The NMR spin-lattice relaxation time, TI, has been measured as a function of temperature for both 7Li and 27Al in pure and doped β-LiAl alloys. Compositions with 7Li concentration in the range 48.3-54.5% and doping in the form Li50Al50-xMx, where M = Ag or In, were studied. The relaxation rates T1-1 for the 27Li and the 27Al resonances were found to be peaked functions of temperature with the maxima for 7Li appearing at composition dependent temperatures. The 27Al maxima always appeared at a lower temperature, independent of composition, and the 27Al maximum relaxation rate was a strong function of composition in contrast with 7Li where the maximum rate was only weakly dependent on composition. The principle relaxation mechanisms are identified as dipole-dipole coupling in the 7Li and coupling of the 27Al quadrupole moment to electric field gradients. The temperature dependence of these rates is attributed to the thermally activated diffusion of vacancies of a non-thermal origin in the Li sub-lattice. These vacancies are also responsible for the fluctuating electric field gradients. The results have been analyzed to give the Li diffusion coefficients with associated activation energies and estimates of the vacancy concentration as functions of alloy composition.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics