General trends in oxygen stoichiometry effects on Tc in Bi and Tl superconductors

We have previously shown that T_c for Bi ₂ (Sr, Ca) _n+1 Cu _nO _{2 n+4+δ} ( n=1, 2 and 3) varies with oxygen stoichiometry δ 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 _nO _{2 n+ m+2+δ} , for both m=1 and 2, revealing two general results for this class of Bi or TI 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 Bi ₂O ₂ or Tl ₂O ₂ 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.