The Gibbs free enthalpy, chemical activity across the transition temperature to superconductivity, Tc in YBa2Cu3O7 − δ is obtained from reciprocally enhanced X-Ray absorbance, XAS and diffraction, XRD data near the Ba L3,2 edges' energy Ea, and orientations in the X-ray beam for preferred Miller indexed [HKL] planes' scattering, that are enhanced near Tc. The standard enthalpy and entropy for the formation of mixed normal metal/superconducting domains above Tc, determined individually across the two Ba L3,2 edges, to better than a percent accuracy: ΔH≠≥ = − 220 meV, and ΔS≠≥ = − 2 meV/K when 121 ≥ T ≥ 92 K ≈ Tc1, indicate that energy is gained, with an increased order by forming a mixture with reduced entropy. Below Tc the standard enthalpy and entropy to form at least two mixed superconducting phases are halved to ΔH≠≤ = − 86 meV, ΔS≠≤ = − 1 meV/K when 92 K ≥ T ≥ Tc2 ≈ 72 K. Thus reciprocal XAS/XRD enhancement at orientations in the X-ray field, of preferred 2D planes, induced by the transition to superconductivity in the layer solid, indicates one, the importance of the 2D-plane electron density scattering near Tc, and two, identifies the 2D-plane chemical activity, by the electron density leading to symmetry allowed excitations, a role similar to that of the electron density in linear bonds for molecular activity.
Juana Vivó Acrivos. "Chemical activity in YBa2Cu3O7 − δ across the normal to superconducting phase transition" Microchemical Journal (2011): 239-245. doi:10.1063/1.1701808