Evaluation of 27Al and 51V Electric Field Gradients and the Crystal Structure for Aluminum Orthovanadate (AlVO4) by Density Functional Theory Calculations

Three sets of crystal-structure data reported for AlVO4 from two powder-XRD studies and a density functional theory (DFT) investigation, employing the Vienna ab initio simulation package (VASP), have been examined and refined using the DFT structure-optimization scheme implemented in the WIEN2k software. The crystal structures are evaluated on the basis of 27Al and 51V quadrupole coupling parameters recently reported for AlVO4, employing the corresponding electric-field gradient (EFG) tensor elements obtained from the DFT calculations. The DFT calculations provide a reliable assignment of the 27Al/51V resonances from three distinct Al and three V environments to the specific crystallographic sites in the asymmetric unit for AlVO4. An improved agreement between experimental quadrupole tensor elements and calculated EFG tensors is achieved after the DFT structure optimizations and consistent results are obtained using the three different structures as starting points. The improvement of the structural data is also supported by an evaluation of the Al−O and V−O bond lengths before and after DFT structure optimization. The 51V nuclear quadrupole moment, |Q(51V)| = 4.8 ± 0.1 fm2, derived from the present analysis, represents a value of higher accuracy than earlier reported Q(51V) values. The origin of the 27Al and 51V EFGs are investigated by an evaluation of the orientations of the EFG tensors in the crystal frame and by an examination of the individual contributions from the valence electrons and the surrounding lattice. The latter investigation shows that the magnitude and orientation of the tensors are largely determined by the p-p(27Al) and p-p, d-d(51V) orbital contributions to the valence electrons, while the lattice part only gives a minor contribution for both nuclei.