X-ray absorption and magnetic resonance spectroscopic studies of LixV6O13

Polycrystalline LixV6O13 samples, 0.5 less than or equal to x less than or equal to 6, were prepared by chemical intercalation in n-butyl lithium and investigated spectroscopically by x-ray absorption, electron paramagnetic resonance (EPR), and Li-7 solid state nuclear magnetic resonance (NMR). Both the EPR results and the vanadium K-edge x-ray absorption fine structure spectra show that the average oxidation state of the vanadium decreases with the addition of Li, and the x-ray results provide evidence of lithium deficient and oxygen deficient impurity phases. The local symmetry of the vanadium atoms first decreases with increasing x from 0 less than or equal to x less than or equal to 1 and then increases with increasing x as the vanadium octahedral environment becomes less distorted. These changes are revealed by both the intensity of the first V-O peak in the radial distribution function and by the decrease in the x-ray absorption pre-edge peak intensity. However, structural correlations beyond the nearest neighbor atoms rapidly decrease with increasing Li content above x=1.5, reflecting increased disorder. The observed increase in the V-O distance implies a modest lattice expansion with intercalated Li, from 1.93 Angstrom at x=0 (in agreement with x-ray diffraction) to 2.11 Angstrom at x=5. Variable temperature Li-7 NMR linewidth and spin-lattice relaxation measurements demonstrate that dynamic processes govern the spin-lattice relaxation when 0.5 less than or equal to x less than or equal to 2, but paramagnetic and homonuclear dipolar interactions reduce the spin-lattice relaxation time as x is increased beyond 2. The Li-7 NMR lineshape confirms the presence of impurity phases. Paramagnetic and diamagnetic chemical shifts imply local magnetic ordering accompanying the structural changes on lithium intercalation. (C) 1998 American Institute of Physics. [S0021-8979(98)07103-5].