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 symmetric stretching upsilon1 mode of nitrate ions (D3h symmetry) in the molten binary systems (M1,Ag) NO3(M1= Li, Na, and K) have been measured. Vibrational and rotational autocorrelation3 functions, Cv(t) and Cr(t), respectively, have been evaluated from Fourier transformation of the spectra. The shifts in the peak frequencies are deeply correlated with the polarizability of silver in these mixtures. The vibrational relaxation rate increases with increasing 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 deviates from additivity3 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 several nitrate ions in these mixtures appreciably restricts the rotational motion of the nitrate ions. On the other hand, rotational motion becomes easier in the mixture system (Li,Ag) NO3 mainly because the different preferential sites of the two cations coordinating to an NO-3ion, and partly because the exchange rates of these cations around the NO-3ion are high.