Phase transitions in KNO3 studied by variable-temperature 15N magic-angle spinning NMR spectroscopy

The phase transitions within the three phases of potassium nitrate (alpha-, beta-, and gamma-KNO3) have been investigated in detail by variable-temperature (VT) N-15 magic-angle spinning (MAS) MMR spectroscopy employing 98% N-15-enriched KNO3. The stability of the metastable gamma-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 beta-KNO3 phase at 160 degrees C. Furthermore, the spectral appearance for the room-temperature alpha-KNO3, phase is found to change after excursions of the sample into the high-temperature beta-KNO3 phase, an observation similar to that made in a recent VT synchrotron X-ray powder diffraction study. (C) 1999 Acndemic Press.