Electron spin resonance study of metallic lithium–ammonia systems

Abstract

Electron spin resonance (ESR) in metallic lithium−ammonia systems has been investigated in the temperature range 12−296°K. In the liquid solutions and in the cubic phase of Li(NH3)4 the conduction−electron−spin resonance (CESR) line shapes are in excellent agreement with Dyson’s theory. The g shift, which is independent of concentration and temperature, is equal to 7.9±0.3×10−4. The spin−orbit coupling constant derived from the g shift is about 10 cm−1. The diffusion time TD is nearly independent of temperature in the liquid state, but it is strongly dependent on concentration and undergoes a fourfold decrease at the freezing point of Li(NH3)4. The spin−lattice relaxation time T1 is weakly dependent on concentration and temperature in the liquid state and undergoes a twofold increase at the freezing point of Li(NH3)4. The data are consistent with the spin−orbit relaxation mechanism and indicate that to a good approximation the solvated ions are the only spin scatterers in the liquid state. The paramagnetic susceptibility of liquid Li(NH3)4 is independent of temperature and equal to 77×10−6 emu/mole. For metallic concentrations below 18 MPM the cubic phase is found below the solid−solid transition temperature, and the relative amount of the cubic phase present increases with decreasing lithium concentration. An ESR signal is also observed in the hexagonal phase of Li(NH3)4. Paramagnetic susceptibility measurements indicate that the hexagonal−phase ESR signal is due to localized moments which appear near the solid−solid transition temperature. The reciprocal paramagnetic susceptibility varies linearly with temperature down to 30°K, shows a broad minimum between 20 and 30°K, and increases below 20°K. No resonance signal could be detected below 12°K. The paramagnetic susceptibility of hexagonal−phase Li(NH3)4 at 54°K is about 1.5×10−6 emu/mole. The results indicate that the concentration of localized moments is small and they order antiferromagnetically below 20°K. The Fermi surface in the hexagonal phase is re−examined on the basis of the spin−orbit coupling constant determined from g shift measurements.