O.J. Zogal

Li colloids created by electron-irradiation of LiF: A great wealth of properties

Abstract Like in lithium oxide, it is easy to nucleate very pure metallic lithium precipitates by electron-irradiation of lithium fluoride crystals. Irradiations performed near room temperature give metallic colloids which are characterized by electron spin resonance (ESR). The metallic character of the precipitates is demonstrated by the temperature behaviour of the ESR line intensity, which follows the temperature-independent Pauli law. A typical value for the metal concentration is about a few percent. Varying the irradiation parameters, i.e. the total fluence and/or the instantaneous flux, affects considerably the ESR properties of the colloids: first, the ESR line width changes by almost two orders of magnitude, indicating that the sizes of the precipitates strongly depend on these irradiation conditions. Second, when measuring the variation of the ESR intensity, proportional to the metal content, two very different behaviours are observed during recovery (dynamic or isochronal). When the flux is high, an intense and narrow metallic line is obtained, which disappears rapidly with annealing (300 °C). For lower flux, the colloids have a broader ESR signal, i.e. a much smaller size, and are not destroyed by annealing until the LiF crystal melts (870 °C). 7 Li NMR measurements extending earlier NMR experiments on neutron-irradiated LiF indicate that the Knight-shifted signal is split into two components with different widths. The position of the narrower component seems to correspond to big lithium particles having the features of bulk metallic lithium. The second component is likely to regroup particles of different sizes and geometry with different hyperfine interaction strength.

Dynamic hydrogen disorder in yttrium-dihydride phases as seen by variable-temperature 89Y CP MAS NMR

Variable-temperature solid-state MAS NMR studies on some yttrium-dihydride phases YH2+x are reported and yield evidence that 89Y CP MAS NMR techniques are an experimentally feasible route to investigate order-disorder phenomena in such metal-hydride phases.

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

Abstract When irradiated with energetic electrons near room temperature, Li 2 O single crystals exhibit metallic Li colloids, as shown by conduction electron spin resonance (CESR) and 7 Li-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 (≪1 μm in diameter); and a Dysonian to much bigger (>1 μm) 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 7 Li-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 Li 2 O crystals investigated by transmission optical microscopy after irradiation contained disc-like structures of up to 30 μm 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.

Low-temperature and high-temperature experiments on electron-irradiated Li2O: Molecular-oxygen freezing and metallic-lithium melting

Magnetization measurements of Li2O crystals between 1.4 and 300 K reveal a diamagnetic-to-paramagnetic transformation of the samples, containing Li-colloids after electron irradiation near room temperature, and exhibit a break near 50 K as well as an antiferromagnetic ordering transition at 38 K. The magnetism is attributed to ⩽1% of molecular oxygen gas, freezing at 50 K, which is contained in micron-size cavities and produced simultaneously with the Li colloids. The earlier 7Li NMR experiments were extended to higher temperatures, in the range 300–520 K. The Knight-shifted Li-metal signal at ∼260 ppm is weakening during heating, due to annealing, but maintains its split double structure. The separation between the two peak components diminishes by ∼10%, after crossing the Li melting point at 453 K. The interpretation of these observations is discussed.

Metallic Li colloids studied by 7 Li MAS NMR in electron-irradiated LiF

7 Li MAS NMR spectra of 2.5 v MeV electron-irradiated LiF crystals have been measured in a field of 9.4 v T. Besides the resonance line of the ionic compound, a second well-separated spectrum is observed 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. When the temperature is increased, line narrowing takes place at first, indicating shortening of correlation times for self-diffusion, independently in both components. Above 370 v K, both lines broaden and approach each other before collapsing into a single line. The high ppm component disappears after crossing the melting temperature of metallic lithium (454 v K). The two lines are attributed to different types of metallic Li: one to bulk-like metal, the other to Li present initially under pressure and relaxing to the former under thermal treatment.