Search of soft modes in crystals having KDP structure

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Abstract
The low-frequency Raman scattering in KDP, RDP and RDA in Y(XY)X configuration has been reinvestigated as a function of temperature with particular emphasis near the transition temperature Tc. The spectra in the region of the overdamped and A-BO4 stretching modes have been fitted to the response function of two coupled damped harmonic oscillators and that of lattice oscillators due to Silverman.8 In both the formalisms, assuming purely real or purely imaginary coupling the overdamped mode softens with temperature well above T c but attains a “hard-core” frequency near the phase transition. These results do not indicate existence of a mode approaching zero frequency in contrast to the earlier work by various authors.