Jean-Pierre Doumerc

The LixV2O5 system: An overview of the structure modifications induced by the lithium intercalation

Abstract Depending on the amount of lithium ( x ) intercalated in V 2 O 5 several structural modifications are observed. For the smallest lithium contents, the α −( x e −(0.35 x 2 O 5 type structure with an increasing puckering of the layers. For the LiV 2 O 5 composition, a gliding of one layer out of two leads to the δ-type structure. All these reactions are fully reversible and the pristine V 2 O 5 phase is recovered upon deintercalation. For intercalation amounts larger than one, the δ-phase is transformed into a γ-one via an irreversible reconstruction mechanism.the γ−Li x V 2 O 5 phase can itself be reversibly cycled in the 0.0≤ x ≤2.0 range while maintaining the γ-type structure. A new metastable variety of V 2 O 5 has been obtained. The intercalation of the third lithium atom in V 2 O 5 leads irreversibly to the formation of an ω-type phase that exhibits a rocksalt type structure. Almost all the lithium can be electrochemically or chemically removed from this material, leading to a unique positive electrode materials for secondary lithium batteries.

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.

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+ ue5f8Cr 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+ ue5f8B 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.

Electrochemical Oxygen Intercalation into Oxide Networks

The electrochemical oxidation of Sr 2 Fe 2 O 5 brownmillerite-type ferrite and La 2 M O 4 ( M = Ni, Cu) oxides is reviewed. The oxidation process appears as a very powerful method for preparing high oxidation states of transition-metal oxides. The characterization of the reaction products shows that they are well described as intercalation compounds of oxygenated anions. The fact that the intercalated species are the same as the host anions distinguishes these materials from most other intercalated compounds, in which guest species usually are chemically different from the host ions. The question of the nature and site of the intercalated species as well as that of their diffusion into the sample bulk is discussed on the basis of various characterizations and physical measurements as well as within the scope of recent considerations of the electronic distribution in oxides of transition elements in a high oxidation state.

Electrochemical and structural characterization of lithium intercalation and deintercalation in the γ-LiV2O5 bronze

Abstract The γ-LiV2O5 bronze has a layer-like structure. Provided that x does not exceed 2, it can insert reversibly lithium without major structural rearrangement but with formation of a new ζ-phase for large x-values. Deintercalation of lithium from γ-LiV2O5 at oxidation potentials larger than 3.5 V leads to formation of a new orthorhombic γ-phase. Complete deintercalation is achieved at a potential of 3.65 V giving rise to a new form of vanadium pentoxide.

Crystal growth and electrical properties of CuFeO2 single crystals

Abstract Delafossite-type CuFeO2 single crystals have been prepared by a flux method: crystals obtained in a Cu crucible with LiBO2 as flux are n-type whereas those prepared in a Pt crucible with a Cu2O flux are p-type. Electrical measurements have revealed that n-type crystals exhibit weak anisotropic conductivities with large activation energies and small mobilities (r.t. values perpendicular and parallel to the c-axis: μ⊥ = 5 × 10−5 and μ‖ = 10−7 cm−2 V−1 sec−1). p-type crystals, less anisotropic, are characterized by low activation energies and higher mobilities (μ⊥ = 34 and μ‖ = 8.9 cm2 V−1 sec−1). A two -conduction-band model is proposed to account for the difference observed between the energy gap value deduced from photoelectrochemical measurements and the activation energy of the electrical conductivity in the intrinsic domain.

Photoelectronic processes in transition-element doped n-type TiO2 electrodes

Abstract Photoelectronic properties of n-TiO2 anodes doped with first row transition elements and giving an absorption in the visible region, have been investigated. The photocurrent has been measured vs wavelength for a given electrode potential and vs time at different anode potentials. Significant photocurrents resulting from visible light excitation are observed for n-type TiO2 anodes doped with large amounts of trivalent chromium. When other transition elements (V3+ or Mn2+) are considered, such a long wavelength photoresponse was not observed. Co2+ and Ni2+ ions could not be introduced into n-TiO2 anodes because of the reduction into Co or Ni metal. The experimental data have been discussed on the basis of a model involving hole tunneling from Cr4+ ions to the valence band.

Photoconduction mechanisms in titanium and rare earth n-type semiconducting electrodes with pyrochlore and perovskite structures

Abstract The photoelectrochemical properties of Ln2Ti2O7 and Ln2/3TiO3 pyrochlore and perovskite anodes have been investigated (Ln = La, Pr, Nd). The pyrochlore samples have a smaller electron affinity but a larger band gap than the perovskite compounds. These results allow an accurate determination of the band energy schemes of the materials.

Sodium Ion Mobility in NaxCoO2 (0.6 < x < 0.75) Cobaltites Studied by 23Na MAS NMR

Various P2 and P3-Na(x)CoO(2) phases, with x ranging approximately from 0.6 to 0.75, have been studied by variable-temperature (23)Na magic angle spinning (MAS) NMR. Signal modification versus temperature plots clearly show that Na(+) ions are not totally mobile at room temperature on the NMR time scale. As the temperature increases, the line shape change of the (23)Na MAS NMR signal differs for the P2 and P3 stackings and is interpreted by the differences of Na(+) ion sites and of sodium diffusion pathways in the two structures.