A first-principles study on the sublimation/decomposition of NH₄ClO₄

We have studied the kinetics and mechanism for the sublimation/decomposition of NH₄ClO₄ by first-principles calculations using generalized gradient approximation in the plane-wave density functional theory. Supercells containing 4, 8, 16 NH₄ClO₄ units were used, the predicted sublimation energy for solid NH₄ClO₄ to gaseous NH₃ and HClO₄ is around 45.0 & plusmn; 1.5 kcal/mol. The proton transfer barrier in the crystal matrix was calculated to be 31.3 kcal/mol and the HO-ClO₃ dissociation in the crystal environment needs 77.3 kcal/mol energy which is close to the values 73.1, 75.0 and 76.9 kcal/mol obtained from AP decomposition by the PCM model, the latter three cases involve 0, 1, and 2 H₂O molecules in the system, respectively. The result of statistical-theory calculation indicates that the decomposition of a H₃N& middot;& middot;HClO₄ molecular complex from the relaxed surface is the rate-controlling step. The decomposition rate constant can be presented as k_dec.= 6.528& times;10₁₂ exp (-28.75kcal/mol/RT) s^-1 which is in reasonable agreement with the experimental data. This calculation further confirms that the activation energy for the sublimation process is significantly lower than the enthalpic change and that the molecular complex of NH₃ and HClO₄ desorbs concurrently as a pair.