FTIR characterization of polymorphic transformation of ammonium nitrate

Solid ammonium nitrate (NH4NO3) exists in five stable polymorphic forms ( designated as phases I, II, III, IV, and V) below its melting point at around 170 degrees C. Phase IV is stable in a temperature range of - 17 degrees C similar to 32 degrees 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 NH4NO3 and how relative humidity ( RH) affects the transition path and the transition temperatures using in-situ microscopic Fourier Transform InfraRed spectroscopy. Two kinds of NH4NO3 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 degrees C (forward) and 48 degrees 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 NH4NO3 aerosol particles are stable in phase IV under ambient conditions.