Single crystals of chevrel-type compounds: Growth, stoichiometry and electrical resistivity

A series of rhombohedral Mo chalcogenides (Chevrel compounds) based on S or Se have been melted under argon pressures up to 100 atm. We have found that single crystals of an appreciable size (1 to 3 cm 3) can be grown by the Bridgman-Stockbarger technique. Density measurements on these crystals, together with x-ray and microscopic observations lead to the conclusion that the formula M xMo 6X 8 describes correctly the great majority of these compounds. The sulfides with M = Pb, Sn as possible exceptions seem to be stabilized by a small excess of Mo relative to the stoichiometric ratio Mo:S=6:8. No evidence for the presence of S defects was found. The electrical resistivity of PbMo 6.2S 8 has been found to vary linearly as a function of temperature in the range T c ≤ T ≤ 50 K, the lowest measured value at the normal state being ϱ = 90 μΩ cm .