Influence of OIII ordering on Tc in YBCO

The results of systematic and comparative investigations, carried out by electron diffraction and resistivity measurements on oxygen-chain-equalized polycrystalline YBCO samples in the existence domain of the OIlI structure, are reported. Experimental evidence for a characteristic To of the OIlI structure at about 75 K is provided. The results, which evidence a crucial role of the structural order in determining To, were expected from the theoretical point of view but were never obtained previously. They show that, in the existence domain of the OIlI structure, the degree of structural order results the most important factor in determining To, since it determines the real efficiency of the oxygen content in determining the hole carrier density in the CuOz layers.