The NaNO3-KNO3 system: the position of solidus and sub-solidus

The existing evidence for the nature of the phase diagram for the binary system sodium nitrate-potassium nitrate is reviewed and in particular whether the system is of the continuous solid solution type, as has often been stated in the last 80 years, or whether this system is of the eutectic type as was earlier believed and has again been asserted recently. Additional evidence from Raman spectroscopy and Raman mapping on the 50:50 mole % (minimum melting point) composition is now presented, supporting the eutectic classification. Abrupt changes in wavenumber, or in the wavenumber-temperature gradient of five Raman bands indicate a solid state transition at about 115 oC and are attributed to a phase transition in KNO3-rich areas. On a fast cooled sample, Raman bands attributed to sodium nitrate-rich and potassium nitrate-rich areas were found to persist up to and slightly beyond the melting point, and although their wavenumber-positions converged, the apparent single band could still be resolved into the two bands which could be attributed to the Na-rich and K-rich areas. On cooling, the reverse change took place quickly. Measurements with the initially slow cooled sample, where these areas were bigger, showed that the spectral bands reverted to the room temperature wavenumber values, after holding at 22 oC for only 60 to 90 minutes.