The effect of (Ba/Sr) substitution on Tl-based superconducting thin films prepared from fluoride precursors

Tl-2212 and Tl-1212 c oriented thin films were prepared by the thalliation of (Ba1-xSrx)(2)Ca2Cu3(O/F)(z) precursor layers, which were deposited on single-crystal CeO2-buffered sapphire substrate by sequential thermal evaporation of BaF2, SrF2, CaF2 and Cu components. The partial defluorination of the precursor did not significantly influence the phase composition and the superconducting properties of the obtained Tl-2212 films. The prepared Tl-2212 layers exhibit T-C(0) 98 K for a sample without Sr and falls to T-C(0) = 78 K for a partially Sr substituted films (x = 0.4). The increasing Sr content in the precursor stabilises the Tl-1212 phase. The samples prepared from defluorinated precursors show T-C(0) values in the range 47-55 K depending on the Sr content. On the other hand the Tl-1212 films synthesized from fluorinated precursors show greater transition widths to the superconducting state and therefore their T-C(0) values are shifted to 38-41 K. This is caused probably by the increased amount of amorphous non-superconducting admixtures, which can be seen by SEM in form of precipitates on the film.