Structure and oxygen exchange in superconducting Nd 2Ce 0.67Sr 0.33BaCu 3O 9−δ

Abstract

Oxidation-reduction of the title compound, a member of the A 2B 2Cu 3O 9−δ family [1] was studied by thermogravimetry and neutron diffraction. A superconducting material, obtained by annealing under 13 atm oxygen, has oxygen content δ=0.08 (average copper valence 2.18). It reversibly loses oxygen by heating under variable oxygen partial pressures. A sample equilibrated under 0.02 atm oxygen has δ=0.29, average copper valence 2.04 and does not superconduct. Neutron diffraction shows that the exchangeable oxygen is located on Cu(1) copper layers similar to the Cu chains in YBa 2Cu 3O 7−δ, but without oxygen ordering, hence without forming long-range CuO chains. The bond distances and valences are very similar to those found in the intermediate (δ≈0.4) phase with T c ≈60K in the YBa 2Cu 3O 7−δ system. The enthalpy of oxidation is approximately 40 kj/mol.