Pretransition phenomena near first-order phase transitions in ion-molecular crystals

Processes of molecular relaxation in Li$_{2}$SO$_{4}$, Na$_{2}$SO$_{4}$, and K$_{2}$SO$_{4}$ sulfates; Li$_{2}$CO$_{3}$, Na$_{2}$CO$_{3}$, and K$_{2}$CO$_{3}$ carbonates; NaClO$_{4}$ and KClO$_{4}$ perchlorates; and Ca(NO$_{3}$)$_{2}$, Sr(NO$_{3}$)$_{2}$, and Ba(NO$_{3}$)$_{2}$ nitrates have been investigated by Raman spectroscopy. It is established that the first-order phase transitions in these systems have a stretched character (structural phase transitions in sulfates, carbonates, and perchlorates and crystal–melt phase transitions in nitrates). The existence of a pretransition region is found in all crystals under investigation.