Amorphous nitric acid hydrates in the concentration range 25 to 50 mol% HNO 3 were prepared by lowtemperature quenching of the respective HNO 3 /H 2 O aerosols. Subsequently, a properly selected thermal treatment enabled the phase distribution of two thermodynamically stable (NAM, b-NAT) and three metastable phases (a-NAD, b-NAD, a-NAT) to be determined between 180 and 232 K by X-ray powder diffractometry. The intriguing phase distribution of the two nitric acid dihydrates was interpreted on the basis of the classical nucleation theory and Ostwald's step rule using known crystallographic data on aand b-NAD.

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Crystallization kinetics of stable and metastable nitric acid trihydrate (NAT) were investigated by time dependent X-ray powder diffraction (XRD) measurements. Kinetic conversion curves were evaluated adopting the Avrami model. The growth and morphology of the respective crystallites were monitored in situ on the cryo-stage of an environmental scanning electron microscope (ESEM) under a partial pressure of nitrogen gas (0.3 Torr, 40 Pa). The results show a close relationship between the presence of ice in the sample and the crystallization mechanism of NAT, which results in different shapes and sizes of NAT crystal particles.

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