Phase Transition and Ignition Sensitivity of Ammonium Periodate Through Density Functional Theory Calculations

Ammonium perchlorate (NH₄ClO₄, APC) is a major oxidizer for civil and military applications, with efficiency and safety combustion performance. A similar material, ammonium periodate (NH₄IO₄, API), is extremely sensitive to friction and impact, which makes it dangerous for any application. Investigations of these materials at the atomic level provide explanations of how such similar materials can have so opposite behaviors. Density functional theory (DFT) calculations revealed small differences that imply important in consequences. The first difference is in the elastic properties, where API has a less uniform strain behavior. Although API is more rigid, with the bulk modulus (K) 25.9 GPa against 21.4 GPa for APC, the shear modulus (G) of API and APC are similar, 9.8 and 9.4 GPa respectively, implying higher Pugh’s ratio (K/G) for API, 2.65 against 2.27 for APC. Great deformations of the lattice structure are correlated with sensitivity. The most significant differences, however, are found in the electronic structure: APC is an insulator with a band gap (bg) of 6.2 eV while API is a semiconductor, with bg of only 2.9 eV. Electronic excitation is the beginning of almost any chemical reaction, and a small bg is traditionally related to the sensitivity of energetic materials. Finally, the DFT calculations indicated a phase transition in the crystalline structure of API, changing from a molecular crystal to a chained structure, with the anions (IO4^-) being connected through an oxygen bridge (API_chain). This phase has an even smaller bg, only 2.09 eV, a highly reactive characteristic. The phase transition under mechanical input may trigger the decomposition.