Formation of a stable shell of YBa₂Cu₄O₈ upon oxygen outdiffusion from superconducting YBa₂Cu ₃O_7-δ oxides

A stable shell of YBa2Cu4O8 (124) forms in powder and sintered bulk YBa2Cu3O7-δ (123) superconducting oxides upon oxygen outdiffusion from the latter and serves as a diffusion barrier to retard further oxygen outdiffusion. The 124 phase formation has been investigated by thermogravimetric analysis and x-ray diffraction. The better thermal stability of the 124 phase can be explained by the stronger bonding configuration of O(1) atoms in a double-chain structure.