MoxCu1−xSr2YCu2Oy (0.3≤x≤1) revisited: Superconductivity, magnetism and the molybdenum oxidation state

A systematic study is reported on the range of stability of molybdenum substituted Sr-based 123 compounds with Mo–Sr–Y–Cu system, synthesized under ambient pressure. All materials crystallize in the space group: P4/mmm and the observed solubility limit of Mo is rather low in this structure because secondary phases start to form as soon as x>0.3 in nominal composition is reached. The antiferromagnetic property in all superconducting multiphase samples can be attributed to the secondary phase Y2Cu2O5. The influence of oxygen annealing on the changes in electronic structure for the pure Mo0.3Cu0.7Sr2YCu2Oy system associated with a non-superconductor to superconductor transition has been investigated by means of X-ray photoelectron spectroscopy, powder X-ray diffraction, magnetic susceptibility and specific-heat measurements. We unambiguously show the predominance of the Mo5+ state over the Mo6+ one on both as-synthesized and annealed phases; annealing under an oxygen atmosphere enhances both the Mo6+ and Cu2+ amounts. We suggest that the enhancement of Mo6+ under oxygen annealing is in close relation with the decrease in the O 2p→Cu 3d charge-transfer energy resulting in superconducting properties.