HIGH-PRESSURE SYNTHESIS AND SUPERCONDUCTIVITY OF A BA FREE MERCURY-BASED SUPERCONDUCTOR (HG0.75RE0.25)SR2CA2CU3OY

A Ba free mercury-based cupric oxide superconductor (Hg0.75Re0.25)Sr2C a2Cu3Oy was prepared at 6 GPa and 1273 K. The quarter substitution of Re for Hg was found to be essential to the stabilization of the Ba fre e phase. It is isostructural to HgBa2Ca2Cu3O8+delta with lattice const ants of a = 3.830 Angstrom, c = 15.227 Angstrom, except for the format ion of a unique superlattice of 2a X 4a X 2c. The origin of the supers tructure has been considered to be ordering of the Re and Hg ions on t he Hg site. As-grown samples were over-doped, and T-c varied systemati cally as a function of temperature of annealing in an evacuated quartz ampoule. The maximum T-c thus attained is 120 K. As opposed to sample s of HgBa2Ca2Cu3O8+delta, the compound (Hg0.75Re0.25)Sr2Ca2Cu3Oy is st able in air at room temperature.