Evidence for two distinct orthorhombic structures associated with different Tc regimes in LnBa2Cu3O7 − δ (Ln = Nd, Eu, Gd and Dy): A study of the dependence of superconductivity on oxygen stoichiometry

Superconductivity in LnBa2Cu3O7 − δ with Ln = Nd, Eu, Gdand Dy has been investigated as a function of δ, closely following the accompanying changes in crystal structure. Orthorhombic GdBa2Cu3O7 − δ and DyBa2Cu3O7 − δ show a T c of ≈ 90 K up to δ = 0.2 and a lower T c plateau (40–50 K) in the δ range 02 to 0.4, similar to that found in YBa2Cu3O7 − δ. The orthorhombic structure II in the lower T c regions is different from the structure I in the 90 K T c (low δ) region. The unit cell parameters of the orthorhombic I structure in the high T c region bear the relationship of a a ≠ b ∼ c/3. This relationship is not seen in the low T c plateau. The low T c plateau region does not distinctly manifest itself in NdBa2Cu3O7 − δ just as in LaBa2Cu3O7 − δ.