PHYSICAL-PROPERTIES OF SEVERAL M2MO6X6 COMPOUNDS (M = GROUP IA METAL - X=SE,TE)

We report the physical properties of the pseudo one-dimensional ternary molybdenum chalcogenides M//2Mo//6X//6 (M equals K, Rb, Cs, Tl; X equals Se, Te) measured on single crystals grown at high temperature. For Group IA metals (Rb, . . , Cs), the transport properties evolve from a metallic to a semiconducting-like behavior on decreasing the temperature. Over the same temperature range, the Group IIIA compound (Tl//2Mo//6Se//6) remains metallic and even superconducts at 5. 1K. The susceptibility of these materials is weakly diamagnetic and does not reveal anomalies correlated to the onset of the broad metal-nonmetal transition, except perhaps for Cs//2Mo//6Te//6. A variation of the electronic interchain coupling is proposed to explain the difference between the Group IA and IIIA metal compounds. In the absence of evidence of three-dimensional ordering at low temperature, we suggest that the broad metal-nonmetal transition is due to a fluctuating Peierls gap that is uncorrelated from one Mo chain to the next.