Anomalous properties of molten alkali nitrates coexisting with aluminum oxides by hetero-phase effect

In order to reveal physicochemical anomalies of molten nitrate melts coexisting with inorganic powder, Raman spectra, electrical conductivity and thermal analysis of α-Al2O3 powder/molten alkali nitrates coexisting systems were measured. The variations of intra-molecular vibration modes of nitrate ion permeated among porous solid material were investigated by means of Raman spectroscopic measurement and IF spectra measured in previous study. The nitrate ν1 mode varied with the specific surface area of the solid phase and the apparent average thickness of the liquid layer. These variations indicate that the nitrate molecular motions are perturbed by the presence of the alumina powder. The Gaussian resolution of the nitrate ν1 mode showed a small asymmetry, which is attributed to the presence of bound nitrate ion in higher wavenumber range. Two Raman lines observed in the range from 450 to 350 cm−1 are assigned to the symmetric stretching νs(AlO) of aluminum nitrate or aluminum nitrite. Although, most of the nitrate ions retained their free D3h structure, a lowering of symmetry from D3h to C2v was observed for samples containing small amount of nitrate with large surface area. These structural changes are in good agreement with some other properties of nitrate melts coexisting with the solid phase, such as the ionic conductivity and the activation energy, and thermal behaviors. The roles of cationic species involved into systems are also discussed.