Structural and Physical Properties of Superconducting La<Subscript>2</Subscript>CuO<Subscript>4+δ</Subscript>: Fid-Bau Portal

Structural and Physical Properties of Superconducting La2CuO4+δ

Chaillout, C. / Marezio, M.

Abstract Among high temperature superconducting oxides, La2CuO4+δ is still one of the most studied compounds as the appearance of superconductivity is not really understood. La2CuO4 can be oxidized either by a treatment at high temperature and under high oxygen pressure, or by using “soft” chemistry methods at room temperature such as electrochemistry. It seems that two regions of extra oxygen concentration have to be considered: first, one with δ≤0.05, which corresponds to a superconducting phase with Tc ≈32K; another one with δ≥0.08, which corresponds to a superconducting phase with Tc ≈45K. The former is obtained by treating La2CuO4 under high oxygen pressure. In this case, below ≈280K, the excess oxygen atoms migrate in order to form antiferromagnetic La2CuO4 domains and La2CuO4+δ ones. The migration of the oxygen atoms is accompanied by anomalies in resisitivity and in thermoelectric power. The extra oxygen is found to be in between two LaO layers in a similar position as the oxygen atoms located between two Nd layers in Nd2CuO4 structure. The insertion of extra oxygen causes the displacement of some of the other oxygen atoms. The nature of the bonding between the excess oxygen and the “normal” oxygen atoms of the structure has not yet been completely elucidated.The more oxidized samples (δ≥0.08) present a larger orthorhombic distortion than the δ≤0.05 samples. It is not clearly established whether or not these δ≥0.08 samples present a phase separation at low temperature. The presence of superstucture spots on electron diffraction patterns suggest an ordering of the incorporated oxygen atoms.