Polymorphism and melting in the alkali nitrates to 40kb with some comments on the alkaline earth carbonates

Abstract

The phase diagrams of the alkali nitrates were determined by differential thermal analysis to 40 kb and 700°C. LiNO 3 shows no polymorphic transitions. The effect of pressure on the gradual transition in NaNO 3 can be expressed by the equation T = 275+6.076 p-0.038475 p 2 ( Tin °C, p in kb). The melting curves of LiNO 3 and NaNO 3 are very similar. The latter flattens considerably at higher pressure and cannot be fitted to the Simon equation. This might indicate an approach to a maximum in the melting curve similar to the one found in KNO 3. The RbNO 3 IV-III and CsNO 3 II-I transitions were followed to 40 kb. The results are in very good agreement with Bridgman's lower pressure data. The melting curves of CsNO 3 I, RbNO 3 III and KNO 3 VI rise steeply with pressure. Comparison is made between the alkali nitrates and the alkaline earth carbonates and a tentative phase diagram is proposed for CaCO 3 based on the analogous KNO 3 diagram and on available experimental data.