Transport properties of Al doped SmBa2Cu3O6+δ superconductor. Part I: oxygen non-stoichiometry and diffusion

The oxygen non-stoichiometry and diffusion in SmBa2Cu3−xAlxO6+δ superconductor were investigated by means of thermogravimetric analysis (TGA) in a wide P ( O 2 ) ( 1 ⩽ P ( O 2 ) ⩽ 10 − 4 atm ) and T ( 400 ⩽ T ⩽ 800 °C ) range. Thermogravimetric data at fixed oxygen partial pressures were collected using temperature steps and long isotherms. The samples used for TG analysis were previously sieved in order to obtain powders of known particle dimensions. This procedure allowed to determine both the oxygen non-stoichiometry and its diffusion coefficient in SmBa2Cu3−xAlxO6+δ. With respect to undoped SmBa2Cu3O6+δ, the corresponding δ values are quite similar in oxidising conditions, while the doped material loses less oxygen than the pure one in reducing conditions. For what concern the oxygen diffusion, aluminium doping does not affect the activation energy for that process; on the opposite the absolute D value is at least one order of magnitude lower than in the pure phase and the D varies as a function of P ( O 2 ) . All the above features are interpreted under the hypothesis that, in the thermodynamic conditions of the experiments, only O4 oxygen ions and sites surrounded by two Cu1 ions are involved into gas–solid equilibria and into diffusion process.