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 po wder diffraction. The detectors used were (i) a curved position-sensit ive detector and (ii) a moving imaging-plate system built for time-, t emperature- and wavelength-dependent powder diffraction. On heating, t he 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 th e cooling rate. With a high cooling rate of 15 K min(-1) from 403 to 3 03 , the gamma-KNO3 phase was obtained as a pure phase at 303 K, but i t was eventually transformed to alpha-KNO3 at this temperature, and th e phase transition at 303 K was complete within 15 min. With a slow co oling rate of 0.5 K min(-1) from 403 to 303 K, the gamma-KNO3 phase wa s formed at 391 K and transformed at 370 K to alpha-KNO3. With a cooli ng rate of 7 K min(-1) from 403 to 303 K, the gamma-KNO3 phase transfo rmed to alpha-KNO3 in a temperature range between 377 and 353 K. The t wo phases could exist simultaneously in temperature ranges that were a pparently dependent upon the thermal history of the sample. The unit-c ell parameters of gamma-KNO3 from 383 K to room temperature are report ed.