NaNO2 + NaNO3 Phase Diagram: New Data from DSC and Raman Spectroscopy.

The binary phase diagram of sodium nitrite and sodium nitrate is currently not well-understood because four earlier determinations of this binary system resulted in considerably different conclusions. Two works classify the system as eutectic and two as a continuous solid solution type, together with different sub-solidus curves. New Raman measurements and differential scanning calorimetry data on solidified mixtures of different compositions have provided support for a simple eutectic diagram with a solidus at 230 °C ranging from 0.25 to 0.80 in mole fraction of sodium nitrate. A sub-solidus transition was detected close to 183 °C and a second one close to 160 °C. The latter was considered to be related to the nitrite component and probably arose from the ferroelectric to antiferroelectric or paraelectric transition of sodium nitrite. The change of slope of the maxima of some Raman lines with temperature also gave support to these transitions.