Prediction of phase transition and ignition sensitivity of ammonium periodate*

The ammonium perchlorate (NH₄IO₄, API) was investigated theoretically to understand the influence of crystalline structure on mechanical sensitivity. Density functional theory (DFT) calculations predicted polymorphism of API, showing a phase change from tetrahedral (API_T) to monoclinic (API_M).  During this transformation, isolated IO₄^- ions are linked to a chain structure. The monoclinic phase is predicted to be slightly less dense than the API_T, with densities of 3.01 and 3.09 g/cm³ respectively, and is very similar in energy, with a phase change enthalpy (ΔH_phase) of 4.8 kJ/mol at 300 K. The predicted optimal phase change temperature is 450 K, where ΔH_phase reaches the minimum of 4.6 kJ/mol. The API_M is found to have many properties correlated with sensitivity, including low resistance to shear, with Young’s modulus of 16.27 GPa against 25.99 GPa of API_T, and a smaller electronic band gap, 3.51 eV against 4.46 eV of API_T. Therefore, the polymorphism induced by shear can be a mechanism of ignition of API.