Light-Scattering Studies of the Soft Ferroelectric and Acoustic Modes of KD2PO4

Light-scattering spectra of the soft-optic and -acoustic modes of K${\mathrm{D}}_{2}$P${\mathrm{O}}_{4}$ have been studied in the 0-0.8-${\mathrm{cm}}^{\ensuremath{-}1}$ spectral region for temperatures from 0 to 10\ifmmode^\circ\else\textdegree\fi{}K above the ferroelectric phase transition. The true spectra of the soft ferroelectric optic mode were extracted from the experimental data obtained in the $x(y,x)y$ scattering geometry by a Fourier-deconvolution procedure and were found to fit a Debye-relaxational-model susceptibility. The spectra obtained in the $x+z(\ensuremath{-}x+z,y)\ensuremath{-}x+z$ configuration exhibited scattering from both the (overdamped) soft ferroelectric optic mode and the soft acoustic mode, and because these modes are piezoelectrically coupled, interference effects in the scattering amplitudes were observed. A phenomenological theory, which is an extension of the theory used for BaTi${\mathrm{O}}_{3}$ by Fleury and Lazay, is developed to describe the scattering from the coupled modes and is shown to be in agreement with the observed spectra.