Abstract: Recent developments in energy technologies have led to an increased effort in scientific research into the use of nitrate salts as an energy storage material in recnet years. However, little is known about on the structure of molten salts and the influence of impurities on the structure of molten salt mixtures have been seldom reported. The presence of impurities can impose impacts on the thermophysical characteristics of molten salts that cannot be neglected. This study examines the addition of SO₄^2- on the structure and hence properties of a mixed KNO₃/NaNO₃ salt with mass ratio of 1∶1 a Raman spectral meter and a X-ray diffractometer were used in the measurements under different temperatures. The results suggests that when the molten salt solidifies and crystallizes, SO₄^2- ions combine with Na⁺ preferentially, and Na⁺ ions exist in the form of solid solution. It was also found that SO₄^2- ions exist in the form of solid solution at the room temperature, and the structure of the mixed salts change significantly at 200 ℃ compared to that at room temperature, suggesting a solid-solid phase change during the heating procedure. The results also show that the state of solid solution of SO₄^2- breaks at about 200 ℃.

Key words: molten salt structure, impurity ion, high temperature raman spectrum, high temperature X-ray diffraction