Establishment of strongly overdoped states in the HgBa₂Ca₂Cu₃O_8+δ superconductor

Strongly overdoped HgBa₂Ca₂Cu₃O_8+δ (Hg-1223) samples with superconducting transitions as low as 97 K were successfully obtained by using HgO, Ca₂CuO₃, CuO, and mixtures of highly oxidized BaCuO_2+∈ (∈ ≈0.13) and/or BaO₂ powders as starting materials for the high-pressure synthesis at 5 CPa 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 CuO₂ planes and the incorporation of oxygen into the HgO_δ layer. The oxygen excess δ as determined by the Cu(+I)/Cu(+II) coulometric titration method, was ∼0.19 in the overdoped sample with T_c=107 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 K was obtained by annealing the same sample in Ar at a higher temperature (∼400°C). Finally, high-pressure synthesis starting from less-oxidized BaCuO_2+∈ (∈≈0.06) yielded a Hg-1223 material with δ≈0.10 and T_c= 132 K.