Chevrel phases: Past, present and future.
Résumé
The ternary molybdenum chalcogenides MxMo6X8 (X = chalcogen), known as Chevrel phases, constitute an outstanding family of materials presenting numerous and spectacular properties. More than 100 examples of these compds. have been synthesized thanks to their versatile crystal structure. Numerous variants are found, from the binary material formed just by the molybdenum clusters Mo6X8 leaving a three-dimensional lattice where the third element M can be inserted, up to a condensation of clusters giving rise to a monodimensional material. The great interest in these compds., discovered more than 40 years ago, came from their superconducting crit. temp. and upper crit. fields (15 K for the former, 50 T at 4.2 K for the latter), both being reasonably high values at the time of their discovery thus opening enormous hopes for their use in the fabrication of magnets. Other fundamental features are found, such as the coexistence of magnetic order with the superconducting state. These features are still of interest for the scientific community, but other potential applications are now foreseen, such as their use in batteries, catalysis and thermopower technol.We recall herein some basic characteristic of Chevrel-phases, mentioning several important families, their crystal structure and mode of elaboration. This contribution being focused on the superconducting properties, we put an accent on some fundamental aspects, such as the structural and electronic transitions, the vortex lattice, their granular behavior, crit. current densities, upper field and anisotropy, to finally discuss the so-called unconventional supercond., classifying these materials among the "exotic superconductors" and making a parallel with other superconductors which, in spite of their quite different electronic and crystal structures, present similar features.Chevrel phases have a long and incredible past as outstanding materials for basic and applied research but, in addn. to that, they have a bright future ahead because of a large range of potentialities.