Synthesis and superconductivity of molybdenum cluster compounds (Chevrel phase)

The discovery of superconductivity in ternary molybdenum sulfides (Chevrel phases) in 1972 has stimulated research on these compounds. Some of the phases show extremely high critical fields Hc2 and might therefore find technical application as high field superconductors. In order to understand the electron-phonon-interaction in these substances, measurements of the superconducting isotope effect in 92-100Mo6Se8, Mo676-82Se8, and 116-124SnMo6S8 have been performed. The corresponding isotope effect exponents β (βmo=0.27 +- 0.04, βSe=0.27 +- 0.05 and βSn <=0.05) have been calculated by fitting the experimental data to the relation TcMsup(β)=const. We have shown that phonon modes connected with Se-displacements as well as modes connected with Mo-displacements couple to the conduction electron system in Mo6Se8. In case of the ternary Chevrel phase SnMo6S8, phonon modes connected with displacements of the Sn-ions have only minor influence on the transition temperature. This result can be explained by the weak overlap of the molybdenum dsub(x)2sub(-y)2 - orbitals with Sn-sites. Furthermore, we report experiments on the synthesis of new Chevrel phase materials. In order to optimize the valence electron concentration in some ternary molybdenum selenide compounds, chalcogen exchange reactions have been performed. A new Chevrel phase superconductor, Cusub(x)Mo6S6J2 with x=0 - 1.2, has been synthesized by copper diffusion into the non occupied channels running between the Mo6S6J2-'molecules' of Mo6S6J2. (orig.)