Influence of firing procedures in the sol-gel synthesis of YBa2Cu3O6+x superconductors

The influence of firing procedures on the crystal chemistry and superconducting properties of YBa2Cu3O6+x has been investigated systematically by firing at different temperatures a citrate derived precursor. While a tetragonal nonsuperconducting phase with oxygen contents of about 6.6 is formed for firing temperatures 750 less than or equal to T less than or equal to 800 degrees C, orthorhombic superconducting phases are formed for T > 830 degrees C. The superconducting diamagnetic signal of the orthorhombic phases increases with increasing firing temperatures. More importantly, the tetragonal phase formed at the intermediate firing temperature (800 degrees C) is found to have significant influence on the structure and properties of the orthorhombic superconducting phases formed by subsequently firing at T > 900 degrees C. It leads to oxygen disordering, lower oxygen contents and inferior superconducting properties in the orthorhombic phases.