Phase Transitions in KNO3 Studied by Variable-Temperature 15N Magic-Angle Spinning NMR Spectroscopy

The phase transitions within the three phases of potassium nitrate (-, -, and -KNO3) have been investigated in detail by variabletemperature (VT) 15N magic-angle spinning (MAS) NMR spectroscopy employing 98% 15N-enriched KNO3. The stability of the metastable -KNO3 phase is found to be highly influenced by the thermal history of the sample, i.e., by the number of heating cycles into the high-temperature -KNO3 phase at 160°C. Furthermore, the spectral appearance for the room-temperature KNO3 phase is found to change after excursions of the sample into the high-temperature -KNO3 phase, an observation similar to that made in a recent VT synchrotron X-ray powder diffraction study.