Ph. Galez

Structural characterization of PbWO4 and related new phase Pb7W8O(32−x)

Abstract Single crystals of PbWO 4 have been obtained by the Czochralski method. Atomic structure has been determined by X-ray diffraction (scheelite type, tetragonal, space group I 4 1 / a , a = 5.456(2), c = 12.020(2) A). With PbO deficiency in the melt, crystallization occurred with a new atomic arrangement determined by X-ray diffraction (tetragonal, space group P 4/ nnc , a = 7.719(2), c = 12.018(2) A). Unit cell parameter a related to the scheelite type by a factor of √2. Oxygen and lead sites not fully occupied gave a formula Pb 7 W 8 O (32− x ) with x = 3.2(8).

A new structural model for Pb-deficient PbWO4

Abstract A previously reported structure proposal for Pb-deficient PbWO4 (Pb7W8O32−x, space group P4/nnc) could be rejected based on neutron powder diffraction data. A new model in agreement with X-ray and neutron diffraction data is proposed (tetragonal, tP48, space group P-4-h10g2dcba, a=7.7208(1), c=12.0417(2) A). It is a superstructure of the scheelite type with partial vacancies on a one-fold lead site, leading to the composition Pb7.5W8O32.

The Zr-Au system

Abstract The Zr-Au system has been reinvestigated by means of differential thermal analysis, X-ray diffraction and electron probe microanalysis. Our main result concerns the discovery of two new intermetallics: Zr3Au2 and ZrAu. The existence of the Au-rich phases, ZrAu4, ZrAu3, ZrAu2 and Zr7Au10, was confirmed and we clarified their type of formation. The stability regions of Zr3Au, Zr2Au and the two new phases have been also determined.

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.

Structural characterization and superconducting properties of (Tl0.5Pb0.5)(Sr2−xBax)Ca2Cu3O9−δ

Abstract Samples of nominal composition (Tl0.5Pb0.5)(Sr2−xBax)Ca2Cu3O9−δ, 0 ≤ x ≤ 0.6, and (Tl0.6Pb0.2Bi0.2)(Sr1.8Ba0.2)-Ca2Cu3O9−δ were characterized by X-ray powder diffraction, scanning electron microscopy and AC susceptibility measurements. In the Bi-free samples, the highest weight fraction of the 1223 phase, 91.3%, was observed for x = 0.4. Above this Ba content, BaPbO3 perovskite appeared as main impurity phase. In the Bi-containing sample the weight fraction was even higher, 92.6%. The Ba content of the 1223 phase, as determined from X-ray diffraction data, was in agreement with the nominal composition of the samples. The unit cell parameters increased when the Sr atoms were progressively replaced by the larger Ba atoms. The refinement of the crystal structure of the 1223 phase revealed a significant off-centering of the Tl site. The distance from the splitted site to the ideal position on the four-fold axis was found to be ∼ 0.26 A for the Ba-containing samples, but only 0.07 A for the Ba-free sample. The distance from the Cu atom to the apical atom of the square pyramid formed by the oxygen atoms increased from 2.29 to 2.50 A as the Ba content was raised. The critical temperature remained between 116 and 120.5 K and was not significantly affected by changes in composition or structure.

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.

Unsubstituted Tl-1223: a possible candidate for high current applications of superconductivity

Abstract The Tl–Ba–Ca–Cu–O system has been investigated. We report on the superconducting properties and the pathway for the formation of the Tl-1223 phase and on equilibrium phase relations. The formation of Tl-1223 implies the double TlO layer phases and occurs at high temperature (890–910°C) by a progressive de-intercalation of TlO layers from Tl-2223. As a consequence of this non-equilibrium process, transient intergrowth phases may be formed and have been unambiguously identified. This path of formation results in a platelet-like morphology for Tl-1223 grains which, in the view of the fabrication of tapes, is much more interesting than the globular shape observed for (Tl,Pb)(Sr,Ba) 2 Ca 2 Cu 3 O z . Moreover, the superconducting properties ( T c and irreversibility line) of the unsubstituted Tl-1223 phase compare well with those of (Tl,Pb)(Sr,Ba) 2 Ca 2 Cu 3 O z . With regard to phase equilibria, we show the existence of a three phase field Tl 2 Ca 3 O 6 –(Tl-2212)–(Ca,Tl) 1− x CuO y which prevents the formation of Tl-2223 for Tl contents higher than two.

Phase diagram of mixed LiK1-xRbxSO4 crystals

Neutron powder thermodiffractometry has been used to determine the phase diagram of LiK 1- x Rb x SO 4 mixed crystals ( x ≤ 0.50), between 50 and 800 K. We observed that, in this temperature range, these crystals undergo practically the same sequence of phase transitions as pure LiKSO 4 crystal. The main effects of the presence of rubidium ions in the potassium sites are seen in the variations of the critical temperatures and in the smearing of the first order transitions. We observed the narrowing of the region of the room temperature hexagonal phase with increasing Rb concentration. Its upper limit temperature decreases, while the lower one increases, favoring the trigonal phase. The low-temperature ferroelastic phase is absent for crystals with Rb concentration higher than c.a. 0.30.

Studies in the BaO2–BaO–CuO system

Abstract The complex ternary system Ba-Cu- O has been re-visited in 0.1 MPa oxygen pressure. It is established that for such pressure, the BaO 2 = BaO+ 1 2 O 2 reaction has an important role for reaching the equilibrium state. The pathway for the phase formation is then described in ternary sections BaO2-BaO-CuO. We have specified the temperatures and the forming reactions of the 3 compounds in the system: Ba2CuO3.04 decomposes by peritectic-like reaction at 1033°C and shows a structural transition from orthorhombic to tetragonal for at 825°C, BaCuO2 is a congruent melting compound at 1035°C and Ba2Cu3O5.5 undergoes a peritectoidic decomposition into BaCuO2 and CuO at 893°C.

Tl-based superconducting films by spray pyrolysis and MOCVD

Tl-1223 and Tl-2223 superconducting films have been prepared using spray pyrolysis and MOCVD for the deposition of the precursor layer and a solid source of thallium for the subsequent thallination step. For the former phase, critical temperatures and critical current densities up to 113 K and 2.5 MA/cm2 (77 K, 0 T) have been obtained after 3 to 6 hours at 860-870°C. Pure Tl-2223 films are by far more difficult to prepare. This can be achieved by operating at 890-900°C or with a fluorine containing source. The resulting films have then poor superconducting properties due to the presence of holes. Working at 875°C yields multiphase films (Tl-2223 + Tl-2212 or Tl-2223 + Tl-1223) with however better properties