Résumé
Syntheses performed in the A3B/MSe2 systems lead to the formation of (B/B')(M/A)5Se9 phases (ferromagnets or superconductors) {A=M= (Nb, V, Ta); B=B'= (Al, Ge, Ga, Sn, Sb)}. The magnetic studies on GaV5Se9,, GaV0.15Ta4.85Se9 and Sn0.94Ga0.05V0.30Nb4.70Se9, compared to that of SnNb5Se9, GaNb5Se9 show that the presence of V modifies the superconducting exchanges and allows ferromagnetic couplings. Through Point-Contact Spectroscopy on SnNb5Se9, it is shown that nanoclusters of the pristine NbSe2 and Nb3Sn exist, so the properties result from a synergy between them and the matrix, and it could be a mesoscopic superconductor. The superconductors' s Tc values vary from 17.5(2) K for SnNb5Se9 to 6.0(2) K for SbNb5Se9, and the ferromagnetic state transition from 22K for GaV5Se9, to 35 K for Sn0.94Ga0.05V0.30Nb4.70Se9. The 119Sn and 121Sb Mössbauer studies of Sn0.94Ga0.05V0.30Nb4.70Se9 and SbNb5Se9 are compared to that obtained for SnNb5Se9 itself; for this last one, 3 types of sites corresponding to 2 species of Sn were observed with a reversible dynamic process between the 2 SnII Sn-Se sites correlated to a modification of the Sn-Nb site(1). SbNb5Se9, at 5 K, shows just a singlet, attributed to Sb surrounded by Se ; from 298 K to 5 K, Sn0.94Ga0.05V0.30Nb4.70Se9, presents one asymmetrical Sn-Se site without any transferred field on Sn and an increased asymmetry when T decreases. Sn0.94Ga0.05V0.30Nb4.70Se9 is a weak and complicated ferromagnet. In both cases, GaV5Se9, and Sn0.94Ga0.05V0.30Nb4.70Se9, the ESR spectrum observed at 300 K can be fitted with 2 lorentzian distributions: for the first case, with axial symmetries, for the second only one remains with an axial symmetry, belonging to the V sites. its vanadium sites are thus less symmetrical than in the first case.