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

lWhen irradiated with energetic electrons near room temperature, Li2O singl e crystals exhibit metallic Li colloids, as shown by conduction electron sp in resonance (CESR) and Li-7-nuclear magnetic resonance (NMR) experiments. These colloids are established as being of two types represented by two sup erimposed signals in a CESR spectrum: a Lorentzian line corresponding to sm all colloids (much less than 1 mu m in diameter); and a Dysonian to much bi gger (> 1 mu m) aggregates. We present isochronal annealing experiments and CESR experiments above room temperature in the range 290-590 K, in which m elting of the Li metal is observed. When investigating such colloid-contain ing 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 resul t either of the presence of two types of colloids, or of one type having tw o different orientations inside the crystal. In the latter case the colloid s should exhibit anisotropic properties as was already suggested by dielect ric constant measurements. Thin Li2O crystals investigated by transmission optical microscopy after irradiation contained disc-like structures of up t o 30 mu m in diameter, which possibly represent oxygen gas bubbles surround ed 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.