Single-crystal conductivity study of the tin dichalcogenides SnS2 –xSex intercalated with cobaltocene

Abstract

Single crystals of the n-type semiconducting tin dichalcogenides SnS_{2 –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 =η⁵-C₅H₅) to give the series of compounds SnS_{2 –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 × 10⁸ 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.