EFFET RAMAN ET THÉORIERAMAN SCATTERING FROM FERROELECTRIC MODES IN THE KDP ISOMORPHOUS PHOSPHATES AND ARSENATES

The low frequency Raman spectra of a number of KH2PO4 (KDP) isomorphs have been recorded over a range of temperatures in the paraelectric phase. We have extended the measurements to frequencies of 250 cm-1 and have included in our model fitting the characteristic interference between the overdamped and the low frequency optical phonon of the same symmetry. We report the first observation of an overdamped mode of B2 symmetry in the Raman spectrum of (NH4)H2PO4. The temperature dependence of the Debye relaxation times for both E and B2 modes in (NH4)H2PO4 are compared with dielectric measurements.