D. Pelloquin

MAGNETORESISTANCE IN THE OXYGEN DEFICIENT LNBACO2O5.4 (LN=EU, GD) PHASES

New “112” phases, LnBaCo2O5.4, with an ordered oxygen deficient perovskite structure, derived from the YBaFeCuO5-type were studied for Ln=Eu, Gd. The appearance of giant negative magnetoresistance in this structural type is demonstrated. Resistance ratio R0/R7 T reaches at least 10 at 10 K, i.e., is significantly larger than those observed in the other cobalt perovskites, such as La1−xSrxCoO3. These properties are linked to an original magnetic behavior of these materials that exhibit two types of transition—antiferromagnetic to ferromagnetic, and ferromagnetic to paramagnetic—as T increases. This magnetic behavior may be related to a possible I.S. and L.S. spin ordering of trivalent cobalt in pyramidal and octahedral coordinations, respectively.

Magnetoresistance and magnetothermopower properties of Bi/Ca/Co/O and Bi(Pb)/Ca/Co/O misfit layer cobaltites

Two new compounds of the Bi/Ca/Co/O and Bi(Pb)/Ca/Co/O systems have been prepared. Their structure is built up from the intergrowth of four rock-salt-type layers and one [CoO2] hexagonal layer. Both cobaltites exhibit large thermopower values (S-300 K similar to 140 muV K-1), low resistivity values (rho(300) (K) 40-60 mOmega cm) and small thermal conductivities (kappa(300) (K) similar to I W K-1 m(-1)). Furthermore, these compounds exhibit a negative magnetoresistance, (MR = rho(H)-rho(H=0)/rho(H=0)), reaching, at 2.5 K, -85% in 7 T for the Bi/Ca/Co/O misfit cobaltite. A large negative magnetothermopower is also found for these cobaltites in the same temperature range. A qualitative explanation of the observed behaviour is proposed.

Perovskite manganites and layered cobaltites: potential materials for thermoelectric applications

Abstract Metal-transition oxides are potential materials for thermoelectric devices operating at high temperatures ( T ≫300 K). In particular, compounds exhibiting small resistivity and large Seebeck coefficient are required. Two kinds of oxides have been retained: the ‘misfit’ cobaltites which are p-doped metallic phases and the electron doped perovskite manganites. For the former, it is found that the Ca for Sr substitution enhances the thermopower values in the Pb/Sr/Co/O misfit cobaltite whereas it has only a moderate effect in the case of the Tl/Sr/Co/O phase. From this study, it appears that the oxidation state of Co in the conducting layers plays a crucial role on the physical properties. Moreover, for the manganites, we show that electron-doped SrMnO 3 can be prepared by a two-steps method. The doping, created via substitution at either the A-site (Pr 3+ for Sr 2+ ) or B-site (Mo 6+ for Mn 4+ ), leads to metallic compounds with large thermopower values.

A 94 K Hg-based superconductor with a “1212” structure Hg0.5Bi0.5Sr2Ca1−xRxCu2O6+δ (R = Nd, Y, Pr)

Abstract A new mercury-bismuth-based cuprate, Hg 0.5 Bi 0.5 Sr 2 Ca 1− x Nd x Cu 2 O 6+δ has been syntahesized for x ranging fro m 0.35 to 1. The high-resolution electron microscopy and the X-ray diffraction analyses give evidence for a 1212-type structure, without any ordering between Hg and Bi. For 0.35⩽ x ⩽0.5, the samples are superconducting with a T c conset of 94 K and a diamagnetic volume fraction of 32%. Annealings at low temperatures under different atmospheres do not modify the superconducting behavior. Substitutions of Pr and Y for Nd allow isotypic oxides to be synthesized with similar superconducting properties.

Thermopower enhancement in misfit cobaltites

The isovalent substitution of Ca2+ for Sr2+ substitution in the recently discovered misfit cobaltite [Pb0.7Co0.3Sr2O3] [CoO2]1.79 has been investigated for its very attractive thermopower properties. Starting from the pure Sr cobaltite with a thermopower S value at 300 K in the range from 100 to 130 μV K−1, it is found that the calcium substitution enhances the thermopower value up to +165 μV K−1 for the pure calcium phase, [Pb0.4Co0.6Ca2O3] [CoO2]1.61. Since these thermoelectric materials are studied for the high T applications, it should be pointed out that this S improvement induced by chemical substitution is even more pronounced for T≫300 K with S700 K=+210 μV K−1. This beneficial effect is interpreted in the light of the clear modification in the geometry of CoO2 layers which could play a role on the cobalt electronic configurations.

A new “1201” mercury cuprate The 27 K-superconductor Hg0.5Bi0.5Sr2−xLaxCuO5−gd

Abstract A new superconductor, Hg 0.5 Bi 0.5 Sr 2− x La x CuO 5− δ with a T c onset of 27 K has been isolated for 0≤ x ≤0.75. This phase crystallizes in the tetragonal system with a ≈ a p ≈3.8 A and c ≈8.9 A. Its X-ray diffraction and high resolution electron microscopy analysis show that it belongs, like the 94 K-superconductor HgBa 2 CuO 4+ δ , to the “1201” series. Its structure can be described as the intergrowth of single perovskite layers with double oxygen deficient rock salt layers built up from mixed layers [Hg 0.5 Bi 0.5 O 1− δ ]∞. This unique “Sr-Hg-1201” cuprate opens the route to the exploration of “mixed” superconductors resulting from the substitution of mercury for different metallic cations such as Tl, Bi, Pb in the “normal” layers that ensure the junction between the superconducting copper layers.

On the strong impact of doping in the triangular antiferromagnet CuCrO2

Abstract Electronic band structure calculations using the augmented spherical wave method have been performed for CuCrO2. For this antiferromagnetic ( T N = 24 K ) semiconductor crystallizing in the delafossite structure, it is found that the valence band maximum is mainly due to the t2g orbitals of Cr3+ and that spin polarization is predicted with 3 μ B per Cr3+. The structural characterizations of CuCr1−xMgxO2 reveal a very limited range of Mg2+ substitution for Cr3+ in this series. As soon as x = 0.02 , a maximum of 1% Cr ions are substituted by Mg site is measured in the sample. This result is also consistent with the detection of Mg spinel impurities from X-ray diffraction for x = 0.01 . This explains the saturation of the Mg2+ effect upon the electrical resistivity and thermoelectric power observed for x > 0.01 . Such a very weak solubility limit could also be responsible for the discrepancies found in the literature. Furthermore, the measurements made under magnetic field (magnetic susceptibility, electrical resistivity and Seebeck coefficient) support that the Cr4+ “holes”, created by the Mg2+substitution, in the matrix of high spin Cr3+ ( S = 3 / 2 ) are responsible for the transport properties of these compounds.

The great ability of mercury-based cuprates to accommodate transition elements eleven new superconductors Hg1−xMxBa2Cam−1CumO2m+2+δ (M V, Mo, W, Ti, Cr)

Abstract The HgBaCuMO and HgBaCaCuMO systems have been investigated for MV, Mo, W, Ti, Cr. Eleven new superconductors Hg1−xMxBa2Cam−1CumO2m+2+δ have been synthesized at normal pressure with the 1201 structure for MTi, Mo, W and Cr, with the 1212 structure for MMo, W, Ti and Cr, and with the 1223 structure with MV, Mo, Ti and Cr. The XRD and ED investigation of these oxides shows that the a parameter of their tetragonal cell is not strongly influenced by the presence of the transition element, whereas the c parameter is significantly decreased with respect to HBC and HBCC systems, due to the partial replacement of Hg by M. The structural anisotropy in the three series of oxides, represented by the c/a ratio varies in the following way: (c/a)Cr

Single crystals of the 96 K superconductor (Hg,Cu)Ba2CuO4+δ: growth, structure and magnetism

Abstract Single crystals of the 1201 ( n = 1) (Hg,Cu)Ba 2 CuO 4+δ mercury based cuprate have been grown by using a simple process without dry box. The as-synthesized crystals exhibit constant T c (onset) of 96 K with sharp superconducting transitions. The electron microscopy coupled with EDX analyses evidence a “1201”-type structure while a mercury deficiency is observed balanced by an excess of copper. The structural refinements based on single-crystal X-ray diffraction data confirm the electron deficiency on the Hg site (0,0,0) and show a splitting of the latter along the c axis correlated to the partial substitution of Cu for Hg. This structural study leads to the following formula Hg 0.84 Cu 0.16 Ba 2 CuO 4.19 . The magnetic study of a large crystal (1.1 × 0.38 × 0.065 m 3 ) shows that the (Hg,Cu)-1201 crystals exhibit an irreversibility line higher than that of the 1201 Hg 0.8 Bi 0.2 Ba 2 CuO 4+δ crystal ( T c = 75 K). From the reversible magnetization, a λ ab (0) = 2470 A value can be extrapolated. Using a 3D-2D decoupling formula, we obtain γ = 29 for the electronic anisotropy of this phase.

Electronic structure and thermoelectric properties of CuRh 1 − x Mg x O 2

Electronic structure calculations using the augmented spherical wave method have been performed for