Relaxation of vibrationally excited states in solid binary carbonate-sulfate systems

The processes of molecular relaxation in solid binary carbonate-sulfate systems, such as Li$_{2}$CO$_{3}$–Li$_{2}$SO$_{4}$, Na$_{2}$CO$_{3}$–Na$_{2}$SO$_{4}$, K$_{2}$CO$_{3}$–K$_{2}$SO$_{4}$, have been studied by Raman spectroscopy. It has been revealed that the relaxation time of CO$_{3}^{2-}$ anion vibration $\nu_{1}$(A) in a binary system is higher than in an individual carbonate. It is shown that an increase in the relaxation rate may be explained by the existence of an additional mechanism of the relaxation of vibrationally excited states of a carbonate anion. This mechanism is associated with the excitation of the vibration of another anion (SO$_{4}^{2-}$) and the “birth” of a lattice phonon. It has been established that the condition for the implementation of such a relaxation mechanism is that the difference between the frequencies of these vibrations must correspond to the region of a rather high density of phonon spectrum states.