Valence studies on various superconducting bismuth and lead cuprates and related materials

OK-edge and CuL23-edge XANES study on the concentration and distribution of holes in the(Pb2/Karppinen1, Kotiranta2, Yamauchi3 et al. 2001Phys. Rev. BSelf Cite214170View full textAdd to dashboardCite

“…This is in contrast to, e.g., fully oxygenated Cu͑Ba, Sr͒ 2 (R 1Ϫx Ca x ͒Cu 2 O 6ϩz and Bi 2 Sr 2 ͑Y 1Ϫx Ca x ͒Cu 2 O 8ϩ␦ systems in which the maximum oxygen content decreases with x. 20,21,24,25 The obtained samples possessed an orthorhombic structure within the whole substitution range studied. With an increasing amount of Ca͑II͒ entering the Y͑III͒ site, the lattice parameters a and b were found to decrease, while the c parameter increased ͑Table I͒.…”

“…This conclusion is consistent with charge-balance considerations and the results of BVS calculations 15,17,19 and chemical analyses. 20,21 Note that, the Pb atom with five nearestneighbor oxygen atoms is believed to be divalent. 20,21 That the CuO 2 plane can keep the holes created through Ca substitution violates against the redox-potential scheme: based on the redox potentials of the Pb͑II͒/͑IV͒, Cu͑I͒/͑II͒ and Cu͑II͒/͑III͒ couples, Pb͑II͒ and Cu͑I͒ should be oxidized more easily than Cu͑II͒.…”

“…20,21 Note that, the Pb atom with five nearestneighbor oxygen atoms is believed to be divalent. 20,21 That the CuO 2 plane can keep the holes created through Ca substitution violates against the redox-potential scheme: based on the redox potentials of the Pb͑II͒/͑IV͒, Cu͑I͒/͑II͒ and Cu͑II͒/͑III͒ couples, Pb͑II͒ and Cu͑I͒ should be oxidized more easily than Cu͑II͒. In a solid matrix, however, the redox-potential scheme applies only when the number of nearest-neighboring counter ions allows the predicted changes in the oxidation state.…”

“…In other words, oxygen loading seems to increase the oxidation states of the charge-reservoir cations only, from ϩI to ϩII for Cu and from ϩII to ϩIV for Pb, as suggested by the results of both BVS calculations 15,17,19 and wet-chemical analyses. 20,21 This conclusion manifests itself by the fact that the oxygenated (Pb 2/3 Cu 1/3 ͒-3212 phase does not superconduct. Also with the oxygen-deficient configuration at z Ϸ0, ͑Pb 2/3 Cu 1/3 ͒-3212 is an antiferromagnetic semiconductor if the Q-metal layer between the two CuO 2 planes in the CuO 2 -Q-CuO 2 block is composed of rare-earth element͑s͒ only.…”

“…The low level of excess oxygen in the synthesized samples was confirmed by Cu͑I͒/͑II͒ coulometric titrations. 20,21 The samples were characterized for superconductivity properties by a superconducting quantum interference device ͑SQUID͒ magnetometer ͑Quantum Design MPMS-5S͒. The ''effective'' T c (e) values were estimated from the -versus-T curves, measured in a field-cooling mode under a magnetic field of 10 Oe from room temperature down to 5 K, by extrapolating the diamagnetic portion of the curves to the ϭ0 level.…”