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We have investigated the UCo2xGe2+x, system for −0.2x0.5. The new ternary germanide U2Co3Ge5 orders ferromagnetically below Tc = 21 K. UCo2Ge2 shows a narrow range of homogeneity on the cobalt-rich side. The stoichiometric germanide orders antiferromagnetically near TN = 160 K. The lattice parameters and the magnetic ordering of the compounds containing an excess of cobalt depend on their thermal treatment. This fact is proposed as an explanation for the ambivalence in the literature concerning the structural and physical properties of this last germanide.

La2 CuO4 samples have been treated under pure fluorine gas at temperatures TF2 ranging from 20 °C to 300 °C. For TF2 = 300 °C, the starting oxide is completely decomposed in a mixture of LaF3 and CuF2, whereas for TF2 ⩽ 230 °C the resulting product crystallizes in an orthorhombic distortion of the K2NiF4-type structure as La2CuO4 starting oxide. After the fluorination process, the weight uptake of the material, the elemental analysis and a noticeable change of the unit cell parameters corroborate the incorporation of fluorine atoms inside the La2CuO4 lattice. These oxyfluorides are superconductors below Tc = 40 ± 1K and exhibit a metallic behaviour above Tc. On the other hand the three-dimensional antiferromagnetic ordering observed in starting oxide near TN ∼- 250 K appears to be destroyed by the fluorine-gas treatment.

When La2CuO4 is treated with pure F2 gas at 200°C, the X-ray diffraction pattern of the resulting product is characteristic of a new single phase derived from the K2NiF4-type structure. An enhancement of the orthorhombic distortion relative to the starting oxide is observed with: a = 5.342 Å; b = 5.436 Å and c = 13.192 Å. 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 Tc = 40 K with Hc1 ⋍ 700 Oe and exhibits a strong diamagnetic susceptibility (χg = -6.13 × 10-3emu/g) at 6 K under an applied field of 1 Oe

The substituted oxides Bi2Sr2−xLaxCuO6+y and Bi2Sr2Ca1−xYxCu2O8+y isostructural to the n = 1 and n = 2 members of the high Tc superconductors have been prepared. The solid solution Bi2Sr2−xLaxCuO6+y shows a superconducting transition for 0.2–0.25<x<0.5 with an onset Tc near 30K and zero resistance temperatures which depend on x and the heat treatment. For x = 1 the isostructural phases Bi2ALaCuO6.5+y with A = Ca or Ba have also been synthesized and are not superconducting. A modelfor the localization of the oxygen atoms is proposed based on neutron diffraction of Bi2Sr1.6La0.4CuO6+y and Bi2CaLaCuO6+y. The Bi2Sr2Ca1−xYxCu2O8+y solid solution exhibits a superconducting transition for 0<x <0.7. The critical temperature remains almost constant (85–90K) for 0<x <0.3 and then decreases rapidly. The oxygen content value 8.65 for Bi2Sr2YCu2O8+y agrees well with the increase of the positive charge when calcium is replaced by yttrium.

The influence of fluorination on series of superconducting oxides YBa2Cu3O7−δ and Bi4(Ca, Sr)6Cu4O16+δ has been investigated. Fluorine gas treatments carried out at low temperature (20°C⩽TF2⩽100°C) yield the formation of a thin film of (oxy) fluoride (or oxyfluoride) on the surface of the material. This layer has been characterized by X-ray diffraction, electron microprobe analysis and Auger spectroscopy. In the case of YBa2Cu3O7−δ, the experimental conditions to achieve an efficient protection of the ceramics against hydrolysis and gas exchange have been determined. The influence of the fluorination treatments on the superconducting properties has been established from ESR, magnetic and conductivity measurements, in particular the improvement of the shielding effect-Meissner effect cycle.

The basic structure of the new superconducting phases in the Bi or Tl/Ca, Sr and or Ba/Cu/O systems appears to be based on alternating Bi2O2 or Tl2O2 double layers and perovskite (Ca, Sr, Ba)OCuO2 sheets. The discovery of the new series of compounds with the general formula Bi2ALaCuO6.5 with A = Ca, Sr or Ba isostructural to the n = 1 member sets the problem to the exact localization of the oxygen atoms in the sub-lattice. The excess of oxygen in such compounds cannot be explained by a structural type based on the intergrowth of rocksalt layers and stoichiometric perovskite sheets in the case of the n=1 member.

The new oxides Bi2Sr2YCu2O8+y and Bi2Sr2(Ca1−xYx)Cu2O8+y have been prepared. The former compound contains 8.65 oxygen atoms per formula and crystallizes in the orthorhombic symmetry with, as lattice parameters, a = 5.431 Å, b = 5.467 Å and c = 30.22 Å. After heating at 940°C in air, this oxide decomposes with Y2O3 and Bi2Sr2CuO6 as the main products. For 0⩽x⩽0.7, the Bi2Sr2(Ca1−xYx)Cu2O8+y solid solutions exhibit a superconducting transition. The critical temperature remains almost constant (85–90 K) for 0⩽x⩽0.30 and then decreases rapidly.

The atmospheric degradation of high T c superconductors YBa 2 Cu 3 O 7−δ can be prevented using fluorine gas processings around room temperature. A thin layer of amorphous fluorides (and/or oxyfluorides) is formed at the surface of the particles, which passivates the material against hydrolysis and gas-exchange processes. A fluorination mechanism has been proposed, which can account for both fluorine analysis and weight uptake. The superconductivity onset does not seem to be affected by fluorination. Furthermore, an improvement of the shielding effect-Meissner effect cycle to 91% reversibility is achieved when F 2-treated oxides are heated at 350°C under vacuum.

The YBa2Cu3Oy and LaBa2Cu3Oy oxides annealed at 500°C in oxygen or under vacuum have been studied using 57 Fe Mo¨ssbauer spectroscopy. The Mo¨ssbauer spectra have been fitted assuming three symmetric doublets. These results are discussed in relation with electric-field gradient (EFG) calculations.