The interpretation of Raman spectra of soft modes in ferroelectric crystals is not easy because of its over-damped line shape and the complicated temperature dependence near the transition temperature. There exist not a few cases which are difficult to determine the transition type is either order-disorder or displacive type. The main purpose of this paper is to point out the characteristics of Raman spectra of polar modes and several cautions for the analysis of soft modes. We discuss on some general properties of the susceptibility functions and also show possible reasons of confusion which might lead to the incorrect conclusions in the specific cases of KH2PO4 (KDP), ferroelectric SrTiO3 and proton–ordered Ice crystals.
Part of the book: Ferroelectric Materials