Phenomenological theory of coupling of debye relaxations with IR‐active phonons

A phenomenological theory of the coupling of dipole relaxations with infrared active long optical phonons in ferroelectric crystals is given. It is shown that whenever there exists a relaxation process in the crystal the phase transition is driven by a soft relaxation state which in an optical spectrum should appear as a central peak. The occurrence of a soft relaxation state is also responsible for observed discrepancies between values of the static dielectric constant measured by different experimental techniques.