Order-disorder transitions and structural relaxation phenomena in crystalswith hydrogen bonds

This work is devoted to the study of structural relaxation in superionic cr ystals with hydrogen bonds at the recovery of the low temperature phase str ucture after fast cooling from the superionic phase. The results of experim ental investigations of this phenomenon in (NH4)(4)H-2(SeO4)(3) crystal by means of Raman spectroscopy are given. It follows that superionic-ferroelas tic transition is connected with the transformation from delocalization of protons to their localization and that the creation of the ordered state of protons with definite space symmetry takes place after their localization. To explain this effect the microscopic approach, based on the proton order ing model and taking into account thermally activated proton hopping and qu asi-polaron effect resulting from the interaction of protons with the optic al phonons, is proposed.