Phonon spectra respond, in principle, to any change of the crystal structure or lattice imperfections of a material. Traditionally optical soft modes, which can show relatively large variations, have been widely used for the analysis of displacive phase transitions. A soft mode is a particular mode of vibration of the high symmetry phase, whose frequency tends towards zero as the critical point is approached. In other cases the variations can be small; typical examples are the weak phonon renormalization of high temperature superconductors at the superconducting phase transition or isosymmetric phase transitions. Typical relative changes Δω/ω of phonon frequencies which...

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