M. Pouchard

Sur deux nouvelles phases oxygenees du cuivre trivalent: LaCuO3 et La2Li0, 50Cu0, 50O4

Resume Two new copper III + compounds have been prepared under oxygen pressure and investigated. The first phase LaCuO3 has a distorded rhomboedral perovskite structure ( a = 5.431 ± 0.002 A ; α = 60°51′ ± 0°02′) and the second phase La2Li0.50Cu0.50O4 has the K2NiF4 structure ( a = 3.731 ± 0.002 A ; c = 13.20 ± 0.02 A ). Whereas LaCuO3 shows metallic behaviour, La2Li0.50Cu0.50O4 is diamagnetic and the properties suggest a low spin d8 state for the Cu3+.

Structural transitions at high temperature in Sr2Fe2O5

Abstract The high-temperature behavior of Sr 2 Fe 2 O 5 has been well characterized by various techniques (DTA, X-ray diffraction, magnetic measurements) paying particular attention to keep the nominal composition constant. The transition from the low-temperature ordered form (with brownmillerite structure) to the high-temperature disordered form (of perovskite structure) has been explained in terms of microdomain formation. A structural model is proposed and discussed.

A new superconductor obtained by electrochemical oxidation of La2CuO4

Abstract The electrochemical oxidation of La 2 CuO 4 in alkaline solution (1N KOH) has been used for preparing the superconducting phase La 2 CuO 4+δ (δ⋍0.07). Crystallographic data show an enhanced orthorhombic distortion and an increase of the unit cell volume. The transport properties reveal a sharp transition from a metallic behaviour to a superconducting state below 44 K which is corroborated by the noteworthy magnetic properties.

Sur de nouveaux composés oxygénés du cobalt +III dérivés de la perovskite

Abstract TCoO3 phases where T is a rare earth (T = Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) have been prepared under high oxygen pressure. LaCoO3 is trigonal (space group R 3 c , D63d). NdCoO3 is pseudo-tetragonal with the orthorhombic symmetry and the GdFeO3 type structure as the following TCoO3 phases. Crystallographic investigations at high temperature, magnetic and electric measurements on YCoO3 and LuCoO3 characterize a transition from the low spin to the high spin state of cobalt +III.

Mise en evidence d'une nouvelle famille de phases de type perovskite lacunaire ordonnee de formule A3M3O8 (AMO2,67)

Abstract Based on the description of the brownmillerite structure, a model of oxygen vacancy ordering in the AMO 3-x phases along the [101] direction of the cubic perovskite cell is proposed. This model is used to describe phases of formulation A n B n O 3−n (n > 2) with a structure intermediate between the brownmillerite and the perovskite type. The phases corresponding to n = 3 (x = 0,33) have been synthetized in Ca 2 Fe 2 O 5 -AMO 3 systems (A = Ca, Sr, Ba, Y, La, Gd ; M = Fe, Ti). Their structure is characterized by a succession along the [010] axis of two sheets of octahedra linked by chains of tetrahedra.

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.

Sur quelques nouveaux composes de structure de type delafossite

Resume The CuCo0.5Ti0.5O2, CuNi0.5Ti0.5O2 and CuNi0.5Sn0.5O2 quaternary oxides have been prepared using direct solid state reaction. AgNi0.5Ti0.5O2 and CuScO2 have been obtained only by exchange reaction. They exhibit a 3R-delafossite type structure. For CuCo0.5Ti0.5O2, a 2H-polytype is also observed. The lattice constant variation with the size of the cation occupying the B-site is similar for both mixed (AB0.5′B0.5″O2) and simple ABO2 delafossites. The evolution with the size of B-site cations of the BO6 or (B′, B″)O6 octahedron D3d-deformation is explained by that of the covalency of the σ(B-O) bonds. No evidence of a B′ B″ ordering has been detected in the materials.

Le cobaltite de strontium Sr2Co2O5 : Caracterisation et proprietes magnetiques

Abstract Stoichiometric strontium cobaltite oxide Sr 2 Co 2 O 5 has been prepared at 910°C in air and characterized by chemical analysis and X-ray diffraction. Temperature dependent magnetic measurements have shown a magnetic transition which is confirmed by DTA and related to a structural modification. At high temperature a brownmillerite-type structure is stabilized by the high spin state of the trivalent cobalt (four free electrons). On the other hand, the magnetic behaviour of the low temperature phase which derives from the 2H-perovskite type may be explained by the simultaneous presence of an intermediary spin state (two free electrons) and a low spin state (no free electron) of trivalent cobalt. These electronic transitions are discussed in connection with the cobalt environment.

Transport and magnetic properties of the superconducting La2CuO4+δ phases (0 < δ < 0.09) prepared by electrochemical oxidation

Abstract A preliminary phase diagram is given for the system La 2 CuO 4+δ , o ≤ δ ≤ 0.09, prepared electrochemically at room temperature. Room-temperature X-ray data distinguish two orthorhombic phase fields, O I for δ M for δ > 0.055. The orthorhombicity of O I decreases with increasing δ, that of O M increases with δ. A spinodal phase segregation into an oxygen-rich superconductive phase and an oxygen-poor antiferromagnetic phase occurs in the temperature interval 220K T ≤ T sp ; below 220 K, the mobility of the interstitial oxygen becomes too low for a further static phase segregation. The phase segregation causes an increase in resistivity and a decrease in the Seebeck coefficient with decreasing temperature in the interval 220 T T sp . A similar transport behavior occurs in the range T c T T ϱ ≤ 120 K in samples 0.02 ≤ δ ≤ 0.04; from comparisons with the La 2− x Sr x CuO 4 system, we suggest that at lower temperatures this behavior is the signature for a further dynamic hole segregation, induced by atomic displacements, into domains more rich in mobile holes within the superconductive phase. The diamagnetic AC susceptibility associated with this superconductivity is independent of the magnetic field below 0.01 mT. In the compositional range 0.06 ≤ δ ≤ 0.09, the Seebeck data indicate a nearly constant concentration of itinerant holes in the superconductive CuO 2 planes. We conclude that in the O M phase oxygen-oxygen interactions within the superconductive layers trap out at oxygen-atom clusters the excess mobile holes Δ P = 2 δ −0.12 from the CuO 2 planes. An H c1 = 64 mT was obtained for δ = 0.09. A step in the resistivity drop at T c signals a temperature interval T c T T on within which the superconductive pairs are confined to small domains. Whether these domains represent a second superconductive phase correlated to additional oxygen ordering that fails to develop with increasing δ or should be considered a pair-fluctuation phenomenon is not resolved.

On magnetic properties of some oxides with delafossite-type structure

Abstract Magnetic properties of ABO 2 (A = Cu, Ag, Pd and B = Cr, Fe) delafossite-type compounds are investigated on the base of a 2D-Heisenberg model using the expansion series method proposed for reciprocal magnetic susceptibility by Rushbrooke and Wood. Exchange integrals have been determined and their evolution correlated to the variation of the Cr 3+ Cr 3+ distance as the A cation varies. 3D-long range ordering which takes place at low temperature has been tentatively determined from the thermal variation of the susceptibility. The investigation leads to the conclusion that interlayer B 3+ B 3+ interactions which occurs through (CuO 2 ) 3− groups are relatively strong in spite of the important structural anisotropy. Magnetic behavior of CuAlO 2 , CuCoO 2 and AgCoO 2 have also been determined and discussed.