Interpretation of the low frequency Raman spectra of molten lithium nitrate and molten silver nitrate

Abstract

Low frequency bands due to librational motions of the nitrate ion are observed in the Raman spectra of molten lithium nitrate (at 143 cm^–1) and molten silver nitrate (at 107 cm^–1). The peak frequencies, shapes and intensities of these bands were studied as a function of dilution with various other salts. It is concluded that the bands originate from localized librational modes and that their presence is no evidence for a quasi-crystalline structure. For lithium nitrate it is suggested that the librational motion is a direct result of transient pairwise ionic association. The intensity measurements for silver nitrate mixtures indicate that silver ions are capable of simultaneous association with more than one nitrate ion. The specific librational motion which occurs in these two cases is distinguished from the angular motions of nitrate ions which give to the low frequency background scattering in molten nitrates.