How does compression reduce the volume of a solid?

In the study of high-pressure phenomena, compression reduces the volume of a solid by forcing atoms closer together, which alters the energy of atomic bonds. This process occurs through three primary mechanisms:12

• Bond compression: This involves the shortening of interatomic distances. The magnitude of this effect depends on bond strength; for example, strong covalent bonds like those in diamond are highly resistant to compression, while weaker bonds in materials like alkali halides compress more easily.1
• Bond-angle bending: In materials composed of framework structures—such as quartz and other silicates—compression often occurs by bending the bond angles between atomic polyhedrons. This mechanism, along with 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 held together by van der Waals forces or hydrogen bonds, volume reduction is primarily achieved through large decreases in the distances between discrete molecules rather than the compression of the molecules themselves.1

Additionally, under sufficiently high pressure, materials may undergo phase transitions to denser, more closely packed atomic arrangements.3