RAMAN-SPECTRA OF DRADP-25 DIPOLAR GLASS - EVIDENCE FOR THE MIXED FERROELECTRIC GLASS PHASE

Raman spectra of a Rb-0.75(ND4)(0.25)D2PO4 (DRADP-25) mixed crystal ha ve been investigated in the 8-300 K temperature range for all fundamen tal polarization geometries. As in the case of DRADP-50, the spectra c an be interpreted in terms of vibrational modes of principal structura l units. Analysis of the temperature dependence of peak positions and halfwidths has shown two characteristic temperature regions (similar t o 200-220 K and similar to 110-120 K) where incipient cluster formatio n and freezing of individual structural units takes place in this comp ound on cooling through the dipolar glass transition. In contrast to t hose for DRADP-50, the Raman spectra of DRADP-25 reveal additionally t he onset of incipient ferroelectric clustering and the existence of a low-temperature mixed ferroelectric-glass phase.