NMR study of the electric field gradient in the paramagnetic phase of [M.sub.3][V.sub.2][O.sub.8] compounds

The NMR spectra and the decay of a spin echo signal from [sup.51]V nuclei in Kagome-staircase [Co.sub.3][V.sub.2][O.sub.8] (CVO) and [Ni.sub.3][V.sub.2][O.sub.8] (NVO) single crystals are measured in the temperature range 30-300 K and a magnetic field [H.sub.0] = 20 kOe. The orientation dependences of the [sup.51]V NMR line shape are used to determine the electric field gradient (EFG) parameters, namely, quadrupole frequency [v.sub.Q] and asymmetry parameter r|. These parameters for NVO and CVO are [v.sub.Q] = 180(10) kHz, η = 0.5(l) and [v.sub.Q] = 130(10) kHz, η = 0.6(1), respectively. A comparison of the results of calculating EFG tensors with a point charge model and the NMR data indicates that the crystallographically equivalent vanadium atoms in the [Ni.sub.3][V.sub.2][O.sub.8] and [Co.sub.3][V.sub.2][O.sub.8] compounds differ in the EFG axis orientation. [M.sub.3][V.sub.2][O.sub.8] crystals are found to have vanadium positions ([V.sub.1], [V.sub.2]) with different orientations of the z axis, which specifies the direction of the principal value of EFG ([V.sub.zz]): these orientations lie in the bc plane and make an angle of either +51(5)° (V1) or -51(5)° (V2) with axis c. In the temperature range 30-300 K, the EFG tensor components and the local symmetry of the charge surrounding of the vanadium positions in NVO and CVO oxides are found to change insignificantly.