Stoichiometry and physical properties of ternary molybdenum chalcogenides MxMo6X8 (X = S,Se; M = Li,Sn, Pb)

Chemical and electrochemical insertion of Li at room temperature, as well as insertion of lead and tin at moderate temperatures (500°C), into the binary phase Mo₆X₈ forms ternary molybdenum chalcogenides M_xMo₆X₈ (X = S, Se). Crystallographic parameters, superconducting properties, and magnetic susceptibility are reported. The stoichiometry x for lead and tin is shown not to exceed x = 1, while for Li, x can reach approximately 4.0. For the lead and tin sulfide series, the hexagonal lattice parameters and superconducting critical temperatures (T_c) are invariant to changes in the nominal composition of 0.8 < x < 1.2, while both an increase in T_c and a small decrease in c_h is observed for the selenides; a narrow homogeneity range exists near x = 1 below 500°C for both these sulfides and selenides, the single-phase region being somewhat larger in the selenides. In contrast, several single-phase regions and large unit cell changes are observed in Li_xMo₆X₈ (0 < x < 3.2). Magnetic susceptibility measurements of the lithiated compounds at x ～ 3.2 reveals a structural phase transition at 140 and 185 K for the sulfide and selenide, respectively; but neither superconducts down to 1.5 K. At lower lithium concentration near x ～ 1.0, the T_c of the sulfide is raised from that of Mo₆S₈ (1.8 K) to 5.2 K but the T_c of Mo₆Se₈ (6.5 K) is depressed to 3.9 K.