The harmonic model usually gives a good starting point for understanding the vibrational thermodynamics of materials. At high temperatures, however, non-harmonic effects can alter substantially the thermodynamics of alloy phases. One effect can be understood as standard "quasiharmonic" behavior, where vibrational frequencies are lowered as a crystal expands against its bulk modulus. Most materials show a second effect from phonon-phonon interactions (PPI), and metals can show a third from electron-phonon interactions (EPI). We have been measuring these effects with inelastic neutron scattering, and calculating them with density functional theory.
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