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

Metallic lithium particles precipitated in lithium hydride by UV irradiatio n are investigated by electron spin resonance (ESR), electron-nuclear doubl e resonance (ENDOR), and ESR at low temperatures under conditions of bistab le Overhauser effect [Phys. Rev. B 47, 15 023 (1993)]. Although the convent ional ESR spectroscopy gives only a single structureless line for conductio n electrons, ENDOR clearly shows the existence of two well-defined populati ons of lithium particles with different crystallographic structures. The pr esence of a quadrupole structure in the ENDOR spectrum indicates that one o f the two populations belongs to the 9R close-packed phase of metallic lith ium, normally stable below 80 K. The other population belongs to a cubic ph ase. 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 d ifferent populations of lithium particles. The presence of a broad hysteres is of the resonance at 4 K is the consequence of a very long T-2, which amo unts to 5 X 10(-6) s and 10(-6) s in the cubic and the noncubic phases, res pectively. These values are much larger than those deduced from the linewid th of the unsaturated ESR line. It is concluded that ENDOR combined with bi stable ESR could provide very selective methods for the investigation of th e lithium anode in lithium batteries.