Strongly overdoped ${\mathrm{HgBa}}_2{\mathrm{Ca}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{8+\mathrm{\delta }}$ (Hg-1223) samples with superconducting transitions as low as 97 K were successfully obtained by using HgO, ${\mathrm{Ca}}_2{\mathrm{CuO}}_3,$ CuO, and mixtures of highly oxidized ${\mathrm{BaCuO}}_{2+\mathrm{\epsilon }}$ $(\epsilon \approx 0.13)$ and/or ${\mathrm{BaO}}_2$ powders as starting materials for the high-pressure synthesis at 5 GPa and 950 °C. The overdoped state was confirmed by observing negative values for the Seebeck coefficient throughout the temperature range from $T_c$ to 320 K in a thermoelectric power measurement. Also, both of the cell parameters $a$ and $c$ were found to decrease with decreasing $T_c,$ reflecting, respectively, an increase in hole concentration in the ${\mathrm{CuO}}_2$ planes and the incorporation of oxygen into the ${\mathrm{HgO}}_{\mathrm{\delta }}$ layer. The oxygen excess δ as determined by the $\mathrm{Cu}(+\mathrm{I})/\mathrm{C}\mathrm{u}(+\mathrm{I}\mathrm{I})\mathrm{}$ coulometric titration method, was $\sim 0.19$ in the overdoped sample with $T_c=107\mathrm{}\mathrm{K}.$ Subsequent reducing annealing in an Ar atmosphere at 280 °C increased the $T_c$ to 131 K. Consistently, only positive Seebeck coefficient values were observed up to 320 K for the Ar-annealed sample. A clearly underdoped material with $T_c=118\mathrm{}\mathrm{K}$ was obtained by annealing the same sample in Ar at a higher temperature $\left(\sim 400°\mathrm{C}\right).\mathrm{}$ Finally, high-pressure synthesis starting from less-oxidized ${\mathrm{BaCuO}}_{2+\mathrm{\epsilon }}$ $(\epsilon \approx 0.06)$ yielded a Hg-1223 material with $\delta \approx 0.10$ and $T_c=132\mathrm{}\mathrm{K}.$

• Received 21 July 1997

DOI:https://doi.org/10.1103/PhysRevB.56.14790