Self-Diffusion in Solid Ammonia

Refined measurements of the rotating frame relaxation time (T_1p) in solid ammonia from 166°K to the melting point (196°K) are presented and interpreted as due to vacancy diffusion. The data are represented by a self‐diffusion coefficient $\mathrm{D}{\text{=(2.2 × 10}}^{\text{−1}}\mathrm{)\hspace{0.17em}}\exp \text{[−(9.4 ± 0.6)/RT]\hspace{0.17em}}{\mathrm{cm}}^2\hspace{0.17em}{\sec }^{\text{−1}}.$ In the calculation of the jump times from the ${\mathrm{T}}_{1_{\rho }}$ data both the intermolecular and intramolecular dipolar interactions are taken into account. It is proposed that similar $T_{1_{\rho }}$ data measured by Van Steenwinkel in benzene, cyclohexane, and hexamethylbenzene are due to vacancy diffusion and not to molecular rotations at a lattice site which are inconsistent with the crystal symmetry of the solid. The observed pre‐exponential factor of D is compared with a theory of self‐diffusion based on a Maxwell‐Boltzmann distribution of molecular velocities in the solid. Entropies of migration and vacancy formation are evaluated for a hypothetical fee form of ammonia. The calculated pre‐exponential factor (0.14) is in good agreement with the observed value (0.22). Lennard‐Jones parameters for ammonia are derived by comparison of the model system with experimental data.