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 monolayer cuprates is studied; three series of oxides have been isolated and characterized by X-ray diffraction: the 1201 phases TlBa[sub 1.2][sub [minus]][sub x]Sr[sub x]La[sub 0.8]CuO[sub 5][sub [minus]][delta] with O [le] x [le] 1.20, the 1212 phase TlBaSrCaCu[sub 2]O[sub 7][sub [minus]][delta], and the 1223 phases TlBa[sub 2][sub [minus]][sub x]Sr[sub x]Ca[sub 2]Cu[sub 3]O[sub 9][sub [minus]][delta] (0.50 [le]x[le] 1.50). The superconducting properties of thesee cuprates are optimized by Ar-H[sub 2] annealing at low temperature. It is demonstrated that, contrary to thallium bilayer cuprates, the critical temperature of thes phases is not significantly affected by the Sr substitution. The 1201 cuprates are all superconducting with very similar T[sub c]'s, close to 40 K, whatever x, i.e., equally for pure barium as for pure strontium phase. Similarly the 1212 phase TlBaSrCaCu[sub 2]O[sub 7][sub [minus]][delta], with a T[sub c] of 100 K, can be considered more interesting than TlBa[sub 1.5]Sr[sub 0.5]Ca[sub 2]Cu[sub 3]O[sub 9][sub [minus]][delta] exhibits the highest T[sub c], 124 K, which has been registered 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 ofmore » the experimental conditions of annealing for the optimization of the hole carrier density. 20 refs., 12 figs., 2 tabs.« less