THE 1201-PHASE OF THE TL-SR-LA-CU-O SYSTEM - HOMOGENEITY RANGE, STRUCTURE AND SUPERCONDUCTIVITY

The systematic investigation of the pseudo-ternary system "Tl-Sr-La-Cu-O" has allowed a large domain for the "1201"-type superconductor to be isolated. This can be divided into two subdomains (A and B) separated by the line TlSr2-yLa(y)CuO5-delta (0.4 less-than-or-equal-to y less-than-or-equal-to 1). The X-ray diffraction and electron microscopy studies confirm that, for this boundary the monolayers are fully occupied by thallium whereas lanthanum and strontium are distributed in the [Sr1-y/2La(y/2)O]-infinity layers. The domain A, which corresponds to the formula Tl1+xSr2-x-yLa(y)CuO5-delta, involves thallium excess but no intergrowth of thallium bilayers is observed by electron microscopy so that excess thallium is substituted over La/Sr sites; moreover their electron diffraction patterns exhibit diffuse streaks which suggest an inhomogeneity in the local composition due to thallium segregation. For the domain B, represented by the formulation Tl1-xSr2+x-yLa(y)CuO5-delta, the electron microscopy exploration shows that most of the crystals exhibit extended defects corresponding to the intergrowth of the "0201" La2CuO4-type structure with the "1201" TlSrLaCuO5-type matrix. This phenomenon partly explains thallium deficiency. However, the XRD patterns, which attest to the absence of an extra phase, impose the presence of Sr or La on the thallium sites in order to respect the phase diagram. The study of the superconducting properties of these phases, with different annealings at low temperatures (O2 and Ar/H-2), shows critical temperatures ranging from 30 to 40 K, which can be interpreted in terms of variations of hole carrier density.