14N MAS NMR spectroscopy and quadrupole coupling data in characterization of the IV ⇆ III phase transition in ammonium nitrate

The first application of N-14 MAS NMR spectroscopy and the associated N-14 quadrupole coupling data for characterizing a phase transition between two solid phases is presented. For NH4NO3, the transition between the phases IV and III at about 35 degreesC and the phases themselves are studied by N-14 MAS NMR using the NH4+ as well as the NO3- ion as probes. Spectra are acquired employing spinning speeds in the range from 2000 to 7000 Hz thereby taking advantage of the frictional heating as the heat source. The N-14 quadrupole coupling constants and asymmetry parameters for both the NH4+ and NO3- ion determined from these spectra show large changes upon transition between the two phases. Also the transition exhibits the effect of hysteresis with a deviation of about 10 degreesC for the transition temperature. The results show great potential for N-14 MAS NMR being a useful tool in characterizing phase transitions for nitrogen compounds from N-14 quadrupole coupling data.