Pressure dependence of the superconducting critical temperature of HgBa2Ca2Cu3O8+y and HgBa2Ca3Cu4O10+y up to 30 GPa.

By performing experiments on samples as pure as presently available, the origin of the reported pressureinduced Tc values well above 150 K in the mercury-based high-Tc superconductors has been investigated. For HgBa2Ca2Cu3O81y ~Hg-1223! maximum Tc values between 142 K and 146 K are obtained under pressure up to 30 GPa, while for HgBa2Ca3Cu4O101y ~Hg-1234! Tc never exceeds 121 K. To verify the reproducibility between different laboratories the pressure dependence of a sample from the same batch as the sample on which Nuñez-Regueiro reported a Tc value of 157 K at 23.5 GPa has also been measured. Below 10 GPa the agreement with his results is rather good and a possible explanation for the disagreement above 10 GPa is given. It appears that the reported high Tc values in non-single-phase Hg-1223 samples are due to the presence of Hg-1234 phase impurities. Also some of the adopted definitions of Tc have led to somewhat optimistic values of Tc : The high-pressure results on Hg-1223 found in the literature are therefore reviewed and reanalyzed. @S0163-1829~96!05830-4#