Anomalous metallic lithium phases: Identification by ESR, ENDOR, and the bistable Overhauser effect

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Metallic lithium particles precipitated in lithium hydride by UV irradiation are investigated by electron spin resonance (ESR), electron-nuclear double resonance (ENDOR), and ESR at low temperatures under conditions of bistable Overhauser effect [Phys. Rev. B 47, 15 023 (1993)]. Although the conventional ESR spectroscopy gives only a single structureless line for conduction electrons, ENDOR clearly shows the existence of two well-defined populations of lithium particles with different crystallographic structures. The presence of a quadrupole structure in the ENDOR spectrum indicates that one of the two populations belongs to the $9R$ close-packed phase of metallic lithium, normally stable below 80 K. The other population belongs to a cubic phase. At low temperatures the saturation of the ESR line produces a bistable Overhauser effect which gives a hysteresis of the ESR trace. This bistable conduction electron spin resonance clearly confirms the existence of two different populations of lithium particles. The presence of a broad hysteresis of the resonance at 4 K is the consequence of a very long T₂, which amounts to 5×10^-6s and 10^-6s in the cubic and the noncubic phases, respectively. These values are much larger than those deduced from the linewidth of the unsaturated ESR line. It is concluded that ENDOR combined with bistable ESR could provide very selective methods for the investigation of the lithium anode in lithium batteries.