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 α-KNO₃ to β-KNO₃ occurs at 401 K. On cooling with a cooling rate of 7 K min⁻¹, the transition from β-KNO₃ to γ-KNO₃ was observed at 388 K. The phase transition from γ-KNO₃ to α-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 K, the γ-KNO₃ phase was obtained as a pure phase at 303 K, but it was eventually transformed to α-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 γ-KNO₃ phase was formed at 391 K and transformed at 370 K to α-KNO₃. With a cooling rate of 7 K min⁻¹ from 403 to 303 K, the γ-KNO₃ phase transformed to α-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 γ-KNO₃ from 383 K to room temperature are reported.