β-VO2 -: a V(IV) or a mixed-valence V(111)-V(V) oxide-studied by 51V MAS NMR spectroscopy

Analysis of high-field (9.4 and 14.1 T) V-51 MAS NMR spectra for the low-temperature non-metallic beta-form of VO2 unambiguously shows the presence of only one V(IV) site for this semiconductor phase. Magnitudes and relative orientation of the V-51 quadrupole coupling and chemical shift tensor have been precisely determined. Variable-temperature V-51 MAS NMR studies (25-90 degreesC) show that the phase transition from non-metallic (beta) to metallic (alpha) VO2 is accompanied by a large change in Knight shift from 2115 to -4788 ppm. Approximate values for the V-51 quadrupole coupling and shift anisotropy are reported for the V(IV) site in alpha-VO2. (C) 2002 Published by Elsevier Science B.V.