We tried to prepare the mercury-based superconductor, HgBa2Ca2Cu3O8+δ (Hg1223), under highly oxidizing atmosphere using a high pressure technique. Samples containing Hg1223 as the main phase with TC=90-122K were obtained. CuO and a new phase were included in the samples as impurities. The a-axes of the samples were shorter than the values ever reported for Hg1223 with TC=135K. These results suggest that our Hg1223 samples had excess hole concentrations, i.e. they are over-doped. The irreversibility field (Birr) was measured for two samples with TC=118 and 94K. They had almost the same normalized temperature (1-T/TC) dependence of Birr.
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