Oxygen-nonstoichiometry effect on the superconducting properties of the Sr-free Bi cuprates.

Abstract

The effects of oxygen nonstoichiometry on normal-state electrical conduction in and the superconducting properties of ${\mathrm{Bi}}_{2}$${\mathrm{Ca}}_{2.5}$${\mathrm{Nd}}_{0.5}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{\mathit{y}}$ and ${\mathrm{Bi}}_{2}$${\mathrm{Ca}}_{2.5}$${\mathrm{La}}_{0.25}$${\mathrm{Pr}}_{0.25}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{\mathit{y}}$ have been investigated. From the temperature dependence of the normal-state resistivity of ${\mathrm{Bi}}_{2}$${\mathrm{Ca}}_{2.5}$${\mathrm{Nd}}_{0.5}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{\mathit{y}}$, it is speculated that oxygen nonstoichiometry controls the metal-to-semiconductor transition. From the dependence of the high-temperature resistivity on oxygen partial pressure and the thermogravimetric behavior of ${\mathrm{Bi}}_{2}$${\mathrm{Ca}}_{2.5}$${\mathrm{Nd}}_{0.5}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{\mathit{y}}$ and ${\mathrm{Bi}}_{2}$${\mathrm{Ca}}_{2.5}$${\mathrm{La}}_{0.25}$${\mathrm{Pr}}_{0.25}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{\mathit{y}}$, it is found that the charge carriers in these cuprates are holes. A long-period modulation is observed by electron diffraction. The modulation is near commensurate with a period equal to 9b.The superconducting properties of ${\mathrm{Bi}}_{2}$${\mathrm{Ca}}_{2.5}$${\mathrm{Nd}}_{0.5}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{\mathit{y}}$ and ${\mathrm{Bi}}_{2}$${\mathrm{Ca}}_{2.5}$${\mathrm{La}}_{0.25}$${\mathrm{Pr}}_{0.25}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{\mathit{y}}$ are dependent on oxygen partial pressure during sintering, but the modulation period remains unchanged. Although the Cu oxidation states in both ${\mathrm{Bi}}_{2}$${\mathrm{Ca}}_{2.5}$${\mathrm{Nd}}_{0.5}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{\mathit{y}}$ and ${\mathrm{Bi}}_{2}$${\mathrm{Ca}}_{2.5}$${\mathrm{La}}_{0.25}$${\mathrm{Pr}}_{0.25}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{\mathit{y}}$, which are sintered at ${\mathit{P}}_{\mathrm{O}2}$=1 atm, are the same, being equal to 2.08, their superconducting properties are found to be different. This may suggest that the effective valences of Cu and O are different from the conventional integer values because of charge transfer between Cu and O.