We have studied the microstructure of melt-growth GdBa2Cu 3O7-δ (Gd123) bulk superconductors with the addition of Gd2Ba4CuMOx (abbreviated as M2411 with M ≤ Mo or Zr) particles by transmission electron microscopy. Gd123 bulks are prepared with nominal compositions Gd123+(0.4-x)Gd211+xM2411+0.1BaO2 in a molar ratio with x ≤ 0-0.2. The addition of M2411 particles induced nanosized particles inside bulks. They are apparently distributed well under seeding positions, where fewer Gd211 particles are located. The observed nanosized particles assigned as M2411 may play the role of flux pinning centres, leading to an enhanced Jc. However, the addition of M2411 with x>0.12 leads to the appearance of excess liquid and macrodefects in the bulks. Enhanced interfacial stress induced by the excess M2411 addition was found, together with modified local chemical compositions. These results account for both the suppression of the superconducting properties and the formation of the band structure in the present system.
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