M.R. Nunes

Effects of synthesis method on stoichiometry, structure and electrical conductivity of CaMnO3−δ

Abstract CaMnO3−δ perovskite samples were synthesised by two different methods: conventional solid-state reaction and the citrate route involving autoignition. The formation of a perovskite phase was significantly influenced by the synthesis and processing conditions: samples obtained by the citrate route have a less distorted structure. These materials are always more oxidised than those prepared by the ceramic route and consequently have a higher Mn+ content. Moreover, they present the highest electrical conductivity values, which is explained by a less distorted structure.

Magnetic properties of Co-doped TiO2 anatase nanopowders

This letter reports on the magnetic properties of Ti1−xCoxO2 anatase phase nanopowders with different Co contents. It is shown that oxygen vacancies play an important role in promoting long-range ferromagnetic order in the material studied in addition to the transition-metal doping. Furthermore, the results allow ruling out the premise of a strict connection between Co clustering and the ferromagnetism observed in the Co:TiO2 anatase system.

Titanium dioxide anatase as matrix for matrix‐assisted laser desorption/ionization analysis of small molecules

The use of inorganic species as assisting materials in matrix-assisted laser desorption/ionization (MALDI) analysis is an alternative approach to avoid interfering matrix ions in the low-mass region of the mass spectra. Reports of the application of inorganic species as matrices in MALDI analysis of small molecules are, however, scarce. Nevertheless, titanium dioxide (TiO(2)) powder has been reported to be a promising matrix medium. In this study we further explore the use of TiO(2) as a matrix for the MALDI analysis of low molecular weight compounds. We present results showing that nanosized TiO(2) anatase and TiO(2) rutile perform better as MALDI matrices than a commercial TiO(2) anatase/rutile mixture. Moreover, when using nanosized TiO(2) anatase as a matrix, high-quality mass spectra can be obtained with strong analyte signals and weak or non-existing matrix interference ions. Furthermore, our results show that the phase type plays an important role in the application of TiO(2) as a MALDI matrix.

Electrical conductivity study of the solubility limits of tin–antimony mixed perovskites

A series of tin–antimony mixed-oxide perovskites of the ABO3 type with A = Ba and B = Sn1–xSbx was synthesized using increasing amounts of antimony. The electrical conductivity of these solids was studied to try to detect a possible solid solution phase and to determine the valence state of the cations.Antimony dissolves in the stannic sites of the stannate perovskite structure up to a mole fraction of 0.17 in a pentavalent state (Sb5+). Above this limit, the variations in electrical conductivity have been ascribed to the formation of an insulating second solid phase whose existence was confirmed by X-ray analysis (formation of the antimonate pyrochlore structure, Ba2Sb2O7).

Electrochemical characterisation of BaSn1–xSbxO3 perovskites

The electrochemical properties of the sintered pellets of the mixed oxide perovskite system BaSn1–xSbxO3 have been investigated by cyclic voltametry and a.c. impedance measurements. The solid-state properties and morphology of the materials have also been investigated and the information used to interpret the electrochemical behaviour. The conductivity of the samples increases with antimony content and for x > 0.05 the oxides behave essentially as porous metal electrodes, exhibiting a relatively wide range of electrochemical stability.

Structural singularities of superconducting Y-Ba-Cu oxides

Singular features of the orthorhombic or tetragonal array assigned to the high-T c superconducting phase YBa 2 Cu 3 O 7−δ are pointed out: order-disorder of oxygen atoms and vacancies, and subsequent formation of anti-phase and out-of-step domains; nature and reversible character of the orthorhombic/ tetragonal phase transition; distortion trends of yttrium coordination polyhedron. Sample preparation conditions - sintering temperature, annealing atmosphere, cooling regime - are commented in terms of bulk crystal chemical characteristics. Calculated bond valence summations for the orthorhombic structure are presented.

New structural data on Zr-replaced YBaCu superconducting oxides

Abstract Partial substitution of Zr for Y in superconducting oxide YBa 2 Cu 3 O 7−δ was studied by magnetic susceptibility and electrical resistivity measurements, X-ray diffraction and SEM-EDS. Nominal compositions Y 1−x Zr x Ba 2 Cu 3 O 7−δ (x ⩽ 0.30) were prepared by classical reaction sintering methods with annealing under oxygen. A.c. susceptibility measurements show a decrease of the diamagnetic signal below 40K due to the presence of a normal paramagnetic phase. The systematic observation of a semiconductor-like behaviour and the fact that zero resistance was never attained indicate the presence of normal regions inhibiting the percolation of superconducting ones. Zirconium does not significantly enter the structure of the high superconductivity Y Ba Cu phase. It partially deproportinates into an Y-rich phase and Ba Zr oxide phase(s) structurally related to perovskite plus BaCuO 2 and CuO, all traced by SEM-EDS and XRD.