Oxygen loading in (Bi,Pb)-2212 and -2223 materials

Abstract

Carrier doping arising from oxygen loading and Pb substitution in (Bi,Pb)-2212 and -2223 polycrystalline materials has been monitored using thermopower and T c for a range of annealing conditions in which oxygen partial pressure and temperature are carefully controlled. The three-layer compound shows a departure from the common thermopower-doping relationship found for other superconducting cuprates. This can be interpreted in terms of a hole-concentration deficiency on the inner Cu–O layer for underdoped samples. Activation energies for oxygen loading are derived for the two- and three-layer phases; these decrease as Pb content is increased, in the case of Bi-2212 by a factor of two from 0.25eV for the Pb-free material. The maximum doping level is only weakly dependent on Pb content, however. This reflects both a decrease in oxygen content with increasing Pb concentration, and a buffering action by phase decomposition at higher oxygen partial pressures. At low oxygen content there is a net increase in doping with Pb substitution, so that the doping range accessible through oxygen loading and unloading becomes increasingly restricted as Pb content increases. The carrier doping equilibria also appear to have a very low dependence on oxygen partial pressure, further evidence that a simple vacancy model for hole doping via oxygen loading is not adequate for these materials.