Novel route for the preparation of nanosized NiFe2O4 powders

Nanosized NiFe2O4 powders were prepared by a novel sol-gel route. The powders calcined at 1200degreesC and quenched to liquid nitrogen temperature showed a mixed spinet phase, with a saturation magnetization of 49 emu/g, crystallite size of 67 nut and hyperfine magnetic field B-1 = 520 kOe, B-2 = 484 kOe and A = 488 kOe.438A5249525

Thermal decomposition and structural reconstruction effect on Mg-Fe-based hydrotalcite compounds

The thermal decomposition and structural reconstruction of Mg-Fe-based hydrotalcites (FIT) have been studied through thermogravimetric analyses, X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy and Mossbauer spectroscopy. The destruction of the layered structure took place at about 300degreesC. The broad peaks observed in the X-ray diffractograms suggest that the resultant oxides constitute a solid solution. For samples treated at temperatures higher than 500degreesC, the formation of the MgO and MgFe2O4 spinel phases is observed. Fe-57 Mossbauer spectroscopy was employed to monitor the Fe chemical environment for the samples annealed at different temperatures (100-900degreesC). In situ XRD experiments revealed that the HTs start an interlayer contraction at about 180degreesC. This phenomenon is identified as being due to a grafting process for which the interlamellar anions attach to the layers through a covalent bond. The reconstruction of the HTs was also investigated and its efficiency depends on the thermal annealing temperature and the Mg/Fe ratio. The structure of the reconstructed samples was found to be exactly the same as the parent structure. (C) 2004 Elsevier Inc. All rights reserved.17793058306

Structural and thermal properties of Co-Cu-Fe hydrotalcite-like compounds

The structural properties and thermal decomposition processes of Co-Cu-Fe ternary hydrotalcites (HT) have been studied through X-ray diffraction, thermogravimetric measurements, Fourier-transform infrared and Mossbauer spectroscopies. Due to the strong Jahn-Teller effect, the Cu-Fe layered system is stabilized only in the presence of Co2+. At low Co2+ contents, additional phases are segregated in the solids. X-ray patterns show the presence of Cu(OH)(2) and CuO. The decomposition process was investigated by in situ X-ray, in situ Mossbauer and FTIR experiments. By increasing the temperature from 25 degreesC up to 180degreesC we observed that the structural disorder increases. This effect has been likely attributed to the Co2+ --> CO3+ oxidation since thermal decomposition was carried out under static air atmosphere. Part of the Co3+ cations could migrate to the interlayer region, thus forming a metastable compound that still has a layered structure. Collapse of the layered structure was observed at about 200 degreesC. By further increasing the temperature the system becomes more crystalline and the formation Of CO3O4 is observed in the X-ray patterns. In Cu-rich HT, some of the carbonate anions are released at temperatures higher than 550degreesC and this phenomenon is attributed to the formation of a carbonate-rich phase. The specific surface area data present its highest values in the temperature range where the collapse of the layered structure takes place. (C) 2004 Elsevier Inc. All rights reserved.178114215

Probing the thermal decomposition process of layered double hydroxides through in situ Fe-57 Mossbauer and in situ X-ray diffraction experiments

The thermal decomposition properties of Mg-Fe hydrotalcites were studied through in situ Fe-57 Mossbauer spectroscopy and in situ X-ray diffraction. Abrupt changes in the quadrupolar splitting measured in the Mossbauer spectra revealed a phase transition from the starting lamellar structure to a new crystalline arrangement. By analyzing the Mossbauer parameters we show that the material is highly disordered in the 300-400 degrees C temperature range. This hypothesis is confirmed by the X-ray results whose diffractograms indicated the collapse of the lamellar structure and the formation of a solid solution.42253453