β-VO2—a V(IV) or a mixed-valence V(III)–V(V) oxide—studied by 51V MAS NMR spectroscopy

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