Superconductivity in Nonstoichiometric Compounds, Layers, and Chalcogen Ternaries.

摘要

The objective of this research was to study the effect of chemical composition on superconductivity in chemical compounds. Four kinds of compounds were investigated: tunnel-structure tungsten bronzes, covalent-frame hexaborides, layered-structure transition metal dichalcogenides, and Chevrel-type molybdenum ternaries. In the tungsten bronzes, it was found that changing electron density was less important than enhancing electron-phonon coupling, by approach to a crystallographic structure change boundary. In the metal hexaborides, it was shown that lattice expansion was less effective than decreasing conduction electron density in depressing the superconducting critical temperature, but even more effective was presence of magnetic ions in the metal sublattice. This was found to be the most likely reason for the very low critical temperature of lanthanum hexaboride. In the layered dichalcogenides, insertion of magnetic ions between the layers was found to be less effective in depressing critical temperature than either substitution of magnetic ions within the layers or departure from stoichiometry. The latter was found to lead to a change in polytype which could be correlated with the onset temperature of a charge density wave. In the Chevrel-type compounds, a new correlation was found between superconducting critical temperature, c/a lattice parameter ratio, and deviation from stoichiometry. (Author)