ternary sulfides. Neutron diffraction data from powders was collected on the 3T2 diffractometer at the Laboratoire LØon Brillouin, Saclay, France, to follow the crystal and magnetic structures of the title compound as a function of temperature. Rietveld refinements made use of the FullProf Rietveld program [18].Variable-temperature Mössbauer spectra with good statistics were measured from 300 K down to 4.2 K using a source of 57 Co in rhodium. The data were collected with the sample in a helium cryostat. The program MOSFUN [19] was used to analyze the data.First principles calculations of the electronic structures were performed using the LMTO-ASA method. A detailed description of the LMTO-ASA method, including its applications, can be found elsewhere [20]. The scalar± relativistic Kohn Sham equations were solved, taking all relativistic effects into account except for spin±orbit coupling. The basis set included the s, p, and d wavefunctions for Fe, Cu, Cr, and S. The S d wavefunctions were treated using the down-folding procedure. The k integrated functions were evaluated by the tetrahedron method on a grid of 735 k points in the irreducible part of the Brillouin zone (1/48 of the BZ). We used lattice constants from the neutron diffraction refinement and constructed a supercell with an ordered arrangement of Fe and Cu on the tetrahedral sites in the space group F43m. For comparison, we have also performed calculations on the end members in the series, that is FeCr 2 S 4 and CuCr 2 S 4 .Because of magnetic ordering, the calculations were performed (using the von Barth±Hedin [21] form of the exchange correlation within the local spin density approximation) with the assumption of a magnetic ground state (spin-polarized calculation) as these, in agreement with experiment, yielded more stable total energies.

BackgroundDelignification pretreatments of biomass and methods to assess their efficacy are crucial for biomass-to-biofuels research and technology. Here, we applied confocal and fluorescence lifetime imaging microscopy (FLIM) using one- and two-photon excitation to map the lignin distribution within bagasse fibers pretreated with acid and alkali. The evaluated spectra and decay times are correlated with previously calculated lignin fractions. We have also investigated the influence of the pretreatment on the lignin distribution in the cell wall by analyzing the changes in the fluorescence characteristics using two-photon excitation. Eucalyptus fibers were also analyzed for comparison.ResultsFluorescence spectra and variations of the decay time correlate well with the delignification yield and the lignin distribution. The decay dependences are considered two-exponential, one with a rapid (τ1) and the other with a slow (τ2) decay time. The fastest decay is associated to concentrated lignin in the bagasse and has a low sensitivity to the treatment. The fluorescence decay time became longer with the increase of the alkali concentration used in the treatment, which corresponds to lignin emission in a less concentrated environment. In addition, the two-photon fluorescence spectrum is very sensitive to lignin content and accumulation in the cell wall, broadening with the acid pretreatment and narrowing with the alkali one. Heterogeneity of the pretreated cell wall was observed.ConclusionsOur results reveal lignin domains with different concentration levels. The acid pretreatment caused a disorder in the arrangement of lignin and its accumulation in the external border of the cell wall. The alkali pretreatment efficiently removed lignin from the middle of the bagasse fibers, but was less effective in its removal from their surfaces. Our results evidenced a strong correlation between the decay times of the lignin fluorescence and its distribution within the cell wall. A new variety of lignin fluorescence states were accessed by two-photon excitation, which allowed an even broader, but complementary, optical characterization of lignocellulosic materials. These results suggest that the lignin arrangement in untreated bagasse fiber is based on a well-organized nanoenvironment that favors a very low level of interaction between the molecules.

The study of structures based on nonstoichiometric SnO 2−x compounds, besides experimentally observed, is a challenging task taking into account their instabilities. In this paper, we report on single crystal Sn 3 O 4 nanobelts, which were successfully grown by a carbothermal evaporation process of SnO 2 powder in association with the well known vapor-solid mechanism. By combining the structural data and transport properties, the samples were investigated. The results showed a triclinic semiconductor structure with a fundamental gap of 2.9 eV. The semiconductor behavior was confirmed by the electron transport data, which pointed to the variable range hopping process as the main conduction mechanism, thus giving consistent support to the mechanisms underlying the observed semiconducting character.

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