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

Carrier doping arising from oxygen loading and Pb substitution in (Bi,Pb)-2 212 and -2223 polycrystalline materials has been monitored using thermopowe r and T-c for a range of annealing conditions in which oxygen partial press ure and temperature are carefully controlled. The three-layer compound show s a departure from the common thermopower-doping relationship found for oth er superconducting cuprates. This can be interpreted in terms of a hole-con centration deficiency on the inner Cu-O layer for underdoped samples. Activ ation 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 b y 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 bufferi ng 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 becom es increasingly restricted as Pb content increases. The carrier doping equi libria also appear to have a very low dependence on oxygen partial pressure , further evidence that a simple vacancy model for hole doping via oxygen l oading is not adequate for these materials. (C) 2001 Elsevier Science B.V. All rights reserved.