Substances densified at pressures of several tens of gigapascals show various behaviors different from those at ambient pressure. Molecules in a solid state dissociate to form an atomic solid or combine to form polymers. Structural transformations with rearrangement of constituted atoms or molecules are common behaviors observed for all substances and often accompanied by electronic and magnetic transitions. High pressure thus develops an exciting world of materials science. Three topics are presented as examples; solid-state polymerization of acetylene, symmetrization of the hydrogen bonds in ice, and phase separation of lanthanum dihydride.
We discuss if hydrostatic conditions are realized in actual high-pressure experiments. Specific arguments are given to low-temperature high-pressure experiments with a helium pressure transmitting medium. We further discuss general problems associated with solid pressure transmitting media.
High pressure research of liquids and amorphous using synchrotron radiation is reviewed. From the systematic research of various liquids, the difference of pressure-induced structural change between liquids and solids has been observed. The phase transitions between meta-stable phases are also reviewed from the experiments under low temperatures and high pressures.
In this article, history of the development of high-pressure X-ray diffraction experiments combined with synchrotron radiation and advances of the study of the Earth's deep interior are reviewed. In 1970s, pressure range for high-pressure and high-temperature X-ray diffraction was limited to only a few GPa but now it is possible to get high quality data even above 300 GPa and we can study the behavior of materials under the entire conditions of the Earth's interior. A future plan for the study using neutron diffraction is also introduced.