Preliminary observation of a central peak in the light-scattering spectrum of KH2PO4

Abstract

High-resolution Brillouin scattering experiments on K${\mathrm{H}}_2$P${\mathrm{O}}_4$ have revealed a quasielastic component whose intensity increases dramatically near the 122 °K ferroelectric phase transition. The two-coupled-mode analysis which has previously been employed in analyzing Raman spectra of K${\mathrm{H}}_2$P${\mathrm{O}}_4$ and its isomorphs, is reviewed and extended to simultaneously include the interacting optic, acoustic, and ferroelectric soft modes. Results of both Brillouin and Raman scattering experiments are analyzed in terms of a relaxing self-energy as proposed by Cowley for the piezoelectric ferroelectrics. We find that $\delta$, the strength of the relaxing self-energy term, is approximately 1 ${\mathrm{cm}}^{-1\mathrm{}}$ at $T_C$. The central peak was too narrow to resolve in this experiment, which implies a lower limit for the relaxation time $\tau$ of ${10}^{-9\mathrm{}}$ sec.