Phase Transition of KNO₃ Monitored by Synchrotron X-ray Powder Diffraction

Abstract

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 o~-KNO3 to /3-KNO3 occurs at 401 K. On cooling with a cooling rate of 7 K min -l, the transition from fl-KNO3 to "y-KNO3 was observed at 388 K. The phase transition from "y-KNO3 to c~-KNO3 occurred at temperatures that strongly depended upon the cooling rate. With a high cooling rate of 15 K min -l from 403 to 303 K, the "y-KNO3 phase was obtained as a pure phase at 303 K, but it was eventually transformed to c~-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 -l from 403 to 303 K, the h'-KNO3 phase was formed at 391 K and transformed at 370 K to o~-KNO3. With a cooling rate of 7 K min -1 from 403 to 303 K, the 3,-KNO3 phase transformed to o~-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 "y-KNO3 from 383 K to room temperature are reported.

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

“…2, we observe that the stability of -KNO at 1253C increases as the sample is heat-cycled through the di!erent phases for a number of times. In this context we note that Christensen et al (2) observed the temperature interval for the appearance of -KNO to decrease on continuing cycling through the di!erent phases, i.e., from the range of 115}973C to the range of 115}1043C for 3 cycles. Also, we should point out that in our experiments we always observe -KNO along with the -KNO phase present in di!erent amounts.…”

“…A metastable phase of KNO has been observed by other methods upon cooling of -KNO . In particular, we note the recent synchrotron powder XRD study (2), which paid special attention to observing the -KNO phase by cooling -KNO . The spectral changes observed at 1253C upon cooling of the -KNO phase propose that the resonance appearing at 359.0 ppm arises from the metastable -KNO .…”

“…On further cooling, the metastable -KNO phase is reported to transform to -KNO at approximately 1003C (373 K). However, the lifetime of -KNO and its transition temperature back to -KNO strongly depends on the thermal history of the KNO sample and on the cooling rate (1,2). In a recent investigation Christensen et al (2) employed variable-temperature synchrotron X-ray powder di!raction to monitor the phase transitions for KNO in an elegant manner and with special attention being paid to the to -phase transition.…”

“…However, the lifetime of -KNO and its transition temperature back to -KNO strongly depends on the thermal history of the KNO sample and on the cooling rate (1,2). In a recent investigation Christensen et al (2) employed variable-temperature synchrotron X-ray powder di!raction to monitor the phase transitions for KNO in an elegant manner and with special attention being paid to the to -phase transition. Furthermore, the temperature dependence of this transition and of the existence of the phase was studied in detail and shown to be strongly a!ected by the thermal history of the sample.…”

Time-Resolved X-ray Powder Diffraction Using a Large-Area CCD-Based Detector and Rietveld Refinement: Solid-State Polymerization of S2N2 to (SN)x

“…Energy-dispersive powder diffraction is well suited for time-resolved studies; however, monochromatic powder diffraction offers better d-space resolution, which is important in multi-phase studies. Monochromatic powder diffraction has been used successfully in a large number of studies using either conventional sources (see, for instance, Bdnard & Lou/Sr, 1995;Oyama, Hasegawa & Takei, 1994;Suda, Yashima, Kakihana & Yoshimura, 1994;Weber & Schweda, 1995) or synchrotron radiation (Pennartz, L6chner, Fuess & Wroblewski 1992: Christensen, Norby & Hanson, 1995Rizzo, Doyle & Wroblewski, 1995;Christensen, Norby, Hanson & Shimida, 1996). These studies have either employed conventional 0-20 scans or one-dimensional position-sensitive detectors.…”