NMR study of the electric field gradient in the paramagnetic phase of M3V2O8 (M = Co,Ni) compounds

The NMR spectra and the decay of a spin echo signal from ⁵¹V nuclei in Kagome-staircase Co₃V₂O₈ (CVO) and Ni₃V₂O₈ (NVO) single crystals are measured in the temperature range 30–300 K and a magnetic field H ₀ = 20 kOe. The orientation dependences of the ⁵¹V NMR line shape are used to determine the electric field gradient (EFG) parameters, namely, quadrupole frequency ν _Q and asymmetry parameter η. These parameters for NVO and CVO are ν _Q = 180(10) kHz, η = 0.5(1) and ν _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 Ni₃V₂O₈ and Co₃V₂O₈ compounds differ in the EFG axis orientation. M₃V₂O₈ crystals are found to have vanadium positions (V1, V2) with different orientations of the z axis, which specifies the direction of the principal value of EFG (V _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.