Microstructure and microchemistry characterizations have been made using analytical transmission electron microscopy in order to correlate extended defect formation to intergranular and intragranular current density values. AC magnetization curves as a function of applied magnetic field at 4.2 and 77 K have been obtained. The results show that a large density of twins favours a high intragranular critical current density. In addition, the formation of cationic non-stoichiometric phases can act as vortex pinning centres. The relatively high intergranular current density is in good agreement with the observed absence of intergranular grain boundary phases.