The defect structure and transport properties of some high-Tc superconductors

The defect structures and transport properties of La2-xSrxCuO4-δ, Nd2-xCexCuO4+δ, and YBa2Cu3-xTixO6+δ were studied by 4-probe DC conductivity and thermoelectric power measurements as functions of temperature, PO2 and doping concentration. The conductivity and thermoelectric power of La2CuO4-δ both follow a PO21/6 dependence at high temperatures, consistent with a defect model which involves equilibration between doubly-ionized oxygen defects and holes. The insensitivity of both properties to temperature implies that the enthalpy of the redox reaction is insignificant. Nd2CuO4-δ is an n-type semiconductor. The electron mobility was found not to be thermally activated. At high temperatures, the electron density is primarily determined by a reduction reaction. YBa2Cu3-xTixO6+δ is metallic below ∼ 400C. Above that temperature the redox reaction dominates carrier generation. At T ∼ 700C in 100 ppm O2, it undergoes a p-n transition. The magnitude of conductivity and its dependence on temperature and PO2 are nearly independent of Ti concentration up to x = 0.1 and are very similar to the undoped YBa2Cu3O6+δ, suggesting a large oxygen disorder