Rotational and vibrational relaxation of the NO3- ion in molten binary systems (M,Ag)NO3 (M = Li, Na and K) by Raman spectroscopy

Polarized and depolarized Raman scattering spectra for the totally symmetri c stretching vl mode of nitrate ions (D-3h symmetry) in the molten binary s ystems (M1,Ag)NO3 (M1 = Li, Na, and K) have been measured. Vibrational and rotational autocorrelation functions, C-v(t) and C-r(t), respectively, have been evaluated from Fourier transformation of the spectra. The shifts in t he peak frequencies are deeply correlated with the polarizability of silver in these mixtures. The vibrational relaxation rate increases with increasi ng mole fraction of silver nitrate in (K,Ag)NO3, but does not vary so much in (Li,Ag)NO3 and (Na,Ag)NO3. The vibrational relaxation time negatively de viates from additivity on the order of (Na,Ag)NO3 < (Li,Ag)NO3 < (K,Ag)NO3. In the systems (Na,Ag)NO3 and (K,Ag)NO3 rotational relaxation time becomes slower with increasing concentration of AgNO3. The Ag+ coordinating to sev eral nitrate ions in these mixtures appreciably restricts the rotational mo tion of the nitrate ions. On the other hand, rotational motion becomes easi er in the mixture system (Li,Ag)NO3 mainly because the different preferenti al sites of the two cations coordinating to an NO3- ion, and partly because the exchange rates of these cations around the NO3- ion are high.