Influence of OIII ordering onT c 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 OIII structure, are reported. Experimental evidence for a characteristic Tc of the OIII structure at about 75 K is provided. The results, which evidence a crucial role of the structural order in determining Tc, were expected from the theoretical point of view but were never obtained previously. They show that, in the existence domain of the OIII structure, the degree of structural order results the most important factor in determining Tc, since it determines the real efficiency of the oxygen content in determining the hole carrier density in the CuO2 layers.