T-O, OII-OIII and OIII-OI Phase Boundary in Oxygen-Chain-Equalised and Order-Stabilised Polycrystalline Pair Samples of YBa2Cu3O6+x

A comparative investigation, performed on Oxygen-Chain-equalised(k) Deintercalated [OCD]k and Intercalated [OCD]k pair-samples (k-pairs) of YBa2Cu3O6+x by resistive Tc and electron diffraction (ED) at steps Δk=0.02-0.03, revealed that equivalent or non-equivalent microscopic oxygen-chain arrangements are produced, at the same oxygen content k, by intercalation and deintercalation of oxygen in the Cu(1)Ox chains. These arrangements manifest in Tc-singularities or Tc-splittings of the k-pairs, whose occurrence can be straightforwardly explained by taking into account the non equivalence of the structural phase diagrams of YBCO, pointed out by electron diffraction, when produced by intercalation of oxygen in the tetragonal T structure or by deintercalation from the orthorhombic OI structure. Tc-singularities are produced in the k ranges for which a superimposition of the phase diagrams occurs, whereas Tc-splitting are pointed out in correspondence of splitted structural regions. The maximum Tc-splitting is observed in proximity of the semiconductor-metal transition with the appearing of superconductivity related to an anti-OIII (OIII*) superstructure in intercalated samples at k=0.30, where the corresponding deintercalated samples are tetragonal and semiconductor. This result confirms that the vanishing of the antiferromagnetic ordering, the emergence of hole doped superconductivity and the symmetry breaking at the T-O transition are nearly coincident phenomena in YBCO. However the related critical oxygen stoichiometry is strictly dependent on the ordering parameter in the Cu(1) plane, i.e. on the formation of extended chains. The OIII* structure, for the first time observed in a systematic way at the T-O transition in intercalated samples, seems to exhibit in analogy with OII and OIII a characteristic Tc.