NMR study of the influence of iodine substitution in the Chevrel compounds Mo₆Te_8-xI_x and Mo₆Se_8-xI_x

We have obtained the ¹²⁵Te and ⁷⁷Se nuclear magnetic resonance spectra of the Chevrel type compounds Mo₆Te_8-xI_x (x=0,1,2) at room temperature and Mo₆Se_8-xI_x (x=0,0.5,1.0,1.5,2.0) over the temperature range 70-300K, in order to study the effect of I doping in these materials. The spectra consisted of the superposition of two separate anisotropic Knight shifts corresponding to the two different crystallographic sites of the Se and Te nuclei within their respective compounds. The distinction between the two lines became more blurred with the increase of the iodine concentration. Analysis of the results tends to favor the supposition that in the case of the Se compound, the iodine favors the replacement of the Se ternary sites, although some temperature dependence on the choice of site was discerned. In the case of the Te compounds, the experimental evidence showed that even for x=2, both the ternary and the non-ternery sites were occupied. An abrupt change in the NMR parameters for Mo₆Se₈ near T=100K was found.