synthesized under high pressure, containing nitrogen dimers and molecular nitrogen, respectively, are a group of compounds quite different from conventional nitrides synthesized under ambient pressure. Transition-metal polynitrides composed of transition-metal and molecular nitrogen have been synthesized above about 100 GPa, and various types of molecular nitrogen have been discovered. Pernitrides containing nitrogen dimers have anisotropic axial compressibility in the bonding direction of the dimers due to the low compressibility of the dimers. In this article, we review recent progress in molecular nitrogen-containing metal compounds and the anisotropic compression behavior of a novel molybdenum–nitrogen compound, Mo₃N₅, containing nitrogen dimers.

Pressure is one of the important factors that promote chemical reactions in solid organic matter. Here we present the pressure-induced chemical reactions of aromatic compounds. At room temperature, oligomerization of benzene and naphthalene was observed at pressures greater than around 15 GPa. Gas chromatography-mass spectrometry analysis of the recovered samples revealed several dimerization processes, including simple dimerization through cycloaddition, dimerization through condensation, and dimerization through coupling reactions, associated with release of hydrogen atoms. Notably, carbon nanothreads, a distinctive one-dimensional sp ³ -bonded nanomaterial, are formed by slow compression and decompression of benzene and aromatic compounds.