Abstract
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.
