in solid-state physics, a quantized particle-like unit of vibrational energy arising from the oscillations of the atoms within a crystal. Any solid crystal, such as ordinary table salt ( sodium chloride), consists of atoms bound into a specific repeating three-dimensional spatial pattern called a lattice. Because the atoms have thermal energy, the lattice vibrates in response to applied forces and generates mechanical waves that carry heat and sound through the crystal. In quantum mechanics a packet of these waves constitutes a phonon, which travels within the crystal with particle-like properties. A phonon is a quantum of vibrational mechanical energy, just as a photon is a quantum of light energy.

In addition to their importance in the thermal and acoustic properties of solids, phonons are essential in the phenomenon of superconductivity—a process in which certain metals such as lead and aluminum lose all of their electrical resistance at temperatures near absolute zero (−273.15 °C; −459.67 °F). Ordinarily, electrons (electron) collide with impurities as they move through a metal, which results in a frictional loss of energy. In superconducting metals at sufficiently low temperatures, however, electrons interact with each other through the intermediate effect of phonons. The result is that the electrons move through the material as a coherent group and no longer lose energy through individual collisions or scatterings. Once this superconducting state has been achieved, any flow of electrical current will persist indefinitely. See also low-temperature phenomena.