Indirect coupling of the soft optic phonon in KH₂PO₄

Abstract

The shape of the low frequency first-order Raman spectrum of KDP single crystals in the x ( y , x)y configuration is discussed. Two and three coupled oscillators modds are used to explain the spectral region up to 270 cm-l. The inadequacy of such models in describing the experimental data is analyzed, and a model in which two optic phonons are coupled via their decay into two acoustic phonons is proposed. The spectra in the para-electric phase are thus well accounted for by a soft optic phonon interacting via the two-phonon continuum with a temperature-independent lattice mode. This picture is found to be consistent with a quite reasonable shape of the joint-density of states for the acoustic phonons. Finally the proton-phonon interaction hypothesis is briefly reconsidered from this point of view.