Oxygen ordering and superconductivity in La(Ba2−xLax)Cu3O7+δ
The observation of superconductivity above 90 K in the Y–Ba–Cu–O system1,2 has prompted a search for other structures that display high-temperature superconductivity. The discovery of superconductivity above 70 K in La3−xBa3+xCu6Ol4+y3 raises some questions concerning its relationship to the YBa2Cu3O7−δ structure and the origins of the high-temperature superconductivity. The original X-ray diffraction study of La3Ba3Cu6O14+y, indicated a tetragonal structure4,5, on the basis of which it has been proposed that La3−xBa3+xCu6O14+y is a new class of high-temperature superconductor which, in contrast to YBa2Cu3O7−δ, contains no Cu–O chains3. More recently, a neutron diffraction study of non-superconducting La3Ba3Cu6O14+y6has indicated that the correct structure of this compound is simply the tetragonal YBa2Cu3O7−δ structure with partial substitution of lanthanum on the barium site7–12. In the present neutron powder diffraction study of a series of La(Ba2−xLax)Cu3O7+δ compounds, we show that the superconducting phase is a similarly disordered isomorph of the orthorhom-bic YBa2Cu3O7−δ structure13–19. The region of solid solubility for this compound extends from x = 0.5 (La:Ba:Cu ratios of 3:3:6) to x≈ 0.25 and does not include the stoichiometric compound x = 0 (LaBa2Cu3O7−δ). The presence of Cu–O chains, and not just a formal copper charge state of ≥2, is vital for the occurrence of superconductivity.
Structural properties of the superconductor LaBa2Cu3-yO7-z in the solid solution system La1+xBa2-xCu3-yO7-z.
Etude par diffraction de neutrons par la methode du temps de vol (TOF), de quatre echantillons, a la temperature ambiante. Les structures cristallines moyennes sont affinees par l'analyse Rietweld qui inclut le raffinement des ocupations des atomes La et/ou Ba. Les structures obtenues sont essentiellement isomorphes avec la structure triperovskite deficiente en oxygene o (groupe d'espace Pmmm). Etude de l'ordre autour des sites La, de la valence des sites Cu et des etats de liaison. Variations du parametre de reseau. Il est significatif d'analyser les donnees de diffraction pour la caracterisation du materiau et l'etude des proprietes physiques des oxydes supraconducteurs dans ces systemes de solutions solides
Crystal Structure of a Ba-Y-Cu-O Superconductor as Revealed by Rietveld Analysis of X-Ray Powder Diffraction Data
The crystal structure of a Ba-Y-Cu-O superconductor was refined by the Rietveld analysis of X-ray powder diffraction data. This material is orthorhombic with lattice parameters a = 3.8845 Å, b = 3.8293 Å, and c = 11.6926 Å. Possible space groups are P222, Pmm2, and Pmmm. Metal atoms have essentially the same configuration as that in the perovskite-type structure. A structural model is proposed in which an oxygen atom at (0, 0, 1/2) is deficient.