Aluminum orthovanadate (AlVO4):: Synthesis and characterization by 27Al and 51V MAS and MQMAS NMR Spectroscopy

Folycrystalline samples of AlVO4 have been prepared by two methods of synthesis and characterized by Al-27 and V-51 MAS NMR spectroscopy at 14.1 T. The MAS NMR spectra clearly reveal that essentially pure samples with minor impurities Of V2O5 and alumina have been obtained. From these samples Al-21 quadrupole coupling parameters and isotropic chemical shifts as well as the magnitudes and relative orientations of the V-51 quadrupole coupling and chemical shift tensors have been determined with high precision for AlVO4. These data have been obtained from a combined analysis of multiple-quantum (MQ) MAS NMR spectra and MAS NMR spectra of the central and satellite transitions. The Al-27 and V-51 NMR data show that the asymmetric unit for AlVO4 contains three isolated VO4 tetrahedra, one pentacoordinated Al site, and two AlO6 octahedra. This is in agreement with the supposition that AlVO4 is isostructural with FeVO4 and with a recent structure refinement for AlVO4 based on powder X-ray diffraction (XRD) data. The favorable agreement between the refined crystal structure from powder XRD and the NMR parameters is apparent from a convincing correlation between experimental V-51 quadrupole tensor elements and calculated V-51 electric field gradient tensor elements obtained by the point-monopole approach. An assignment of the Al-27 NMR data is obtained from similar calculations of the Al-27 electric field gradients and by estimation of the distortion of the AlO6 octahedra.