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 pressure-induced T-c values well above 150 K in the mercury-based high-T-c superconductors has been investigated. For HgBa2Ca2Cu3O8+y (Hg-1223) maximum T-c values between 142 K and 146 K are obtained under pressure up to 30 GPa, while for HgBa2Ca3Cu4O10+y (Hg-1234) T-c 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 Nunez-Regueiro reported a T-c 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 T-c 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 T-c have led to somewhat optimistic values of T-c: The high-pressure results on Hg-1223 found in the literature are therefore reviewed and reanalyzed.