Superconducting properties in relation to chemical pressure in YBa 2 Cu 3 O 9-δ

Abstract

Abstract Based on oxygen analyses and investigations by X-ray and neutron diffraction of single phase samples, structural consequences of substitution of La or Sr for Ba in YBa2Cu3O9−δ are rationalized in terms of a chemical pressure concept. Due to the size imbalance, a strain is introduced into the [Cu3O9−δ7−]n network upon the substitution. The resulting contraction is rather isotropic in case of Sr, while upon the La substitution, the c axis is contracted preferably. As a result of this anisotropy, the orthorhombic distortion of the structure diminishes (vanishes at 16 % La substitution for 9−δ = 6.97, being accompanied by redistribution of oxygen towards octahedral coordination for Cu. A decrease in the distortion is observed if the oxygen content itself is varied from 9−δ ≅ 6.97 to both directions. Tc decreases both with La (≈4.6 K/%) and Sr (≈0.3 K/%) substitutions. Independent of the actual degree of the orthorhombic distortion, Tc increases with increasing oxygen content up to 9−δ ≥ 7.0 if the additional oxygens are brought by La. At a constant oxygen content, Tc is coupled to the Jahn-Teller distortion of the Cu-coordination octahedra, and thus approximately to a ratio of the (orthorhombic) unit cell dimensions: D JT = 100 2c 3(a+b)]−1 (%) .Possible analogies in another systems are discussed.