Ch. Bertrand

The Pr(Ba1-xPrx)2Cu3O7+δ solid solution. A crystal structure and phase diagram study

Samples of nominal composition Pr(Ba1−xPrx)2Cu3O7+δ with −0.15≤x≤0.6 and Nd(Ba0.6Nd0.4)2Cu3O7+δ were characterized by X-ray powder diffraction combined with Rietveld analysis, differential thermal analysis (DTA), thermogravimetry (TG), and AC susceptibility measurements. Pr(Ba1−xPrx)2Cu3O7+δ is formed by the peritectic reaction Pr2CuO4+L→Pr(Ba1−xPrx)2Cu3O7+δ at 1060±10°C with x≈0.35 and the homogeneity range extends from x≈0.06 at 975±10°C to x≈0.45 at 1012±10°C which is consistent with the X-ray diffraction results. As for the Nd(Ba1−xNdx)2Cu3O7+δ solid solution, three distinct domains were found. In the Ba-rich region (0<x<0.15), the structure is tetragonal or orthorhombic depending on oxygen content. On the contrary, the structure of the 1212 phase is always tetragonal in the intermediate region (0.15≤x≤0.30) and always orthorhombic in the Pr-rich region (0.30<x≤0.45). In addition, the composition range with respect to oxygen is significantly reduced compared to RBa2Cu3O7−δ. The transition from the tetragonal intermediate domain to the orthorhombic Pr-rich domain results in the expected eightfold coordination for Pr atoms on Ba site when the Pr/Ba ratio for this site approaches unity. No evidence of a superstructure was found for either Pr- or Nd-containing 1212 phases.

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.

Formation pathways in the synthesis and properties of (Tl0.5Pb0.5)(Sr0.9Ba0.1)2Ca2Cu3Oz and (Tl0.5Pb0.5)(Sr0.8Ba0.2)2Ca2Cu3Oz-1223 superconductors

The formation pathway of (Tl0.5Pb0.5)(Sr0.9Ba0.1)2Ca2Cu3Oz and (Tl0.5Pb0.5)(Sr0.8Ba0.2)2Ca2Cu3Oz was studied by neutron and x-ray diffraction. The following reaction pathway was proposed: thallium oxide and lead oxide react with Sr-rich (Sr1-xCax )CuO2 and Ca-rich (CaxSr1-x)O to form Sr4Tl2O7 and (CaxSr1-x)PbO3, respectively. The thallate and the plumbate compounds then form the (Tl0.5Pb0.5)-1212 phase starting at a temperature of 600 ◦C. Finally, between 850 and 900 ◦C, the 1223 phase is formed from (Tl0.5Pb0.5)-1212, (CaxSr1-x)2CuO3 and CuO. Parallel experiments to fabricate the (Tl, Pb)-1223 superconductor resulted in specimens with critical temperatures of 117.5 K and 116 K, respectively, and transition widths of 2 K. Differences between the pathways for the formation of Pb-doped, Sr-rich and Pb-free, Ba-rich Tl-1223 superconductors are discussed.