P. Dordor

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.

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.

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.

Magnetic and transport properties of pure and carbon‐doped divalent RE hexaboride single crystals

Single crystals of pure and carbon‐substituted divalent hexaborides EuB6 and YbB6 have been prepared by the Al flux method. Lattice parameters, density measurements, and microprobe analysis show that the single crystals are very close to stoichiometry. EuB6 is ferromagnetic (TC=12.5 K and ϑp=15 K) whileYbB6 is diamagnetic. Resistivity and Hall effect measurements carried out over a wide temperature range show that both pure hexaborides are semiconductors with a small intrinsic gap. The carbon‐substituted hexaborides RE B6−xCx (RE=Eu, Yb) have a metallic behavior due to the donation of electrons to the conduction band by carbon atoms. Metallic conduction, at low temperature, in the ’’pure’’ hexaborides is attributed to the formation of an impurity band containing more than 1018 electrons per cm3.

Sur la stabilisation dans un reseau oxygene du nickel (+III) a spin fort en octaedrique

Abstract The authors show that increasing the size of the octahedral site in K 2 NiF 4 − and perovskite-type oxides favours the high spin configuration of Ni (+III). This electronic configuration appears for the first time in BaLaNiO 4 and Sr 2 NbIn 0 . 9 Ni 0 . 1 O 6 thanks to the size increase brought about by the presence in the lattice either of baryum or indium. A low spin-high spin transition is observed at rising temperature by an EPR investigation and discussed in terms of covalency and symmetry.

Growth and electrical properties of pure and Ni-doped Co3O4 single crystals

Abstract Single crystals of pure and Ni-doped Co 3 O 4 spinel-type phases have been grown by C.V.T. Electrical conductivity and thermopower measurements have been performed between 25 and 950 K. Ni-doping gives rise to a significant increase of electrical conductivity at room temperature by three orders of magnitude. The obtained results suggest that the Ni atoms are located in the octahedral sites of the spinel lattice with both oxidation states 2+ and 3+, as illustrated by the formula : Co 2+ 1−y Co 3+ y [Co 3+ 2−x Ni 2+ y Ni 3+ x−y ]O 4 .

Thermopower analysis of substituted nickel manganite spinels

Abstract Information about the distribution of cations between tetrahedral (A) and octahedral (B) sites in the spinel structure of NiMn2O4 and some substituted spinel phases are derived from thermopower measurements in connection with some ion distribution models. The cubic spinel NiMn2O4 is characterized by a high degree of inversion coupled with disproportionation of MnIII with formation of MnII on tetrahedral and MnIV on octahedral sites. Spinels of the series LizNiMn2−zO4 show increased concentrations of MnIV on the B-sites and the MnII content on the A-sites is reduced. FeNiMnO4 is dominated by partial Fe octahedral site occupation connected with Mn disproportionation.

Conduction mechanism in polycrystalline Na1−xSrxNbO3 niobium bronzes

Abstract Electrical conductivity and thermoelectric power of polycrystalline Na1−xSrxNbO3 have been measured between 5 and 300 K for various compositions. The results suggest that the density of states is finite at the Fermi level, whilst the carrier mobility is activated. The observed behavior is attributed to an Anderson localization of states at the Fermi level due to disorder resulting from either a random distribution of Sr2+ and Na+ ions or from the polycrystalline character of the samples or from both effects.

Transport properties and magnetic behavior in the polycrystalline lanthanum-deficient manganate perovskites (≈La1−xMnO3)

Abstract The rhombohedral lanthanum manganates ≈La1 − xMnO3 (x = 0.0, 0.05, 0.1) were prepared by a soft chemistry route to test first as catalysts. These three weak ferromagnets exhibit a structural transition at Tt slightly below Tc. ≈La0.9MnO3 undergoes a group-subgroup R3c ↔ R3c transition most likely of second order. ≈LaMnO3 does not show any insulator-to-metal transition, but does show a semiconductor-semiconductor at Tt ~ 130 K. Analysis of the transport data (conductivity and thermopower) indicates hopping of polarons at both low and high temperature. An effective magnetic moment of 3.50 μB/Mn is deduced from the spontaneous magnetization value at 5 K. The slight difference from the spin-only value (3.67 μB) is assigned to a spincanting effect which is reflected in the divergence of the FC and ZFC susceptibility curves at low fields (H ≤ 10−2 T) and low temperature. Compared with recent results on thin films, the compositions x = 0.05, 0.1 are expected to show magnetoresistive effects as well as a semiconductor-to-metal transition. If it is verified on bulk samples which all contain a similar Mn4+ content of 35%, the structural transition at Tt could be considered as magnetically driven.

Influence of structural defects on the electrical properties of α-Fe2O3 ceramics

Abstract The electrical properties of α-Fe 2 O 3 ceramics depend strongly upon the nature of the precursor and on the sintering conditions. The ceramics obtained from fresh precursors are characterized by the presence of extended defects, which tend to order as the sintering goes on. The p -type conductivity of those ceramics is mainly due to the presence of cationic impurities. The appearance of localized defects occurs during the sintering period of aged precursors. As these defects are associated to donor states, a predominant n -type conductivity is observed in the corresponding ceramics.