Vibrational spectra of ion-molecular carbonate crystals in the pretransition region near structural phase transitions

Molecular relaxation processes in lithium carbonate (Li$_{2}$CO$_{3}$), sodium carbonate (Na$_{2}$CO$_{3}$) and potassium carbonate (K$_{2}$CO$_{3}$) were studied by Raman spectroscopy. It has been established that in crystalline carbonates Li$_{2}$CO$_{3}$, Na$_{2}$CO$_{3}$, and K$_{2}$CO$_{3}$, the structural phase transition of the first kind is stretched (diffuse phase transition). The existence of the pretransition region in the studied carbonates Li$_{2}$CO$_{3}$, Na$_{2}$CO$_{3}$, and K$_{2}$CO$_{3}$ was found.