NMR interaction tensors of V-51 and Pb-207 in vanadinite, Pb-5(VO4)(3)Cl, determined from DFT calculations and single-crystal NMR measurements, using only one general rotation axis

Orientation-dependent NMR spectra of a single crystal of the mineral vanadinite, Pb-5(VO4)(3)Cl, were acquired using only one rotation axis with a general orientation in the hexagonal crystal lattice (space group P6(3)/m). The chemical shift (CS) tensors for the Pb-207 on Wyckoff positions 6h and 4f, and both CS and quadrupole coupling tensor Q for V-51 at the positions 6h were determined by including the NMR response of symmetry-related atoms in the unit cell (and in case of Pb-207 at 4f, also the isotropic shift from MAS NMR spectra). This previously suggested 'single rotation method' greatly reduces the necessary amount of data acquisition and analysis. The precise orientation of the rotation axis could not be found by X-ray diffraction experiments because of the high linear absorption coefficient of vanadinite, which is chiefly due to its high lead content. The axis orientation was therefore included into the multi-parameter data fit routine. This NMR-based approach is widely applicable, and offers an alternative way of orienting single crystals. The NMR parameters derived from the tensor eigenvalues are delta(iso) = (-1729 +/- 9) ppm, Delta delta = (-1071 +/- 5) ppm, eta(CS) = 0.362 +/- 0.008 for Pb-207 at positions 6h, and delta(iso) = (-1619 +/- 2) ppm, Delta delta = (-780 +/- 58) ppm, eta(CS) = 0.06 +/- 0.08 for positions 4f. For V-51, delta(iso) = (-509 +/- 3) ppm, Delta delta = (-37 +/- 2) ppm, eta(CS) = 0.78 +/- 0.09, with the quadrupolar coupling described by chi = (2.52 +/- 0.01) MHz and eta(Q) = 0.047 +/- 0.003. In contrast to the precisely determined tensor eigenvalues, the orientation of the eigen-vectors in the crystal ab -plane of the vanadinite system could only be resolved by resorting to data obtained from density functional theory (DFT) calculations.