Problems in the low-temperature synthesis of 92K Superconducting YBCO

Il Nuovo Cimento D - YBCO with high granulometric perfection and low dispersion can be obtained by a low-temperature reaction (≈800°C) of atomic-scale dispersed powders, produced through...     

Y-type hexagonal ferrites containing zinc,copper and cadmium: Magnetic properties and cation distribution

A complete magnetic and Mössbauer characterization has been worked out for the Y-type hexaferrites series with (Ba, Sr)₂Me₂Fe₁₂O₂₂ (Me = Zn, Cu, Cd) composition. The data relative to the dependence of the saturation magnetization, anisotropy, Curie temperature and cation distribution on composition and temperature could be logically interpreted as leading to the following conclusions. The magnetic order in Zn₂-Y is characterized by the presence of a “random flip” for which a quantitative model has been developed. The substitution of zinc with cadmium increases the spin flip, which, on the other hand, is reduced or eliminated by substitution of zinc with copper or of barium with strontium: the possibility of partially controlling the value of the saturation magnetization will follow. Also the planar anisotropy of Zn₂-Y can be controlled and finally turned into an axial one by substituting zinc with copper.     

Growth and optical, magnetic and transport properties of (C4H9NH3)2MCl4 organic-inorganic hybrid films (M = Cu, Sn)

Films of (C₄H₉NH₃)₂MCl₄ (M=Cu and Sn) organic-inorganic hybrid perovskites have been deposited in-situ by a single-source thermal ablation technique on glassy, crystalline and polymeric substrates. Independently of the substrate, the films were well crystallized, c-axis oriented and with a narrow rocking curve of the (0010) reflection (full width at half maximum <1°). The (0 0 ℓ) reflections were consistent with those of the bulk orthorhombic phases and the “c” lattice parameters were 30.85±0.05 and 32.35±0.05 Å, for the Cu- and the Sn-compound, respectively. (C₄H₉NH₃)₂CuCl₄ films had an optical absorption peak at 375 nm at room temperature. From the magnetic point of view they act as layered nanocomposites with a dominant ferromagnetic component localized in planes (2D magnetism). T_c was 7.3±0.1 K and a moderate easy-plane anisotropy was observed. The photoluminescence spectra of typical (C₄H₉NH₃)₂SnCl₄ films at 12 K had a broad yellow band, which did not correspond to any significant peak in the absorption spectrum. The films were semiconducting down to 250 K or, in the case of the best samples, down to 200 K and became insulating at lower temperature. The resistivity of the best films was (5±1) 10⁴ Ω cm at 300 K, and the energy gap was 1.11 eV. PACS 81.07.Pr; 81.15.Kk; 81.16.Dn     

Metallic excess determination in nonstoichiometric oxides and chalcogenides

A very simple polarographic procedure is described for determining the excess of a metallic element in nonstoichiometric oxides and chalcogenides. The method was based on the possibility of changing the reduction potential of an element in the presence of a strong complexing agent (EDTA). After sample dissolution, metals were complexed with an EDTA amount according to an exact stoichiometry; the solution was analyzed by a stripping voltametric procedure with a suitable preelectrolysis potential and the uncomplexed excess of the element was measured. Solutions as dilute as 10^?6M in free cation were examined with an accuracy better than 10% in the presence of a 10³- to 10⁴-times larger concentration of masked cations. With a simple modification the method can also be applied to stoichiometric analysis of cation-defective samples.     

Effect of humidity on the a.c. impedance of CH3NH3SnCl3 hybrid films

Impedance spectroscopy measurements show that complex conductivity of thermally ablated CH₃NH₃SnCl₃ films is strongly enhanced when humidity increases. Coplanar two-electrode test devices are modeled through an equivalent circuit comprising one resistance and two constant phase elements. It is shown that the influence of ambient humidity is mainly resistive. The dynamic responses of the devices to humidification/dehumidification cycles point out that the a.c. current varies by more than three orders of magnitude when humidity is varied between dry air and 80% relative humidity. The rise times are few hundred seconds while fall times are as short as few tens of seconds. This observation suggests that impedance variations are determined by mechanisms involving loosely bound water molecules physisorbed at the surface of the hybrid film. The results obtained are discussed in terms of protonic conduction.