**1. Introduction**

Historically, phase transitions in ferroelectric crystals have been explained by the *order–disorder type* mechanism that is similar to magnetic crystals. In the1950s, however, the idea of soft mode for *displacive-type* structural phase transitions was proposed by Anderson and others [1]. Since the1960s, with the advent of laser technology, laser Raman spectroscopy became one of the most powerful experimental methods for the study of soft mode particularly in ferroelectric crystals, partly because soft modes below *Tc* are always Raman-active [2]. An excellent review of soft mode spectroscopy in the early stage was given by Scott [3]. A theoretical textbook, *Scattering of Light by Crystals*, by Hayes and Loudon was published in 1978 [4].

A displacive-type transition is induced by the softening (decreasing frequency) of a particular lattice vibrational mode (phonon) toward the transition temperature *Tc* and eventually the

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freezing of the mode pattern changes the symmetry of the crystal. The interpretation of a soft mode, however, is not easy because its Raman spectrum is often observed without any definite peak and becomes a broad quasi-elastic peak near *Tc*. Therefore, interpretations of Raman spectra of polar modes, such as soft modes in ferroelectric crystals must be carefully done. Otherwise, incorrect conclusions might be obtained. In the following sections, several exam‐ ples related to this problem are shown for the cases of KDP, ferroelectric SrTiO3, and protonordered Ice crystals.
