THE CRYSTAL STRUCTURE OF THE „451-PHASE"

Räth, S. and Schmahl, W. W.

Inst. f. Mineralogie, Petrologie u. Geochemie, Univ. Tübingen, Germany.
e-mail: wwschmahl@uni-tuebingen.de

Keywords: 451-Phase, 3321-Phase, 231-Phase, BSCCO, HTSC.

For the fabrication of 2223-HTSC wires or tapes, Pb-doped 2223 gives best results, due to promotion of crystal growth probably by a Pb-rich melt. The multiphase precursor powder for the PIT-process is usually reported to contain 2212, Ca₂PbO₄ and other minor phases. However, Majewski [1] and Jeremie [2] described the formation of a „451-phase", now recognized as Pb_3,5Sr_5,5Cu_1,2O_13,7-x, in their precursor powders, which had previously been overlooked or mistaken for Ca₂PbO₄.

The „451-phase" and its wide solid solution range was first described by Kitaguchi [3]. Reported compositions differ very much from each other: Kim et al. [4] determined by iodometric titration Sr_4,8Pb_3,2Cu_1,6O_11,76 and Sr_4,8Pb_3,2Cu_0,7O_12,3, Majewski supposes from EDX-measurements Pb₄Sr₅CuO₁₀ and Jeremie assumes (Bi,Pb)₃Sr₃Ca₂CuO_x.

The structure and crystal chemistry of this substance (also called „231" or „3321"-phase have hardly been the subject of detailed examinations. Kim et al. proposed the spacegroup P-62m on the base of single-crystal X-ray-data, but their two copper-sites, which are just 0.9 apart, with site occupancies of 0.197 or 0.134, respectively, are questionable and should be investigated closer. After Rietveld analysis of X-ray data (Fig. 1), we suppose only one copper-site with full site occupancy.

Fig. 1: Rietveld refinement plot of the sample Sr_4,5Ca₁Pb_3,5Cu_1,2O_13,7-x.

To determine the correct atomic position of Cu and the preferential sites for Ca and Bi substitution in the „451"-solid-solution, we examined three phase pure samples of composition Sr_5,5Pb_3,5Cu_1,2O_13,7-x, Sr_3,5CaPb_3,5Cu_1,2O_13,7-x and Sr_5,5Pb_2,5BiCu_1,2O_13,7-x by X-ray Rietveld refinement. The data were collected on a Huber 644 Guinier-diffractometer (Cu-K1-radiation, 40kV/20mA, 0.01°, 9-90° 2) with Si as an internal standard. Ca occupies the Sr2-Position (3g), Bi the Pb-Position (3f) of Kim et al. [4].

1. Majewski et al.: The use of phase Diagrams for the engineering of flux pinning centres in Bi₂Sr₂CaCu₂O₈ ceramics, Applied Superconductivity Vol.3, No. 5 (1995) 289-301.
2. Jeremie et al.: Effect of carbon impurities on Bi,Pb(2223) phase formation and critical current densities in silver-sheathed Bi,Pb(2223) tapes. Physica C 255 (1995) 53-60.
3. Kitaguchi et al.: Equilibrium phase diagrams for the systems PbO-SrO-CuO and PbO-CaO-SrO, J. Mater. Res. Vol. 5, No. 7 (1990) 1397-1402.
4. Kim et al.: A New Phase in the Sr-Pb-Cu Oxide System: The Crystal Structure of Sr_5-xPb_3+xCu_yO_12-d. Journal Of Solid State Chemistry 85 (1990) 44-50.