FTIR Characterization of Polymorphic Transformation of Ammonium Nitrate

Solid ammonium nitrate (NH 4 NO 3 ) exists in five stable polymorphic forms (designated as phases I, II, III, IV, and V) below its melting point at around 170°C. Phase IV is stable in a temperature range of −17°C ∼ 32°C and is the only phase that has been considered by the atmospheric research community until recently. In this study, we examine the IV ↔ III phase transition of NH 4 NO 3 and how relative humidity (RH) affects the transition path and the transition temperatures using in-situ microscopic Fourier Transform InfraRed spectroscopy. Two kinds of NH 4 NO 3 samples, powder produced from grinding commercially produced chemicals and single particles obtained by efflorescence of droplets on PTFE filters, were studied. The powder samples exhibit the IV↔III phase transition and the transition temperature depends on the RH while the single particle samples exhibit only the IV↔II transition at about 52°C (forward) and 48°C (reverse), bypassing phase III, with transition temperatures independent of the RH. However, grinding of the particles produced through efflorescence results in the IV↔III transitions. Differences in crystal structure and moisture content may explain the distinct phase transition behaviors of the two types of samples. These results suggest that solid and pure NH 4 NO 3 aerosol particles are stable in phase IV under ambient conditions.

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