To understand the physical properties of the interiors of the Earth and planets, it is necessary to determine the crystal structures of these constituent materials at high pressure and high temperature. High-pressure experiments have been performed using two typical devices. One is a handy diamond anvil cell capable of generating pressures the center of the Earth. The other is a large-volume multi-anvil apparatus capable of generating isotropic pressures and uniform high temperatures. As an example of neutron diffraction studies at high temperature and high pressure, the effects of nickel and silicon on the hydrogenation of iron and the light element composition of the Earth’s core are presented. Moreover, studies on the crystal structure of salt hydrates that appear under high pressure in relation to planetary interior materials are introduced.

This report presents the V‒V dimerization phenomena observed in ilmenite-type vanadium oxides（AVO₃）, synthesized using a high-pressure-high-temperature method. Crystal structure refinements and physical property measurements indicate that V‒V dimerization arises from the formation of covalent bonds between vanadium ions. This phenomenon is accompanied by significant changes in the crystal structure, as well as metal-insulator and magnetic-nonmagnetic phase transitions. The temperature at which V‒V dimerization occurs is influenced by the A-site ionic radius and structural distortions. In this report, we discuss the mechanism from the perspective of crystal structure. By gaining a deeper understanding of the crystal structure and electronic states of ilmenite-type vanadium oxides, we aim to establish the fundamental principles governing this material.

A laser-heated diamond anvil cell（LHDAC）combined with in-situ observation is a very powerful tool for exploring novel materials under high pressure. In this study, silicon, germanium, and tin reacted with nitrogen at pressures above approximately 60 GPa, and pyrite-type pernitrides（AN₂, A＝Si, Ge, Sn）were synthesized. The zero-pressure bulk moduli of the synthesized pernitrides were found to be systematically higher in the order of SiN₂, GeN₂, and SnN₂. The synthesis process and crystal chemistry of the newly synthesized materials are described.

Pressure is one of the variables to determine crystal structures and functionalities as well as temperature. Pressure dependance of crystal structures of monatomic inorganic compounds including mixed anion compounds can usually be explained by compressibilities of anions. High pressure crystal structures of molecular anion compounds are expected to be related to morphology and orientation of the anions as well as size. In this paper, high pressure crystal structures of tetragonal BaNCN and orthorhombic Ba_0.9Sr_0.1NCN, which contain dumbbell-like carbodiimide anion NCN²⁻, has been investigated. A small bulk modulus of the tetragonal BaNCN was related to a large wavelength-shift of red photoluminescence from BaNCN：Eu phosphor in response to pressure. Pressure induced phase transition of the orthorhombic Ba_0.9Sr_0.1NCN has been firstly demonstrated in carbodiimide compounds. This phase transition is analogous to B1 to B2 phase transitions in monatomic compounds, such as barium chalcogenides, but the linear morphology of the carbodiimide anion induced the phase transition at approximately 0.3 GPa which is an order of magnitude lower that those required in monatomic compounds.

We report materials synthesis by applying high pressure to relatively complex crystals of highly conjugate molecules and metal-organic complexes, including metal-organic frameworks. Metal-organic complexes are decomposed by heating, and applying the high pressure hinders the diffusion of the atoms to form unique materials with very small clusters of metals or metal carbides embedded in carbon. They work as excellent catalysts. Highly conjugated aromatic molecules undergo polymerization at high pressure. The results of in-situ X-ray diffraction in a diamond anvil cell using synchrotron radiation are presented.