HIGH PRESSURE STUDY ON QUASI-ONE-DIMENSIONAL COMPOUNDS M//2Mo//6X//6.

The M//2Mo//6X//6 compounds, with M being a nontransition metal element and X a chalcogen atom, crystallize in a hexagonal lattice consisting of linear chains of stacks of Mo//6X//6-clusters formed by a Mo-triangle surrounded by chalcogen-triangles. We have investigated resistively the M//2Mo//6X//6 polycrystalline compounds under pressures up to 160 kbar down to 1 K. The broad M-I transition and the activation energy during the transition are suppressed monotonically by pressure. Superconductivity has been induced in K//2Mo//6Se//6, K//2Mo//6Te//6, Na//2Mo//6Se//6, Na//2Mo//6Te//6 and Rb//2Mo//6Se//6 by pressure with a transition temperature T//c between 1 and 4K. A large resistance-increase prior to the superconducting transition is always observed. While the critical pressure for the induction of superconductivity depends on M, the T//c-behavior under pressure is affected only by X.