Effects of Adsorbed Polyaniline on Redox Processes on As(2)O(3) Surfaces

Polyaniline deposited on As(2)O(3) surface resulted in a new material, which was characterized by infrared spectoscopy, thermogravimetry, differential scanning calorimetry, scanning electron microscopy, X-ray diffraction, and cyclic voltammetry. The mass percentage of polymer deposited on oxide surface is approximately 13%. The scanning electron microscopy images as well as the X-ray diffraction patterns provided conclusive evidence that the oxide surface is coated by the polymer. The cyclic voltammograms of the polyaniline adsorbed on As(2)O(3) surface showed that the adsorbate exerts remarkable effects on redox processes on this oxide. The pure oxide exhibited two oxidation/reduction peaks at 0.25/-0.06 and 0.47/-0.25 V attributed tentatively to the processes As(2)O(3)(s)+6H(+)+6e(-)=2As(s)+3H(2)O and As(s)+3H(+)+3e(-)=AsH(3)(g), respectively. The polyaniline-coated sample exhibited a better-defined voltammogram in which the first oxidation peak of the oxide had its intensity increased about four times. Copyright 2000 Academic Press.     

Mixed-valence manganites

Mixed-valence manganese oxides (R₁-_χA_χ)MnO₃ (R=rare-earth cation, A=alkali or alkaline earth cation), with a structure similar to that of perovskite CaTiO₃, exhibit a rich variety of crystallographic, electronic and magnetic phases. Historically they led to the formulation of new physical concepts such as double exchange and the Jahn-Teller polaron. More recent work on thin films has revealed new phenomena, including colossal magnetoresistance near the Curie temperature, dense granular magnetoresistance and optically-induced magnetic phase transitions. This review gives an account of the literature on mixed-valence manganites, placing new results in the context of established knowledge of these materials, and other magnetic semiconductors. Issues addressed include the nature of the electronic ground states, the metal-insulator transition as a function of temperature, pressure and applied magnetic field, the electronic transport mechanisms, dielectric and magnetic polaron formation, magnetic localization, the role of cation disorder and the Jahn-Teller effect. Sample preparation, and the properties of related ferromagnetic oxides are also discussed.     

Magnetic properties of carbonado diamonds

Carbonados are porous polycrystalline diamonds of uncertain origin, which are found at locations in Bahia (NE Brazil) and in the Central African Republic. Their combination of extreme hardness and toughness is ideal for drilling and cutting tools. A variety of elements including Fe, Ni, Cu and Ag are associated with the diamonds. Following a suggestion that Mn-doped diamond may be intrinsically ferromagnetic, we have analysed six samples from Brazil which show ferromagnetic moments of up to 3 A m² kg⁻¹, which is reduced by acid leaching. X-ray diffraction on the crushed powder showed a cubic diamond structure, with no secondary phases visible. Scanning electron microscopy shows an inhomogeneous distribution of heavy metals and Mössbauer spectroscopy indicates that iron is present as native iron. We find no evidence from the carbonados for the existence of transition-metal doped ferromagnetic carbon. The intrinsic diamagnetic susceptibility of carbonados is −4.5×10⁻⁹ m³ kg⁻¹, similar to the accepted value for diamond,−5×10⁻⁹ m³ kg⁻¹.     

57Fe Mössbauer Studies of Sr2−x Ca x FeReO6 Double Perovskite Compounds

Hyperfine Interactions - We present a 57Fe Mössbauer study of the Sr2?x Ca x FeReO6 double perovskite series, with x=0, 0.1, 0.2, 0.5, 1.0, 1.5 and 2.0. The analysis at 300 and 16 K...     

Magnetic soils from mafic lithodomains in Brazil

Tropical soils typically retain high amounts of iron oxides and some of them are magnetic, in the sense that their spontaneous magnetization σ > 1JT^-1 kg^-1. The two major orders forming on mafic domains, namely, dusky red Oxisol and Alfisol, generically referred to as terrae rossae, are the most representative magnetic soils and cover as much as 3.9%(∼330 000 km² of the Brazilian land area. In this paper, an up‐to‐date overview is presented, dealing with selected magnetic soils forming on four representative examples of mafic lithology. Some aspects of their iron oxide mineralogy and field and laboratory methodologies of study are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.