51V NMR Studies of Crystalline Divalent Metal Vanadates and Divanadates

Abstract 51V static and magic-angle spinning (MAS) NMR spectra of crystalline divalent metal vanadates, M3(VO4)2 (M = Mg, Sr, Ba, Zn, Pb) and Mg2M′(VO4)2 (M′ = Sr, Ba, Pb) and divanadates, M2V2O7 (M = Mg, Ca, Sr, Ba, Zn, Pb) have been measured. In vanadates and dichromate-type divanadates, the 51V isotropic chemical shifts decrease linearly with decreasing electronegativity of divalent metal atom adjacent to the VO4-tetrahedron. This was interpreted by considering that the electron density and the symmetry of VO4-tetrahedron increases with decreasing electronegativity of divalent metal atom. This effect was more significant in vanadates than in divanadates. In divanadates the isotropic chemical shifts of the thortveitite-type configuration appeared at lower frequencies than that of the dichromate-type configuration. This was explained by assuming that the degree of s-hybridization of the oxygen orbitals in the V–O–V bonds is higher in the thortveitite-type than in the dichromate-type.