Raman spectra of crystals

Polarized Raman spectra of crystals (x = 0.1 and 0.7) were studied in the temperature range 10-300 K. Thorough site-symmetry analysis of internal and vibrations combined with factor-group analysis based on hexagonal pseudosymmetry allowed us to make an assignment of the observed modes. The spectra of the x = 0.1 sample are compatible with those of pure including the features of the disorder in the paraelectric phase above 220 K (the broad central peak of the relaxator type and high mode damping) and ordering in the ferroelectric phase. The spectra of the x = 0.7 sample show a much smaller disorder at high temperatures (no broad central mode and lower mode damping) due to the lower ammonium content, but clearly confirm the onset of the dynamic dipolar glass transition (short-range correlations inducing the appearance of long-lived dipolar clusters) near 220 K.