The perovskite-like manganites R 1−x A x MnO 3 , where R is a trivalent rare earth or Y and A is a divalent alkaline earth element, are characterized by a strong interplay of magnetism, electric transport and crystallographic distortion. At doping levels 0.15 < x < 0.45 the materials exhibit colossal magnetoresistance near the concomitant ferromagnetic and insulator-metal transitions. At a fractional doping level, such as x = 0.5, the crystallographic and magnetic environment is strongly modified and charge ordering between Mn 3+ and Mn 4+ or phase separation takes place. In this work, the polarized Raman spectra of the orthorhombic and rhombohedral phases of parent RMnO 3 compound were analyzed in close comparison with results of lattice dynamic calculations. We argue that the strong high-wavenumber bands between 400 and 700 cm −1 , which dominate the Raman spectra of rhombohedral RMnO 3 and magnetoresistive La 1−x A x MnO 3 are not proper Raman modes for the R3c or Pnma structures. Rather, the bands are of phonon density-of-states origin and correspond to oxygen phonon branches activated by the non-coherent Jahn-Teller distortions of the Mn 3+ O 6 octahedra. The reduction of these bands upon doping of La 1−x A x MnO 3 and their disappearance in the ferromagnetic metallic phase support the model. The variation with temperature of the Raman spectra of La 0.5 Ca 0.5 MnO 3 is also discussed. The results give a strong indication for charge and orbital ordering and formation of superstructure at low temperatures.
The mixed-valence perovskitelike manganites are characterized by the unique interrelation of Jahn-Teller distortions, electric and magnetic properties. The Jahn-Teller distortion follows the Mn 3ϩ →Mn 4ϩ charge transfer with some delay. Its development depends on the lifetime of Mn in the 3ϩ state, governed by the Mn 4ϩ /Mn 3ϩ ratio and magnetic correlations. The noncoherence of Jahn-Teller distortions in orthorhombic mixed-valence manganites and rhombohedral RMnO 3 (Rϭrare earth͒ results in oxygen disorder. We demonstrate that the Raman spectra in this case are dominated by disorder-induced bands, reflecting the oxygen partial phonon density of states ͑PDOS͒. The PDOS origin of the main Raman bands in insulating phases of such compounds is evidenced by the similar line shape of experimental spectra and calculated smeared PDOS and disappearance of the PDOS bands in the ordered ferromagnetic metallic phase.
A microwave plasma-assisted method for synthesis of advanced carbon nanostructures is presented. The method is based on the injection of gas/liquid carbon precursors into a surface-wave sustained argon plasma environment, where the decomposition of molecules takes place. Gas-phase carbon atoms and molecules created in the "hot" plasma diffuse into colder zones of plasma reactor and aggregate into solid carbon nuclei. The main part of the solid carbon is gradually withdrawn from the "hot" plasma region in the outlet plasma stream where flowing carbon nanostructures assemble and grow. Selective synthesis of free-standing graphene sheets and diamond-like nanostructures was achieved. The engineering of the graphene sheets’ structural qualities and the control of the number of atomic layers per sheet is achieved via synergistic tailoring of the "hot"plasma environment and thermodynamic conditions in the post-discharge zone. The produced sheets (1-5 atomic layers) are stable and present much better qualities than those of graphene assembled via conventional methods, while being synthesized without the need of metal/crystal substrates. The synthesized structures have been analyzed by Raman spectroscopy, scanning electron microscopy, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. It is worth noting that milligrams of high quality self-standing graphene sheets in a readily dispersive form can be obtained within a minute, through a single step process at atmospheric pressure conditions. The method is widely open for scale-up by using large-scale configurations of wave driven plasmas.
Acknowledgements: This work was supported by Fundação para a Ciência e a Tecnologia, under 2015 Strategic Project.
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