Pathway for the formation of the Tl-2223 phase: anin situneutron powder diffraction study

Abstract

The reaction pathway for the formation of Tl 2 Ba 2 Ca 2 Cu 3 O z (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 Tl 1.7 Ba 2 Ca 2 Cu 3 O z and Tl 2.3 Ba 2 Ca 2 Cu 3 O z . We find that, under these conditions, the formation path implies Tl 2 BaO 4 , Tl 6 Ba 4 O 13 and Tl 2 Ba 2 CaCu 2 O 8 (Tl-2212) according to the sequence: precursors/oxides → Tl 2 BaO 4 → Tl 6 Ba 4 O 13 → 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 TlO x vapour and the free BaO resulting from the decomposition of BaCuO 2 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.