Chevrel-phase solid solution Mo6Se8-xTex. Study of its superconducting, magnetic and NMR properties

The Chevrel-phase solid solution Mo6Se8-Mo6Te8 was studied by X-ray diffrac tion, AC and DC magnetic susceptibility and Se-77 and Te-125 NMR spectrosco py. From the smooth, evolution of the lattice parameters and superconductin g critical temperatures, a progressive substitution of selenium atoms by te llurium is shown, on the whole range of composition 0 less than or equal to x less than or equal to 8, in the formulation Mo6Se8-xTex: the unit-cell v olume increases linearly because of the larger ionic size of tellurium, whi le T-c decreases rapidly (from 6.45 down to 0 Id) because of the different formal oxidation states of the anions and a probable evolution of the Fermi level in the density of states. Results of magnetic susceptibility support this model and suggest the inhibition of the intrinsic metallic behavior w ith increasing x. The NMR spectra of the binaries Mo6Se8 and Mo6Te8 reveal two significant features, attributed to two different chalcogen positions i n the R (3) over bar symmetry. At low Se contents in Mo6Se8-xTex (x = 7.5, 7 and 6), selenium first fills the two X(2) sites along the three-fold axis (2c positions), and then it becomes statistically distributed over the gen eral bf positions, leading to broad Se-77 NMR lines. On the other hand, sub stitution of Te atoms in Mo6Se8 seems to occur in a random way, creating la rge perturbations on the Te-125 NMR spectra, over the whole range of x. The oretical analysis based on the presence of two anisotropic lines (of axial and non-axial symmetries, respectively) allowed us to estimate their anisot ropy factors and to perfectly simulate the frequency response of both Mo6Se 8 and Mo6Te8 binaries. Analysis of the Knight shift anisotropy leads us to conclude about the importance of the molybdenum z(2) molecular orbital cont ribution which controls the Mo-X dipolar interactions. (C) 2000 Elsevier Sc ience B.V. All rights reserved.