However, as the magnitude of the Seebeck coefficient increases with phonon drag, it may be beneficial in a thermoelectric material for direct energy conversion applications.
They lose momentum by interacting with electrons (or other carriers) and imperfections in the crystal.
If the phonon-electron interaction is predominant, the phonons will tend to push the electrons to one end of the material, losing momentum in the process.
This region of the Seebeck coefficient-versus-temperature function is highly variable under a magnetic field.
Kittel, Charles (1996) Introduction to Solid State Physics, 7th Ed., John Wiley and Sons, Inc.