Metal atom incorporation studies on A<SUB>x</SUB>Mo<SUB>6</SUB>S<SUB>8</SUB> Chevrel phases

Group IIIa metals and Nb, Hg and Pb containing A<SUB>x</SUB>Mo<SUB>6</SUB>S<SUB>8</SUB> Chevrel phases have been synthesized by low temperature reaction (420-430&#176;C) frorn A metal and Mo<SUB>6</SUB>S<SUB>8</SUB>. Some of these phases are hitherto unknown. Electrolysis method of preparation of A<SUB>x</SUB>Mo<SUB>6</SUB>S<SUB>8</SUB> reported by Sch&#246;llhorn 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<SUB>x</SUB>Mo<SUB>6</SUB>S<SUB>8</SUB> 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<SUB>c</SUB> studies show that for x &#8804; 0.8 in Pb<SUB>x</SUB>Mo<SUB>6</SUB>S<SUB>8</SUB>, the compounds behave more like Mo<SUB>6</SUB>S<SUB>8</SUB> and only for x &#8773; 1.0 or (Pb---Bi)<SUB>1.0</SUB>, the inherent behavior of the ternary Chevrel phase is exhibited. Seebeck coefficients are small and positive for Mo<SUB>6</SUB>S<SUB>8</SUB>, 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