We have synthesized nearly single-phase HgBa2CuO4+δ samples with various oxygen contents, and systematically investigated the relationship between the lattice parameters, the superconductivity transition temperature (Tc) and the oxygen content determined by iodometry. Furthermore, the nominal copper valence values were calculated from the oxygen content data by assuming constant valence values of +2.00 and -2.00 for mercury and oxygen, respectively. Both the a and c lattice parameters were found to decrease monotonically with the Cu valence over a wide range of +2.05-2.28. The obtained Tc versus Cu valence curve exhibits a "bell shaped" correlation with the maximum in Tc at the Cu valence of ∼ +2.18.
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