Phonons are quantized units of vibrational energy that travel through a crystal lattice, much as photons are quantized units of light. When a crystal's atoms are jostled, they do not move independently: because each atom is bonded to its neighbors, a disturbance propagates outward as a collective wave, and quantum mechanics restricts the energy of that wave to discrete steps, giving rise to the phonon quasiparticle. The concept was introduced around 1932 by Igor Tamm and Yakov Frenkel as physicists worked out how heat and sound move through solids. Two families of phonon modes exist: acoustic phonons, where neighboring atoms move mostly in phase like ordinary sound, and optical phonons, where neighboring atoms move out of phase and which can couple to light in ionic crystals. In insulators and semiconductors, phonons are the primary carriers of heat, and phonon-phonon scattering is what limits thermal conductivity as temperature rises.

Characteristics

• Phonon concept introduced by Igor Tamm and Yakov Frenkel around 1932.
• Typical atomic spacing in a crystal lattice is about 0.2-0.5 nm.
• Speed of sound (acoustic phonons) in solids typically ranges 2000-6000 m/s, e.g. ~5960 m/s in steel.
• Diamond's exceptionally stiff lattice carries acoustic phonons at roughly 12000 m/s.
• Acoustic phonons move atoms in phase; optical phonons move neighboring atoms out of phase.
• Phonons are the dominant heat carriers in electrical insulators and semiconductors.
• Debye temperature marks the crossover between classical and quantum lattice vibration: ~2230 K for diamond, ~105 K for lead.
• Phonon-phonon (Umklapp) scattering limits thermal conductivity at higher temperatures.