NMR study of the electric field gradient in the paramagnetic phase of M_3V_2O_8 (M = Co, Ni) compounds

摘要

The NMR spectra and the decay of a spin echo signal from 51V nuclei in Kagome-staircase Co_3V_2O_8 (CVO) and Ni_3V_2O_8 (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 ~(51)V 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 Ni _3V_2O_8 and Co_3V_2O _8 compounds differ in the EFG axis orientation. M3V 2O8 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.