Magnetic resonance of micrometer size Li-metal colloids in electron-irradiated Li2O crystals

lWhen irradiated with energetic electrons near room temperature, Li2O single crystals exhibit metallic Li colloids, as shown by conduction electron spin resonance (CESR) and Li-7-nuclear magnetic resonance (NMR) experiments. These colloids are established as being of two types represented by two superimposed signals in a CESR spectrum: a Lorentzian line corresponding to small colloids (much less than 1 mu m in diameter); and a Dysonian to much bigger (> 1 mu m) aggregates. We present isochronal annealing experiments and CESR experiments above room temperature in the range 290-590 K, in which melting of the Li metal is observed. When investigating such colloid-containing crystals by Li-7-NMR we observe a Knight-shifted signal to the metallic colloids, which consists of two components. This duality is due to a result either of the presence of two types of colloids, or of one type having two different orientations inside the crystal. In the latter case the colloids should exhibit anisotropic properties as was already suggested by dielectric constant measurements. Thin Li2O crystals investigated by transmission optical microscopy after irradiation contained disc-like structures of up to 30 mu m in diameter, which possibly represent oxygen gas bubbles surrounded by metallic Li corresponding to the large colloids detected by magnetic resonance: this confirms the strong anisotropy of the metallic colloids. (C) 1999 Elsevier Science B.V. All rights reserved.