Mona Mouallem-Bahout

Re-examination of the non-stoichiometry and defect structure of copper(II) oxide or tenorite, Cu1±zO or CuO1±ϵ: A short review

Abstract Four sets of compositional data with a departure z or ϵ from the stoichiometry of tenorite Cu 1± z O or CuO 1± ϵ are re-examined for possible point defects. Correlations z ( p O2 ) at equilibrium solid⇔pure di-oxygen – when they can be computed – indicate the presence of either oxygen vacancies or copper interstitials or clusters of them. Some recent data after quenching are in favor of copper interstitials. Few data concerning tenorite prepared by decomposition of a salt, by oxidation of copper and by CVT in sealed tubes have generally been interpreted in the literature as indicating copper deficiency. In those oxides a large departure from the stoichiometry indicates a complex behavior likely due to several point defects possibly ordering at short and/or long range, and including chemical impurities in the case of sealed tube preparation. It is, however, in fact likely that the defect structure of such samples involves the two sub-lattices. Cu 3 O 2 - or Cu 4 O 3 -like clusters can even be envisaged.

Anisotropic thermal expansion and Grüneisen coefficients of the bismuth cuprate 2-2-1-2

Abstract The structural evolution of the superconductor Bi 2 Sr 2 CaCu 2 O 8 has been analyzed by X-ray diffraction measurements of the cell parameters a and c between 5 and 220 K. A tetragonal modeling already applied to YBa 2 Cu 3 O 7 is used in order to characterize the evolution of the anisotropic thermo-elastic properties of this bismuth cuprate by means of the Debye model and Gruneisen coefficients. Similarly to the case of YBa 2 Cu 3 O 7 , a strong peak γ a ≈ 2.8 at 45 K for the Gruneisen coefficient in the a , b plane is evidenced ( γ c = 0.85). The mean value γ a = 1.2 is determined at 220 K. The evolution of γ a ( T ) indicates that microstructural effects might induce an anomalous expansion below T c .

Peculiar magnetic properties of (Dy,Ca)MnO3

The magnetic properties of the Dy12xCaxMnO3 solid solution (0 # x # 0.60) have been studied as a function of temperature and magnetic field. A paramagnetic state, with the Curie – Weiss temperature Q changing from 2 25 to þ14 K, is observed from x ¼ 0t ox ¼ 0.3. At higher calcium concentration, charge ordering effects seem to dominate. Dy – Dy interactions present in the pure DyMnO3 manganite (TN ¼ 9.5 K) persist in the solid solution up to x ¼ 0.3. The Mn sublattice orders ferromagnetically at Tc , 80 K. The ZFC/FC magnetization cycles show a ferrimagnetic-like system, in which negatively-polarized dysprosium moments behave as free spins (T . TN) under the internal field of the ordered Mn sublattice. The local field at a given Dy site depends on the exchange (negative) interaction between both sublattices, resulting in a reversal of the magnetization when the magnetic moment of the Dy sublattice exceeds the one of the ferromagnetic network. q 2002 Elsevier Science Ltd. All rights reserved.

Magnetic and structural characterization of new alkaline copper iron sulfide Rb2Cu3FeS4

Abstract Rb2Cu3FeS4 has been studied by Mossbauer spectroscopy and NPD, as a function of temperature. The Mossbauer spectra show two components corresponding to two different iron environments. The first one orders around 150 K and is assigned to an isotropic Fe(III) surrounding, suggesting that all four of the nearest tetrahedral positions are occupied by iron ions. The second component orders around 50 K and is assigned to an inhomogeneous occupation of the neighboring sites. As a contrast to the Mossbauer data, no chemical order between Cu and Fe could be observed via NPD owing to the low contrast between the scattering amplitudes of those two elements. The crystal structure, refined with a mean transition element, is of ThCr2Si2-type (space group I4/mmm) consistent with the previous X-ray characterization. Antiferromagnetic ordering is observed below TN≈150 K, with a magnetic unit cell 4a×4a×c, whereas no anomaly could be detected in the neutron powder diagrams around 50 K.

Two Quaternary Sulfides Rb2Cu3FeS4 and Rb2Cu3MnS4 Magnetic Interactions and Electrical Behavior

The two quaternary transition metal chalcogenides Rb2Cu3 MS4 (M = Fe, Mn) are based on a common structural pattern consisting of two-dimensional (2D) arrangements of sulfur tetrahedra linked by edges, with the ThCr2Si2-type structure. They are prepared from alkali metal carbonate Rb2CO3 and transition metal oxides CuO, Fe2O3 or MnO by CS2/Ar sulfurization at 850°C  T  900°C. The magnetic susceptibilities are directly measured as a function of the temperature. Rb2Cu3MnS4 presents a magnetic transition from a high spin (Mn2+) to the low spin state (μeff ≍ 1.5 μ B ), while the iron analog Rb2Cu3FeS4 orders antiferromagnetically at T N ≍ 6K under an applied field of one kGauss.

Microstructural, inductive and electrical characterizations of granular Bi2Sr2CaCu2O8/Pb or PbO composites

Abstract Granular composites made from the high T c superconductor Bi-2212 and lead are prepared by two ways, whether pressing and heating just above the melting temperature of Pb (as called pressed composites) or pressing and heating at medium temperature near 650°C (as called sintered composites). In this latter case, a large reaction occurs during the sintering between Pb, PbO and the ceramics, as the microstructural study shows. Inductive measurements give evidence of the existence of an imaginary part in the a.c. susceptibility, which is the indication of a coupling between grains and the percolation of electronic currents through the sample. The percolation threshold occurs for a composition Bi-2212 vol.%≈25, as assessed by magnetization measurements. From the electrical response, it is established that pressed composites exhibit a semiconducting electrical behaviour in contrast to similar Bi-2212/tin composites previously studied. No 3D-percolation threshold of the normal conduction by lead is observed. However, for the sintered composites, the superconducting transition exists but it is not complete and is interrupted by a semiconducting behaviour on the surface of grains, which becomes predominant below 45 K upon cooling.