RAMAN-SPECTROSCOPIC INVESTIGATION ON THE PHASE-TRANSITIONS IN RB3H(SEO4)(2) AND RB3D(SEO4)(2) SINGLE-CRYSTALS

A temperature dependent Raman spectroscopic investigation of Rb3H(SeO4 )(2) and Rb3D(SeO4)(2) single crystals is carried out from liquid nitr ogen temperature up to 620 K. The observed spectra show that the phase transitions are due to restructuring of the system of hydrogen bonds. The ferroelastic phase transition observed in the protonated compound at 451 K is of second order, which corresponds to a rise of symmetry from the room temperature monoclinic to the trigonal phase. The higher wavenumber for the v(Se-O) band at the onset of the transition indica tes a positional disorder of protons. In the high temperature phase, t he protons are in a highly disordered state followed by fast tunneling . The absence of a frequency shift in the low temperature spectra of t he deuterated compound and the invariance of the symmetry during the t ransition show that the antiferroelectric transition at 95 K is second order and of the order-disorder type.