MOLECULAR RELAXATION IN BINARY SYSTEMS NaNO3 - NaNO2, KNO3 - KNO2

In the present study, the molecular relaxation processes in crystalline sodium nitrate NaNO3, crystalline sodium nitrite NaNO2, crystalline potassium nitrate KNO3, crystalline potassium nitrite KNO2, and in solid binary salt systems sodium nitrate - sodium nitrite NaNO3 - NaNO2 and potassium nitrate - potassium nitrite KNO3 - KNO2 were studied with the Raman spectroscopic methods. We have found that the relaxation time of the fully symmetric vibration of. 1 (A) of the molecular nitrate anion NO3- in the solid binary salt systems "nitrate - nitrite" NaNO3 - NaNO2 or KNO3 - KNO2 is lower than in the individual crystalline nitrate NaNO3 or KNO3, respectively. It is shown that the increase in the rate of intramolecular relaxation is explained by the presence in the binary system of an additional relaxation mechanism of the vibrationally excited states of the nitrate ion NO3-. This relaxation mechanism of the nitrate ion NO3- in a binary system is associated with the excitation of a lower-frequency vibration of another anion (nitrite ion NO2-) and the "birth" of a lattice vibration (phonon). The frequency of this phonon is equal to the difference in the vibration frequencies of the nitrate ion NO3- and nitrite ion NO2-. It is established that the condition for the realization of such a relaxation mechanism is that the difference in the frequency of these oscillations of the nitrate ion NO3- and nitrite ion NO2- must correspond to the region of a sufficiently high density of states of the phonon spectrum of the solid binary salt system under study. The maximum frequency of the phonon spectrum of the system is determined by its Debye temperature. For ionic salt systems, the Debye temperature is always higher than 200 K, and often more than 300 K. Therefore, in the solid binary salt systems, the proposed relaxation mechanism is almost always relevant.