Novel solid-state and solution-phase processes for the modification of high-temperature oxocuprate superconductors: cation dependence of reduction by iodide and by tetrathiafulvalene

Summary

The insertion of lithium(I) and zinc(II) into oxocuprates of superconducting compositions was achieved by novel solid-state reactions of LiI and ZnI₂, in which copper is (formally) reduced and I₂ liberated. In comparable insertions from solution, in which the accompanying oxidation of tetrathiafulvalence (yellow → red) acts as reaction indicator, magnesium(II), strontium(II), barium(II), manganese(II), zinc(II) and cadmium(II) are shown to be taken up. Nonincorporation of sodium(I) and larger M⁺ into the oxocuprates Y-123 and Bi-2212 (as well as a limitation of zinc(II) uptake into Y-123 cf. lithium(I), from the iodides) are attributable to cationic charge/size effects. XRD changes match those resulting from alternative copper-reduction processes. The relative redox potentials of lattice and complementary redox couple involved are important in governing the extent of cation uptake.