SINGLE-CRYSTAL CONDUCTIVITY STUDY OF THE TIN DICHALCOGENIDES SNS2-XSEX INTERCALATED WITH COBALTOCENE

Single crystals of the n-type semiconducting tin dichalcogenides SnS2-x(Se)x (x = 0, 0.3, 0.5, 1.3, 1.85 and 2), which have a two-dimensional layered structure, have previously been intercalated with cobaltocene (CoCp2, Cp = eta-5-C5H5) to give the series of compounds SnS2-x(Se)x (CoCp2)0.33. Four-contact resistivity measurements have been carried out on these host and intercalate samples. The sulphur-rich intercalates (x = 0.0, 0.3, 0.7 and 1.3) were found to be semiconducting, whereas the selenium-rich intercalates (x = 1.85 and 2.0) were found to be metallic. These findings confirm earlier observations made in a photoelectron spectroscopy study. The resistivity of the semiconducting intercalates closely follows the functional form exp[(T0/T)1/4], characteristic of variable-range hopping. The values of T0 were in the range 3.4-3.7 x 10(8) K. The metallic intercalates show a transition to a superconducting state at 6.1 K for x = 2.0 and 5.7 K for x = 1.8. In the diselenide case this has been confirmed by observation of a magnetic response appropriate to a type II superconductor below 6 K. This is the highest reported T(c) for a two-dimensional layered structure intercalated by an organometallic guest molecule.