# What are the physical implications of phonon softening?

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1. Feb 25, 2015

### fisher468

I am interested in how phonon softening would lead to changes in a crystals elastic properties but I don't understand what actually is the consequence of this. What would be affected by a reduction in energy to phonons in a crystal?

My best guess would be a reduction in energy to phonons causes more phonon scattering events be it electron-phonon or phonon-phonon because they are easier to excite. This would affect things like carrier mobility and thermal conductivity.

2. Feb 27, 2015

Phonon softening means that the energy of one of phonon branches $\omega(k)$ becomes zero at some point k0 of Brillouin zone. Phonon is a dynamical object related to the atomic vibrations. When its frequency goes to zero the vibrations become static, resulting in general in new crystal structure. This is called displacive phase transition. If k0 is not zero the new crystal structure must be described by symmetry with new bigger unit cell. After this kind of structure transition the properties of crystal might drastically change. Well known example is BaTiO3.