Pathway for the formation of the Tl-2223 phase:: an in situ neutron powder diffraction study

The reaction pathway for the formation of Tl2Ba2Ca2Cu3Oz, (Tl-2223) has been investigated by in situ neutron powder diffraction. The experiments were carried out in an initially evacuated closed system on two samples with nominal compositions Tl1.7Ba2Ca2Cu3Oz and Tl2.3Ba2Ca2Cu3Oz. We find that, under these conditions, the formation path implies Tl2BaO4, Tl6Ba4O13 and Tl2Ba2CaCu2O8 (Tl-2212) according to the sequence: precursors/oxides --> Tl2BaO4 --> Tl6Ba4O13 --> Tl-2212 --> Tl-2223. The difference with respect to the sequence observed in an open system (precursors/oxides-->Tl-2201--->Tl-2212-->Tl-2223) is explained by the low temperature reaction (780 K) between the TlOx vapour and the free BaO resulting from the decomposition of BaCuO2 used as a precursor. The conversion from Tl-2212 to Tl-2223 occurs at 1075 K and does not involve the formation of a liquid phase but the solid-state diffusion of the Ca and Cu species and the restructuring of the Tl-2212 lattice. However, this conversion does not occur when the Tl content is 2.3, in agreement with previous works on phase equilibria. The reacted samples were characterized by high-resolution neutron powder diffraction, ac susceptibility measurements and SEM/EDX. Some original results on crystal structures and, to a lesser extent, on phase equilibria are also given.