B. Chevalier

Stabilization of a new superconducting phase by low temperature fluorination of La2CuO4

Abstract When La 2 CuO 4 is treated with pure F 2 gas at 200°C, the X-ray diffraction pattern of the resulting product is characteristic of a new single phase derived from the K 2 NiF 4 -type structure. An enhancement of the orthorhombic distortion relative to the starting oxide is observed with: a = 5.342 A; b = 5.436 A and c = 13.192 A. Both weight uptake and increase of the c unit cell constant could be consistent with an incorporation of fluorine atoms in the lattice. This new compound is superconducting at T c = 40 K with H c1 ⋍ 700 Oe and exhibits a strong diamagnetic susceptibility ( χ g = -6.13 × 10 -3 emu / g ) at 6 K under an applied field of 1 Oe

Superconductivity of La2CuxO4 − y

Abstract Bulk superconductivity is present in La 2 Cu x O 4 − y for 0.98 ⩽ x ⩽ 1.04 and for an y value which is strongly reduced by annealing under high pressure of oxygen. Two superconducting temperatures are observed T 2 and T c ; the first one, T 2 = 37 K, corresponds to the resistivity fall; the second one, T c , corresponds to the appearance of diamagnetism. T 2 and the critical fields of samples stay unchanged whatever the normal state resistivity is, even if it varies by a factor 40000 from a semi conducting to a metallic state. Then a conventional mechanism of pairing is unconceivable. The existence of a bump in the resistivity at T = 220 K of samples which undergo a superconducting transition suggest a possible correlation between coherence effects among copper spins and superconductivity.

Superconductivity in chlorine-treated La2CuO4

Abstract A superconducting transition is observed in La2CuO4 treated under chlorine pressure. The resulting compound obtained at 300°C and for Pcl2 ≈ 6 bar shows an X-ray diffraction pattern which can be indexed in an orthorhombic symmetry with the following unit-cell parameters: a = 5.354 A , b = 5.378 A and c = 13.190 A . The diamagnetic susceptibility of ZFC samples is about −1×10−3 emu g−1 at 5 K under 100 Oe applied magnetic field and the superconducting transition occurs at 32±1 K. Parasitic phases, present after chlorine treatment, are easily removed by a washing process which does not alter the critical temperature of the sample. When air-annealed above 150°C, the Cl2-treated compound is irreversibly transformed into stoichiometric La2CuO4 with a consequent loss of superconductivity. Similarities with high oxygen pressure treatments have been pointed out.

EXAFS study at the La LIII X-ray absorption edge of superconducting materials obtained by fluorination of La2−xSrxCuO4 oxides (0≤x≤0.15)

Abstract The influence of a fluorine-gas treatment on both structural and superconducting properties of the La 2− x Sr x CuO 4 (0≤ x ≤0.1 5) solid solution has been studied. This treatment leads to the occurence of superconductivity at 40 K and 36 K for x =0 and x =0.02, respectively. In contrast, only a small change of the critical temperature is observed for x =0.10 and x =0.15 after fluorination of the compounds. Such a behaviour is consistent with the increase of the hole concentration arising from the incorporation of anionic species into the lattice. The study of the F 2 -treated La 2 CuO 4 by neutron diffraction reveals that the occupancy rate of the anionic interstitial site O(4))( 1 4 , 1 4 , 0.2646) is n =0.089(6), corresponding to the formula La 2 Cu(O,F) 4, 18 for the resulting oxyfluoride. The analysis of the EXAFS spectra recorded at the La L III -edge shows an increment of the anionic species around lanthanum ions.

Transport and Magnetic-properties of U(2)m(3)si(5) Silicides (m=co,Rh,Ru)

We report and discuss experimental data on the electrical resistivity p, thermoelectric power S, thermal conductivity kappa, specific heat C and magnetic susceptibility chi of polycrystalline U(2)M(3)Si(5) silicides (M = Co, Rh, Ru). The magnetic and transport properties of these compounds depend strongly on the nature of the transition metal M. In U2Co3Si5, p(T) shows features that could be identified to Kondo systems while S(T) shows a two-peak structure. U2Rh3Si5 shows an antiferromagnetic ordering at T-N similar to 25 K, which is accompanied by steep changes in both p(T) and S(T). U2Ru3Si5 behaves like a nonmagnetic compound and displays distinct anomalies in the low-temperature domains of p(T, H) and S(T, H). The lattice thermal conductivity varies as T-2 at low temperatures, suggesting a dominant scattering of the phonons by the conduction electrons.

Anionic Intercalation in La2cuo4 Oxide By Fluorine Or Chlorine Treatment

The influence of a halogen-gas (Cl-2 or F-2) treatment on both structural and physical properties of La2CuO4 has been studied. In ail cases, the resulting product contains an excess of anionic species and shows superconducting properties below 32K and 40K respectively for the chlorine- or fluorine-treated sample. But these treatments have a different influence on the evolution of the: structural properties comparatively to those observed for the starting oxide : after chlorination a decrease of the orthorhombic distortion is detected, whereas an opposite trend is shown after fluorination. The Cl-2-gas treatment leads to an oxygen-excess phase La2CuO4.08, whereas the fluorination yields an oxyfluoride La2Cu(O,F)(4.17).

Occurrence of Superconductivity in La2-xbaxcuo4 (0-less-than-or-equal-to-x-less-than-or-equal-to-0.04) and La2-xndxcuo4(0-less-than-or-equal-to-x-less-than-or-equal-to-0.3) Systems After Fluorination

The influence of a low temperature fluorine gas treatment on structural, superconducting and transport properties of oxides with the K2NiF4 structure has been studied. When materials with the composition La2-xBaxCuO4 where 0 less-than-or-equal-to x less-than-or-equal-to 0.04 are treated under 1 bar of F2 gas at 200-degrees-C, an increase in the orthorhombic distortion (b - a) is observed and the materials become superconductors. The insertion of anionic species in the lattice is confirmed by the observation of an increase in weight and in the c-parameter and by the elemental analysis. After annealing at 200-250-degrees-C under vacuum or argon atmosphere, La2-xBaxCu(O,F)4+delta oxyfluorides lose their superconducting properties. This change is accompanied by a structural orthorhombic-to-orthorhombic transition for values of x up to 0.02 and by an orthorhombic-tetragonal-orthorhombic sequence where x = 0.04. Similar behaviour is also observed in the T/O solid solution in the La2-xNdxCuO4 SYStem (0 less-than-or-equal-to x less-than-or-equal-to 0. 3).

Correlations between fluorination and transport properties in the La1.85−xNdxSr0.15CuO4 system

The influence of fluorination on ceramic samples of La1.85-xNdxSr0.15CuO4 solid solutions has been studied. The presence of small amounts of fluorine species in the interior of the grains which has been evidenced by Auger electron spectroscopy significantly influences the structural and transport properties. It has been shown that the intercalation of fluorine into the structure reinforces the metallic-like behaviour and superconducting properties by hindering the tendency to the low-temperature structural transition and by increasing the charge carrier density.

Correlations between superconducting properties and chemical homogeneity of samples in the La1.85 - xNdxSr0.15CuO4 system

In the La1.85-xNdxSr0.15CuO4 system, phases deriving from the K2NiF4-type structure are found in the 0 less-than-or-equal-to x less-than-or-equal-to 0.98(5) composition range. Their superconducting properties are strongly influenced by the presence of chemical microinhomogeneities revealed by microprobe analysis and are dependent on the synthesis method, as discussed in more detail for x = 0.6. Electrical and magnetic measurements performed on samples of this system show that bulk superconductivity is observed only for x less-than-or-equal-to 0.4. The occurrence at low temperature of a structural transition explains the disappearance of the superconducting properties. This change induces a peculiar behaviour in the transport properties of these oxides.

Thermal-conductivity enhancement in copper oxide compounds

An enhancement of the in-plane thermal conductivity in the superconducting state is observed in most high-T-c superconductors. Here the observation of a similar enhancement of the thermal conductivity in copper-oxide compounds, which is not related to the superconducting transition, is reported. Since this anomaly is also accompanied by a significant variation of the electrical conductivity it indicates that a strong electron-phonon coupling exists and that thermal-conductivity enhancements in copper-oxide compounds should be attributed to an increase of its lattice component.