Rare earth based chevrel phases REMo6X8: Crystal growth, physical and superconducting properties

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.

Chevrel phases: Past, present and future

Enhancement of CdSe film electrode PEC characteristics by metalloporphyrin/polysiloxane matrices

Superconducting thin films of YBa 2 Cu 3 O 7−x have been prepared by d.c. sputtering deposition followed by a suitable thermal treatment. The deposition parameters were optimized in order to obtain ideal 1:2:3 composition of the films. A wide variety of substrates were tested, most of them giving evidence for a strong chemical interaction with the film when annealed. To better determine the contamination effects of potential substrates on the properties of YBa 2 Cu 3 O 7−x , a number of foreign elements were diluted in sintered samples for the superconducting matrix. Their influence was evaluated from a.c. susceptibility or magnetization (SQUID) measurements. Superconducting films were thus obtained, mainly on zirconia. BaZrO 3 or YSZ (yttrium stabilized zirconia) sintered plates, with the onset of superconducting transition occuring near 90 K. The width of the transition strongly depends on the crystalline state of the film, which is connected to the applied thermal treatment Les parametres de depot ont ete optimises afin d'atteindre la stoechiometrie ideale 1:2:3. Des supports de nature variee ont ete testes: la plupart d'entre eux presentent une forte interaction chimique avec la couche lors du recuit. Afin de mieux definit les effets de contamination des divers supports sur les proprietes supraconductrices de YBa 2 Cu 3 O 7−x , des elements etrangers ont ete volontairement introduits en petites quantites dans des pastilles frittees de ce materiau. Leur influence a ete etudiee par des mesures magnetiques

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R. Horyn

Papers

A very simple dc sputtering device for chemistry laboratory use