Metal atom incorporation studies on A_xMo₆S₈ Chevrel phases

Abstract

Group IIIa metals and Nb, Hg and Pb containing A_xMo₆S₈ Chevrel phases have been synthesized by low temperature reaction (420-430°C) frorn A metal and Mo₆S₈. Some of these phases are hitherto unknown. Electrolysis method of preparation of A_xMo₆S₈ reported by Schöllhorn does not yield the desired phases when A = Pb or Ag. Stoichiometry, thermal stability, electrical and superconducting properties of the compounds have been examined. In the Pb_xMo₆S₈ system, a series of phases with varying x have been prepared and studied. Group IIIa metal and Nb-compounds are found to be semiconductors with low energies of activation exhibiting p-type behavior in the range 77-300 K. Hg-phase behaves as a metal or a degenerate semiconductor and does not become superconducting above 4.2 K. X-Ray, resistivity and T_c studies show that for x ≤ 0.8 in Pb_xMo₆S₈, the compounds behave more like Mo₆S₈ and only for x ≅ 1.0 or (Pb---Bi)_1.0, the inherent behavior of the ternary Chevrel phase is exhibited. Seebeck coefficients are small and positive for Mo₆S₈, Cu and Pb-containing phases in the range 77-300 K. The results are discussed in the light of available band structure of these materials.