Oxygen desorption and structure in CaBaLaCu3Ox

The thermodynamics and kinetics of oxygen desorption in the tetragonal superconductor CaBaLaCu3Ox are examined. Equilibrium oxygen contents are determined by thermogravimetric analysis (TGA) for temperatures from 50 to 900degC. The results demonstrate that CaBaLaCu3Ox (x≤6.85) loses oxygen more slowly than YBa2Cu3Ox (x≤7) in flowing O2 as the temperature is increased. The resistance to oxygen loss is linked to a higher enthalpy for oxygen desorption in CaBaLaCu3Ox of ΔH=1.74 eV/atom O (compared to ΔH≅1.1 eV/atom O in YBa2Cu3Ox), as measured by differential scanning calorimetry (DSC). If the superconductor is sufficiently depleted of oxygen, the half-order reflections observed in transmission electron microscopy (TEM) diffraction patterns disappear. This result confirms that oxygen ordering is responsible for unit cell doubling. Finally, upon reducing CaBaLaCu3Ox completely to metallic Cu and elemental oxides - a two step reduction reaction occurs. It appears that the oxidation states of Cu and Ba are first reduced; then this reaction is followed by the conversion of Ba(OH)2 to BaO. (orig.)