GENERAL TRENDS IN OXYGEN STOICHIOMETRY EFFECTS ON TC IN BI AND TL SUPERCONDUCTORS

We have previously shown that T(c) for Bi2(Sr, Ca)n+1Cu(n)O2n+4+delta (n = 1, 2 and 3) varies with oxygen stoichiometry-delta determined by annealing in a variety of oxygen partial pressures and temperatures. Annealing results are now also presented for several Tl-superconductors in the series Tl(m)(Ba, Sr)2Ca(n-1)Cu(n)O2n+m+2+delta, for both m = 1 and 2, revealing two general results for this class of Bi or Tl superconductors: (1) effects on T(c) due to labile oxygen occur for all m = 2 compounds but are absent in m = 1 compounds with Sr. m = 1 compounds with Ba do have variable oxygen but the effects on the c-axis are the opposite to m = 2 compounds. This suggests for the latter that the labile oxygen resides in interstitial sites within the Bi2O2 or Tl2O2 bi layers; (2) the hole concentration per Cu decreases in progressing from n = 1 to 2 to 3 in either class so that the parent n = 3 and n = 2 compounds lie respectively on the low- and high-hole concentration sides of the peak in T(c) while the n = 1 compounds extend out into the non-superconducting domain at very high hole concentration.