51V MAS NMR Investigation of 51V Quadrupole Coupling and Chemical Shift Anisotropy in Divalent Metal Pyrovanadates

Magnitudes and relative orientations of 51 V quadrupole coupling and chemical shift tensors have been determined from 51 V magic-angle spinning (MAS) NMR spectra at 14.1 T for seven divalent metal pyrovanadates: α- and β-Mg 2 V 2 O 7, Ca 2 V 2 O 7, α-Zn 2 V 2 O 7, Cd 2 V 2 O 7, BaCaV 2 O 7, and α-BaZnV 2 O 7 . This has been accomplished by least-squares fitting of the integrated spinning sideband intensities observed for the central and satellite transitions employing spectral widths up to 4 MHz. Numerical error analysis of the optimized data reveals that the five NMR parameters characterizing the magnitudes of the quadrupole coupling and chemical shift tensors are obtained with high precision while somewhat larger error limits are observed for the three Euler angles, describing the relative orientation of the two tensors. The optimized data exhibit a significantly higher precision when compared to earlier reported parameters for some of the pyrovanadates, determined from 51 V static-powder or MAS NMR of the central transition only. The 51 V chemical shift parameters indicate that the different conformations for the V 2 O 7 4- ion in thortveitite-type and dichromate-type pyrovanadates can be distinguished by the sign for the chemical shift anisotropy (δ σ = δ iso − δ zz ), negative and positive, respectively. A linear correlation is observed between the principal elements for the 51 V quadrupole coupling tensors and calculated electric field gradient tensor elements, obtained from point−monopole calculations. This correlation is used to assign the NMR parameters for the two different crystallographic 51 V sites in the asymmetric units for α- and β-Mg 2 V 2 O 7, Ca 2 V 2 O 7, BaCaV 2 O 7, and α-BaZnV 2 O 7 .