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.
Oxygen Disorder Effects in High-Tc Superconductors
It is now well established that oxygen plays a crucial role in the structural, magnetic and transport properties of high Tc materials. We present here a brief review of the effect of the oxygen deficiency and vacancy ordering on the superconducting and magnetic properties of various superconductors.
Sur une Nouvelle Phase Oxygénée du Cuivre + III: SrLaCuO4
A new copper (+III) compound SrLaCuO 4 has been prepared under oxygen pressure and investigated. It has the K 2 NiF 4 structure ( a = 3.765 ± 0.002 A c = 13.27 ± 0.02 A). A comparison with La 2 Li 0.50− Cu 0.50 O 4 suggests a low-spin d 8 state for copper +III, but with associated Cu O bonding that is linear rather than square-coplanar. A cooperative displacement of the oxygen ions is inferred, extensive short-range order occurring within planes, but no order between planes.