Induction of holes not only in the superconductive CuO 2 plane but also in the Bi 2 O 2+δ charge reservoir of the Bi 2 Sr 2 (Y 1-x Ca x )Cu 2 O 8+δ superconductor upon Ca II -for-Y III substitution is evidenced by means of two independent techniques, i.e., high-resolution x-ray-absorption near-edge structure (XANES) spectroscopy measurements and coulometric redox titrations. The absolute values derived for the CuO 2 -plane hole concentration from the Cu L 2,3 -edge XANES spectra are in good agreement with those obtained from the coulometric redox analysis. The CuO 2 -plane hole concentration is found to increase from 0.03 to 0.14 concomitantly with the increase in the BiO 1+δ/2 -layer hole concentration from 0.00 to 0.13 as the Ca-substitution level, x, increases from 0 to 1. The threshold CuO 2 -plane hole concentration for the appearance of superconductivity is determined at 0.06, while the highest T c is obtained at the hole concentration of 0.12. In the O K-edge XANES spectrum, the increases in the CuO 2 -plane and BiO 1+δ/2 -layer hole concentrations with increasing x are seen as enhancement in the relative intensities of the pre-edge peaks at ∼528.3 and ∼530.5 eV, respectively. : 61.10.Ht; 74.62.Dh; 74.72.Hs The high-T c superconductive copper oxide, M m A 2 Q n-1 Cu n O m+2+2n±δ or M-m2(n-1)n, is believed to possess an antiferromagnetic insulating ground state related to its "undoped parent phase". By increasing the CuO 2 -plane hole concentration the phase undergoes an insulatormetal transition and starts to show superconductivity with a transition temperature, T c , that strongly depends on the concentration of induced holes. In the multi-layered structure of an Mm2(n-1)n phase the superconductive Q n-1 Cu n O 2n block containing the CuO 2 plane(s) is sandwiched with two AO layers and an M m O m±δ "charge reservoir" block with a layer sequence of AO-CuO 2 -(Q-CuO 2 ) n-1 -AO-(MO 1±δ/m ) m . 1 Among the variety of known M-m2(n-1)n phases (M = e.g. Cu, Bi, Pb, Tl, Hg, Al, Ga, B; m = 0 -3; A = e.g. Ba, Sr, La; Q = e.g. Ca, rare-earth element R; n = 1 -9), 1 only a limited number of phases, e.g. (La,Sr) ((Tl,Pb)-1212), 2-6 allow us to experimentally observe the actual appearance of superconductivity adjacent to the insulator-metal boundary. This is because many of these phases are structurally rather weak to sustain doping within a sufficiently wide range. Another difficulty arises from the fact that no universal experimental tool to accurately probe the local The accurate oxygen contents were determined by coulometric Cu + /Cu 2+ redox titration. 1,7 This experiment yields the total amount of high-valent copper and bismuth species, i.e., Cu III and Bi V , and thus the oxygen content of the sample. Upon dissolving the sample in 1 M HCl containing a known amount of Cu + ions both Cu III and Bi V oxidize Cu + to Cu 2+ according to reactions,
PACSandOnce the reactions given by Eqs.(1) and (2) are completed the amount of remaining Cu + ions is accurately analyzed through coulometric titration, i.e., anodic...
