What are the three main ways that solids compress under high pressure?

High-pressure X-ray crystallographic studies have identified three primary mechanisms by which solids compress under high pressure:

• Bond compression: This involves the shortening of interatomic distances. It occurs to some degree in all compounds, with the magnitude of the effect depending on the strength of the chemical bonds. Strong covalent or ionic bonds are generally more resistant to this type of compression than weaker bonds.1
• Bond-angle bending: In materials composed of frameworks of atomic polyhedrons—such as many rock-forming minerals—compression can occur by bending the bond angles between these polyhedrons. This process, which often involves the collapse of interpolyhedral spaces, typically results in a volume change an order of magnitude greater than that caused by bond-length changes alone.1
• Intermolecular compression: In molecular solids, which are held together by weak forces like van der Waals forces and hydrogen bonds, compression occurs primarily through large decreases in the distances between discrete, rigid molecules rather than through the compression of the molecules themselves.1