Phase Diagrams of Sodium Nitrite and Potassium Nitrite to 40 kbar

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Abstract
The phase diagrams of NaNO2 and KNO2 were determined to 40 kbar. The NaNO2 phase diagram consists of five solid phases and the liquid. The ferroelectric Curie point (the III—II phase transition) and the Néel point (the I—II transition) were detected by differential thermal analysis (DTA) and determined to 40 kbar. The results are in good agreement with the work of Gesi and his co-workers to 10 kbar. Points on the III—IV boundary were determined by the volume-discontinuity method. The transition manifests itself as a break in the slope of the compression curve of NaNO2 and seems to be a second-order transition. The transition pressures are lower than those estimated previously. The melting curve increases with pressure. A new high-pressure solid-phase, NaNO2 V, wad discovered. The I—V phase boundary branches off the melting curve at the liquid—I—V triple point at 9.8 kbar and 343°C. The KNO2 phase diagram consists of six solid phases and the liquid. The I—II and IV—V phase boundaries, determined by DTA, rise very slowly with pressure. The II—IV, I—IV, and I—V boundaries were determined by the volume-discontinuity method with application of piston rotation. Two new high-pressure phases, KNO2 V and KNO2 VI, were discovered. The melting curve of KNO2 exhibits a maximum at 7.0 kbar and 466°C, meets the I—V boundary at a triple point at 15.2 kbar and 413°C, and then increases steeply with pressure.

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