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Actual critical current densities (Jc) obtained by most of the groups working on superconducting PbMo6S8 wires are about 2 × 108A/m2 at 4.2 K and 20 T. But Jc fluctuations along the wire length are still large. We present here a two-step heat treatment technique that greatly improves the Jc homogeneity and also a new and promising route for producing PbMo6S8 below 500°C.

An increase of about 50-100 kG of the upper critical field is observed in Sn1.2(1-x)EuxMo6.35S8 and Pb1-xEux- Mo6.35S8 with respect to the compounds without Eu. This increase, due to the Jaccarino-Peter compensation effect, pushes Hc2 up into the vicinity of 700 kG.

Using slowly pulsed magnetic fields up to 510 kG it is shown that some lead-rich ternary molybdenum sulphides have critical fields above 500 kG, whereas the Sn-rich compounds have critical fields of the order of 300-400 kG.

An interpretation of results on the structure of SnMo 6 S 8 is correlated with the variation of the critical temperatures of Sn x Mo 6 S 8 and M y Sn x Mo 6 S 8 (M  La, Ce, Pr, Eu, Gd, Ho, Lu, Y, In, U). The doping of Sn x MO 6 S 8 permits the determination of the optimal conditions for obtaining critical temperatures higher than 14 K.

We have studied the correlation between the lattice parameters and the superconducting critical temperature T c in PbMo 6 S 8 and the series M x -PbMo 6 S 8 and M x Pb 1− x Mo 6 S 8 ( M = Cu , Ag, Ba, Sn, Eu, La, Gd, Lu, Y, Th, U). A net correlation was found between the volume V H and T c when M is trivalent or tetravalent. We also found that the high T c modification of PbMo 6 S 8 has a maximal occupation of the lattice site centred about the origin of the rhombohedral lattice cell and that this can be obtained by insertion of trivalent and tetravalent elements.

Using a gas diffusion technique, we have produced PbMo6S8 in the form of wires. The critical temperature of these wires is strongly dependent on the reaction conditions. Optimal conditions give a Tc of 14.4 K (ΔTc ≈ 0.25 K). The lower limit for the critical current density at 4.2 K is 4.104 A cm−2 in a field of 4.2 T.

Mössbauer study on Yb170 and magnetic susceptibility measurements show that YbMo6S8 contains both Yb2+ and Yb3+ ions with nearly equal proportions. The Yb2+ ions are submitted to a very strong electric field gradient with axial symmetry and move preferentially in the plane perpendicular to the gradient axis (which is probably the hexagonal axis c). The Yb3+ magnetic moments order perpendicularly to the c axis below TN=2.75 K and their magnitude is 1.7 μB near zero Kelvin. Ytterbium impurities also appear as Yb2+ and Yb3+ ions in TmMo6S 8, where the Yb3+ g-tensor is slightly anisotropic (g ∥ = 2.32, g〉 = 3.64). Tm 169 Mössbauer measurements on TmMo6S8 show that only one Tm site is present.

A simple analysis of the critical field of ternary molybdenum chalcogenides with and without magnetic impurities is presented.

In 1971, we reported the existence of a new, large series of ternary molybdenum chalcogenides with their structural characterization of the general formula MxMo6X8 [1]. In 1972, Matthias showed that several of these new phases were superconducting with high critical temperatures [2] and a year later, two teams, Fischer [3] and Foner [4] found that many of these compounds were extremely high field superconductors. In 1973, we prepared the corresponding selenides [5] and in 1975–76, the rare earth molybdenum chalcogenides [6]. In 1979, we reported new compounds showing the curious trend to the one dimensional condensation of the octahedral cluster Mo6 [7], Since 1973, the physical properties have been intensively investigated, especially the striking superconducting properties with their high critical fields, long range magnetic order, coexistence of magnetism and superconductivity, soft lattice modes, transitions from metallic to semiconducting behavior, fast ion transport and critical currents.

The preparation and the structural properties of MxMo6X8 (X = chalcogen) compounds are reviewed. The very large facility of substitution of M, Mo, or X elements often involves a drastic change in the physical properties. The new clusters Mo9, Mo12, (Mo6/2) ∞ 1 resulting from a condensation of octahedral Mo6 clusters are exhibited.

In the process of elaboration of composite superconducting wires bases on the Chevrel phases compound SnMo 6 S 8 , a niobium barrier should be introduced between the stabilizer (Cu) and the superconductor to prevent Cu diffusion which degrades the critical properties. During subsequent heat treatments Sn diffusion in the Nb barrier arises and many Nb Sn phases are formed. We studied extensively the structure of the Nb barrier by the AC susceptibility and identified the Nb Sn resulting phases.