Hyperfine Interactions in Dichalcogenides Cr_xNbSe₂ (x = 0.33, 0.5): A ⁹³Nb NMR Study

For the first time, an ⁹³Nb NMR study of dichalcogenides Cr_xNbSe₂ (x = 0.33, 0.5) in the paramagnetic state was performed. Analysis of the ⁹³Nb NMR spectra revealed the presence in Cr_xNbSe₂ of three magnetically nonequivalent niobium positions, whose immediate environment contains 0, 1, and 2 chromium ions, respectively. For each Nb position with a different number of chromium atoms in the immediate environment in Cr_xNbSe₂ (x = 0.33, 0.5), the values of the components of the magnetic shift and electric field gradient tensors at the position of the niobium nuclei were determined. Evidence was obtained of the formation in Cr_0.33NbSe₂ of the ordering of chromium ion positions in the ab plane into a \(\sqrt 3 \) a₀ × \(\sqrt 3 \) a₀ superstructure. On the other hand, in Cr_0.5NbSe₂, no obvious indications of the formation of any superstructure of chromium ion positions were found. It has been established that the overlap of the 4d and 5s shells of niobium ions and the 3d orbitals of chromium leads to the appearance of a positive hyperfine field induced by the magnetic moments of chromium on Nb nuclei. From the temperature dependences of the shift and susceptibility in Cr_0.5NbSe₂, an estimate of these induced hyperfine fields is made.