Topochemistry of Ammonium Perchlorate: How Surface Morphology Mediates Its Sublimation.

Understanding surface-defect reactivity in ammonium perchlorate (AP) is essential for describing its reactivity. Using density functional theory, we compare sublimation pathways from pristine AP(001), and use density-functional tight binding for step edges, kinks, corners, and screw dislocations. Several proton-transfer pathways on flat AP(001) are close in energy, reflecting the complexity of the molecular-ionic crystal. Free-energy calculations show that kink defects lower the proton-transfer barrier by nearly a factor of 5 relative to the flat surface, with NH3 having lower activation energy than HClO4, and migrating to terraces. This indicates the possibility of intermediate stages that may limit the sublimation rate. A one-component kinetic model shows that kink density controls the regime: low kink density gives detachment-limited rates, whereas high kink density gives concentration-independent rates controlled by detachment and terrace desorption. Comparison with experiments highlights the role of surface morphology in AP reactivity.