Cationic Self‐Diffusion in the Highest‐Temperature Solid Phases of Ammonium Chloride and Bromide Studied by 1H NMR

Ionic motions in the highest‐temperature solid phases (Phase I) of NH4Cl and NH4Br having the NaCl‐type cubic structure were investigated by measuring 1H NMR spin‐lattice and spin‐spin relaxation times (T1, T1ϱ and T2). A long T2 of ca. 1 ms observed at 750 K in Phase I of both salts was attributed to the excitation of the rapid cationic self‐diffusion and isotropic rotation suggesting the plastic nature of this phase. The observed relaxation times could be explained well using the monovacancy‐diffusion model proposed by Sholl. The activation energy of self‐diffusion was determined as 120 kJ mol−1 for both salts, which value is much larger than those observed in ionic‐plastic phases with the CsCl‐type cubic structure. The diffusion mechanisms of the NH4+ ions in Phase I, and Phase II having the CsCl‐type structure, were compared.