Cation Diffusion Study in NH4NO3 by Proton Spin Relaxation

Measurements of the proton spin—lattice relaxation times (T₁, T_1ρ) have been made to study the translational and reorientational motions of the ammonium ions in ammonium nitrate in several solid phases. At the plastic‐to‐brittle phase transition at 398°K, the correlation time for translational diffusion of the ${\mathrm{NH}}_4^{+}$ ion increases by more than three orders of magnitude: the increase is from ${\text{2 × 10}}^{\text{−14}}\exp \text{(12.1\hspace{0.17em}}{\mathrm{kcal\; mole}}^{\text{−1}}\mathrm{/\; RT)}$ to ${\text{2 × 10}}^{\text{−16}}\exp \text{(24.3\hspace{0.17em}}{\mathrm{kcal\; mole}}^{\text{−1}}\mathrm{/RT)}$ sec. The self‐diffusion constants for ${\mathrm{NH}}_4^{+}$ in phases II and I obtained from these correlation times are in close agreement with the diffusion constants reported from electrical conductivity measurements in phases II and I.