Phase transition of KNO3 monitored by synchrotron X-ray powder diffraction

The solid-state phase transitions of KNO3 were studied at atmospheric pressure in the temperature range 303 to 533 K by synchrotron X-ray powder diffraction. The detectors used were (i) a curved position-sensitive detector and (ii) a moving imaging-plate system built for time-, temperature- and wavelength-dependent powder diffraction. On heating, the transition from alpha-KNO3 to beta-KNO3 occurs at 401 K. On cooling with a cooling rate of 7 K min(-1), the transition from beta-KNO3 to gamma-KNO3 was observed at 388 K. The phase transition from gamma-KNO3 to alpha-KNO3 occurred at temperatures that strongly depended upon the cooling rate. With a high cooling rate of 15 K min(-1) from 403 to 303 , the gamma-KNO3 phase was obtained as a pure phase at 303 K, but it was eventually transformed to alpha-KNO3 at this temperature, and the phase transition at 303 K was complete within 15 min. With a slow cooling rate of 0.5 K min(-1) from 403 to 303 K, the gamma-KNO3 phase was formed at 391 K and transformed at 370 K to alpha-KNO3. With a cooling rate of 7 K min(-1) from 403 to 303 K, the gamma-KNO3 phase transformed to alpha-KNO3 in a temperature range between 377 and 353 K. The two phases could exist simultaneously in temperature ranges that were apparently dependent upon the thermal history of the sample. The unit-cell parameters of gamma-KNO3 from 383 K to room temperature are reported.