Chemical Activity in YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ Acrosst the Transition to Superconductivity

Changes in the Gibbs free enthalpy, chemical activity across the transition temperature to superconductivity T$_{c}$ in YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ is described by enhanced element X-Ray absorption XAS and diffraction XRD [HKL] reflections. Critical oscillations in the index of refraction within the XAS line width ($\pm $ 2.5eV) at the Ba L2, L3 and Y K-edges observed $\sim $30K above T$_{c}\approx $93K in [HKL] reflections indicates their activity. Enhanced absorbance A versus T obtains the activation enthalpy and entropy: $\Delta $H$^{\ne }_{>}$=-220 meV, $\Delta $S$^{\ne }_{>}$=-2 meV/K (121\underline {$>$}T\underline {$>$}93K) for mixed normal and superconducting phases, which compensates the reported O atom ordering activation energy near T$_{c}$ by 50 meV. The activation needed to mix differently ordered superconducting phases: $\Delta $H$^{\ne }_{<}$= -67 meV, $\Delta $S$^{\ne }_{<}$ = -1 meV/K (60K\underline {$<$}T\underline {$<$}93K) indicates lattice ordering persists to 60K. Enhanced XRD scattering induced near the transition to superconductivity in 3D solids indicates that the role of 2D reactive [HKL] planes is similar to the chemical activity of reactive linear bonds in molecular reactions.