ROLE OF ISOVALENT SUBSTITUTION OF STRONTIUM FOR BARIUM IN THE SUPERCONDUCTING PROPERTIES OF CUPRATES WITH THALLIUM MONOLAYERS

The isovalent substitution of strontium for barium in thallium monolay er cuprates is studied; three series of oxides have been isolated and characterized by X-ray diffraction: the 1201 phases TlBa1.2-xSrxLa0.8C uO5-delta with 0 less-than-or-equal-to x less-than-or-equal-to 1.20, t he 1212 phase TlBaSrCaCu2O7-delta, and the 1223 phases TlBa2-xSrxCa2Cu 3O9-delta (0.50 less-than-or-equal-to x less-than-or-equal-to 1.50). T he superconducting properties of these cuprates are optimized by Ar-H- 2 annealing at low temperature. It is demonstrated that, contrary to t hallium bilayer cuprates, the critical temperature of these phases is not significantly affected by the Sr substitution. The 1201 cuprates a re all superconducting with very similar T(c)'s, close to 40 K, whatev er x, i.e., equally for pure barium as for pure strontium phase. Simil arly the 1212 phase TlBaSrCaCu2O7-delta, with a T(c) of 100 K, can be considered more interesting than TlBa2CaCu2O7-delta, which is very dif ficult to obtain as a perfectly pure phase. The cuprate TlBa1.5Sr0.5Ca 2Cu3O9-delta exhibits the highest T(c), 124 K, which has been register ed up to now in the 1223 series, showing that the multiplicity of the thallium layers does not play a predominant role for superconductivity . Attention is drawn to the crucial role of the experimental condition s of annealing for the optimization of the hole carrier density.