The crystal structure and stability of Ba4CaCu3O8+δ have been investigated by neutron powder diffraction, differential thermal analysis and thermogravimetry. It is found that the phase is not stable below 1065K in p(O2)=1bar and decomposes according to the eutectoid reaction Ba4CaCu3O8+δ+x O2⇒Ba2CuO3.4+CaO+2BaCuO2. However, the equilibrium with the outer gas is not reached for sintered ceramics so that Ba4CaCu3O8+δ can be obtained in a metastable state after normal cooling conditions. In this case, the crystal structure is cubic (Im-3m, a=8.1452(1)Å, δ=0.68, Z=2, Rwp=2.5%, RBragg=5.4%) as reported in the literature. In reduced oxygen partial pressure (p(O2)<10−6bar), Ba4CaCu3O8+δ is stable down to room temperature and has a tetragonal structure with a significant lower oxygen content (P4/mmm, a=8.1976(3)Å, c=8.0709(3)Å, δ=-0.81, Z=2, Rwp=2.8%, RBragg=5.1%). The difference between the two crystal structures is discussed in terms of oxygen content, copper formal valence and cation coordination. The influence of the oxygen pressure on the stability of Ba4CaCu3O8+δ is also discussed.

The crystal structures of the two ternary compounds, Ba4CaCu3O8+δ and Ba6CaCu3O10+δ, have been investigated by means of X-ray diffraction combined with Rietveld analysis. We found that the Ba4CaCu3O8+δ phase has a cubic structure (Im–3m, a=8.1515(1) Å, δ=+0.8) for high oxygen content as reported in the literature but undergoes a transformation into a tetragonal structure (I4/mmm, a=8.1888(1) Å, c=8.0634(1) Å for δ=−0.3) when the oxygen content is lowered. The crystal structure of Ba6CaCu3O10+δ (I4/mmm, a=4.0463(1) Å, c=21.7322(4) Å for δ=+0.2) is confirmed. Changes in the c value with sintering conditions suggest a variable oxygen content with no structure transformation.

The temperature dependence of the unit cell parameters of Tl2Ba2Ca2Cu3O10, Au, BaCO3, CuO and Tl2Ca3O6 have been investigated by means of neutron powder diffraction in the 443–1012K range. The measured values are compared to those reported in the literature. For each compound, polynomial fits are given together with the extrapolation to room temperature.

La structure cristallographique et la stabilite des deux composes ternaires Ba 4 CaCu 3 O 8+δ et Ba 6 CaCu 3 O 10+δ ont ete etudiees par diffraction des rayons X, analyse thermique differentielle et analyse thermogravimetrique. Nous trouvons que la phase Ba 4 CaCu 3 O 8+δ a une structure cubique (Im-3m, a = 8.1515(1) A, δ = + 0.8) pour des contenus en oxygene eleves (8.7-9.3) comme cela est reporte dans la litterature mais subit une transition vers une structure tetragonale (14/mmm, a = 8.1888(1) A, c = 8.0634(1) A, δ= - 0.3) lorsque ce contenu est diminue (7.7-8.7). Ces deux composes ternaires sont metastables a basse temperature sous p(O 2 ) = 1 bar. Les courbes d'ATD/TG indiquent en effet un retour a l'equilibre entre 300°C et 400°C qui correspond a une decomposition en BaO 2 , CaO et Ba 2 Cu 3 O 6 .

The Nd(Ba1−xNdx)2Cu3O7+δ solid solution, Nd123ss, has been investigated by neutron powder diffraction and Rietveld analysis. It is confirmed that the crystal structure of its Nd-rich limit, Nd(Ba0.55Nd0.45)2Cu3O7.33, is satisfactorily described in the space group Bmmm (a=7.7679(3), b=3.8542(1), and c=22.9590(9)Å). The fourfold superstructure with respect to the orthorhombic cell of YBCO is due to ordering between Ba and Nd atoms in the bridging layer. Differences with previous works concern exclusively the distribution of O atoms in the ‘chain’ layer. Our results give strong indications that ordering also occurs for lower Nd contents.

The reaction pathway for the formation of Tl2Ba2Ca2Cu3Oz (Tl-2223), a promising compound for applications in passive microwave devices, has been investigated by in situ neutron powder diffraction. The experiments were carried out in an initially evacuated closed system on two samples with compositions Tl1.7Ba2Ca2Cu3Oz and Tl2.3Ba2Ca2Cu3Oz. We find that, under the conditions specified above, the formation path implies Tl2BaO4, Tl6Ba4O13 and Tl-2212 according to the sequence: precursors/oxides → Tl2BaO4→ Tl6Ba4O13 → Tl-2212 → Tl-2223. The difference with respect to the sequence observed when the experiments are carried out in an open atmosphere (precursors/oxides → Tl-2201→ Tl-2212 → Tl-2223) is explained by the low temperature reaction between the TlOx vapour which becomes appreciable at 700 K in closed system and the free BaO resulting from the decomposition of BaCuO2. The conversion from Tl-2212 to Tl-2223 occurs at 800 °C 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. This conversion however does not occur when the Tl content is equal to 2.3 in agreement with previous reports on phase equilibria.

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.

The crystal structure of Ca4.78Cu6O11.60 (crystal system, monoclinic; space group P2/c, Z=4, ρ=4.48(2)g/cm3, a=10.9456(4) Å, b=6.3192(2) Å, c=16.8408(5) Å, and β=104.952(2)°) has been solved and refined using X-ray and neutron powder diffraction combined with Rietveld analysis. It is closely related to the NaCuO2-type structure. The phase stoichiometry and the displacements of atoms with respect to their positions in the previously reported substructure (crystal system, orthorhombic; space group, Fmmm, a=2.807(1) Å, b=6.351(2) Å, and c=10.597(3) Å) are explained by the minimization of Ca–Ca repulsion and by a relaxation toward a more regular octahedral environment for Ca atoms. The substitution of Tl atoms for Ca atoms results in an increased thermal stability and in incommensurate structures with modulation vectors depending strongly on the Tl content.

The pathway for the formation of the thallium-based TlBa 2 Ca 2 Cu 3 O 9 superconductor has been studied by X-ray Diffraction and a-c Susceptibility for samples prepared either by solid-state reaction in air of elemental or mixed oxides or oxidation in oxygen flowing of the metallic TlBa 2 Ca 2 Cu 3 alloy at temperatures determined by DTA. It is shown that independent on the used procedure, the reaction path implies the double-layer Tl-O series, Tl 2 Ba 2 CuO 6 and Tl 2 Ba 2 CaCu 2 O 8 at the temperature of 800°C and Tl 2 Ba 2 Ca 2 Cu 3 O 10 at 875°C. The Tl-1223 phase is formed for temperatures higher than 905°C. This path-way is understood on the basis of phase equilibrium diagram.

Magnetic properties of R2Fe2Si2C (with R = Nd, Tb) have been studied by means of neutron diffraction. An antiferromagnetic behaviour have been evidenced below 15K for the Nd based compound and 45 K for the Tb based compound. The magnetic cell is twice the crystallographic one, with a propagation vector k = (0, 0, 1/2). The main original feature of both structures is the non-colinearity between both R and Fe sublattices, the magnetic moments being nearly perpendicular.

AbstractThe title compounds are prepared by arc melting and induction techniques starting from stoichiometric amounts of the binary compounds.

X-ray diffraction studies of single crystal DyFe2SiC were used to establish the crystal structure, with space group Cmcm and lattice constants a = 3.712(0) Å, b = 10.531(3) Å, c = 6.863(1) Å. The crystal structure is derived from the YNiAl2 type (Re3B-type derivative) with an ordered replacement of aluminium atoms by iron atoms and of nickel atoms by silicon atoms and with an addition of carbon atoms between the infinite columns of silicon centred DyFe2 prisms.