Transition path selection between ammonium nitrate solid phases IV, III and II

Abstract

Study was made of factors affecting the transition paths between ammonium nitrate solid state phases IV, III and II. The effect of the sample preparation method was investigated by differential scanning calorimetry (DSC), transition mechanisms were explored with simultaneous DSC and Raman spectrometry developed for in situ investigation of the transitions, and the relationship between phase IV structure and the transition paths was studied by X-ray powder diffraction (XRD) and DSC. The results were analysed by partial least-squares regression (PLS) and principal component analysis (PCA). Three categories of transitions were observed: a) the principal transitions IV ↔ III ↔ II and IV ↔ II, b) the parallel transitions IV → III → II and IV → II on heating or II → III → IV and II → IV on cooling and c) the forward-and-back transitions IV → II (→ III → II) on heating and II → IV (→ III → IV) on cooling. The phase transition paths were shown to be influenced by the sample preparation method, which evidently affects the degree of disorder of phase IV. The existence of an intermediate phase II* between phases IV and III was confirmed. The life-time of II* was seven minutes maximum and decreased with decreasing transition temperature. The multivariate analysis of XRD and DSC data showed that a relationship exists between the XRD patterns and the transition paths and confirmed that PLS could be used to predict the transition path that the sample would undergo on heating. PCA indicated seven per cent of the variation in the XRD patterns to be related to the structural factors connected with the phase transition path selection.