V-51 MAGIC-ANGLE-SPINNING NMR AND ELECTRIC-FIELD GRADIENT CALCULATIONS IN V2O5 AND GAMMA-LIV2O5 CRYSTALS

V-51 Magic-angle-spinning (MAS) NMR has been applied to V2O5 at two di fferent magnetic field strengths (4.7 and 7.1 T), Both the magnitude a nd relative orientation of the quadrupole and chemical shift (CS) tens ors have been determined by iterative fitting of the V-51 MAS NMR line shapes at the two magnetic field strengths, The reliability of the res ults is discussed, Moreover, it is shown that previous low-field singl e-crystal data are fully consistent with the high-field powder-sample MAS NMR results provided that a slight noncoincidence between the CS t ensor and the crystal frame axes is considered. The electric field gra dient tensor at the vanadium and lithium sites is subsequently used to test several electronic structure calculation at an ab initio Hartree -Fock level in V2O5 and gamma-LiV2O5 crystals. It is shown that a wide distribution of oxygen charges must be considered to describe the par ticular environment of each type of oxygen atoms, Furthermore, this an alysis supports the fact that the vanadyl bond is likely a short ionic bond. NMR is found to be a valuable experimental tool to get insight into the nature of chemical bonds in vanadium oxides, (C) 1997 Elsevie r Science B,V.