Effect of milling of V_2O_5 on the local environment of vanadium as studied by solid-state ~(51)V NMR and complementary methods

摘要

Milling of V_2O_5 in a ball mill increased the surface area from 7 to a maximum of 32 m~2/g. Simultaneously, XRD line broadening suggested a reduction of the microcrystallites. A color change during milling indicated oxygen loss, and Raman spectroscopy provided evidence for the formation of shear structures such as V_6O_(13). The number of V~(4+) ions produced was determined by ESR spectroscopy and that of V~(3+) ions was obtained from magnetic susceptibility measurements. The local environment of the vanadium nucleus in V_2O_5 after milling in a ball mill was characterized by the combination of static (wide line) and MAS ~(51)V NMR techniques together with theoretical simulations of NMR spectra. Important information on the quadrupole and chemical shielding tensors, including the relative tensor orientation and the distribution of magnetic resonance parameters, is discussed for the vanadium nucleus in V_2O_5 after milling. Special attention was given to the formation of paramagnetic V~(3+) and V~(4+) ions and their influence on ~(51)V NMR spectra of diamagnetic V~(5+) in milled samples. It was shown that paramagnetic V~(3+) ions are responsible for the loss (up to about 70%) of the intensity in ~(51)V NMR spectra. The influence of other V~(4+) paramagnetic ions is significantly smaller.