We studied the normal and superconducting states of the title compounds by measuring the conductivity and magnetization of single crystals and powder samples. From the upper and lower critical fields we deduced the characteristic lengths and thermodynamical fields. These results are borne out by our specific-heat measurements. We recognize in these compounds many features of the Chevrel-phase superconductors, including very small coherence lengths and strong-coupling-like effects. However, we show that the electron system is much more anisotropic and still less delocalized in these materials where the ${\mathrm{Mo}}_6$${\mathrm{Se}}_8$ clusters have condensed in ${\mathrm{Mo}}_{6\mathit{n}}$${\mathrm{Se}}_{6\mathit{n}+2\mathrm{}}$ finite chains. This condensation is accompanied by an enhancement of the magnetic response whereas the lengthening of the chains leads to a counteracting reduction of the density of carriers. This indicates that superconductivity is built upon highly correlated molecular states. Reviewing the available data on the other Chevrel-cluster-based superconductors confirms this picture and suggests that the small coherence lengths reflect the local character of the electron pairing. This comparison also shows that forming finite chains of ${\mathrm{Mo}}_6$${\mathrm{Se}}_8$ clusters makes the electron correlations more repulsive and pushes the electron system near the borderline between superconductivity and magnetism. In this respect these compounds could provide valuable complementary information on issues which are at the center of the research upon high-${\mathit{Y}}_{\mathit{c}}$ superconductivity.

• Received 21 April 1995

DOI:https://doi.org/10.1103/PhysRevB.52.4481