7Li and 6Li magic angle spinning NMR studies of two types of metallic Li colloids formed by electron irradiation in LiF

The 6Li and 7Li magic angle spinning NMR spectra of 2.5-MeV electron-irradiated LiF crystals have been measured in a field of 9.4 T. Two samples which had received doses of 1.17 C/cm2 [low dose (LD)] and 2.33 C/cm2 [high dose (HD)] were studied. Besides the resonance line of the ionic compound, a second well-separated spectrum is observed which lies in the region of the Knight shift value for metallic lithium. At room temperature, the latter can be decomposed into two components with different Knight shift and linewidth values. For the LD sample, they are located at 271.7 ppm and 264.3 ppm with 1600 and 610 Hz widths, respectively. In the HD specimen, the positions and linewidths are 267.2 and 264.3 ppm and 2580 and 470 Hz, with a much larger contribution of the broad component to the total intensity. When the temperature is increased line narrowing takes place at first, indicating a shortening of correlation times for self-diffusion, practically independently in both components. Above roughly 373 K, both lines broaden and approach each other before collapsing into a single line caused by exchange between the two sets. We attribute the higher-ppm component to metallic lithium under higher than atmospheric pressure, while the other corresponds to Li under normal conditions. The thermal evolution of the spectra indicates that annealing processes take place during heat treatment, with partial transformation of the 'high-pressure' lithium into normal metal. The former disappears completely after crossing the melting temperature of metallic lithium (454 K), due to stress relaxation. Simultaneously, the Knight-shifted spectrum moves towards lower ppm values, reducing the separation from the central ionic line by nearly 4 ppm at the Li melting point