Selfdiffusion studies in sodium iodide single crystals by NMR relaxation. (Ionic conduction)

Abstract

Measurements of the nuclear magnetic resonance relaxation times T1, T1p, and T2 made on pure and Cd2+ doped single crystals of NaI, over a temperature range 385 to 900 K, are reported. The process responsible for relaxation is Na+ selfdiffusion. The cation migration enthalpy is found to be 0.45+or-0.02 eV, and the activation energy for the intrinsic diffusion process is found to be 1.6+or-0.05 eV, in good agreement with ionic conductivity and tracer selfdiffusion studies. An expression for T1p due to dipolar coupling between unlike spins is obtained, and is used to relate the T1p measurements to the sodium ion jump frequency. A comparison of the sodium ion jump frequency obtained from the relaxation time measurements, and from ionic conductivity measurements is made. This indicates that at high temperatures diffusion takes place predominantly via cation single vacancies. At lower temperatures impurity-vacancy complexes contribute to the relaxation indicating that the free and complexed vacancies have similar diffusion rates. At the lowest temperatures impurity precipitation takes place and the total concentration of Cd2+ as a function of temperature is calculated from the T1p measurements, and is in good agreement with that calculated from conductivity studies.