Hole concentration in the three-CuO2-plane copper-oxide superconductor Cu-1223

Strongly overdoped samples of the three-CuO₂-plane copper-oxide superconductor, CuBa₂Ca₂Cu₃O_8+z or Cu-1223, were obtained through high-pressure synthesis and post-annealed to various hole-doping levels so as to have the value of T_c range from 65 to 118 K. A concomitant decrease in the average valence of copper from ∼2.20 to ∼2.05 was evidenced by means of wet-chemical and thermogravimetric analyses and Cu L-edge X-ray absorption near-edge structure (XANES) spectroscopy. The valence value as low as ∼2.05 that corresponds to the highest T_c (=118 K) may be understood by taking into account multiple ways for holes to be distributed among the different Cu-O layers. In terms of actual chemical composition of the Cu-1223 phase, both Cu L-edge and O K-edge XANES results suggest that some portion of charge-reservoir copper atoms may have been replaced by CO, i.e., (Cu_1−xC_x)Ba₂Ca₂Cu₃O_8+x+z. The variation range of excess oxygen was estimated at Δz≈0.3.