Navegando por Autor "Jesus Filho, M. F. de"

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Mössbauer and X-ray diffraction studies of the Morin transition in tabular hematites.
(1991) Costa, Geraldo Magela da; Silva, E. Galvão da; Jesus Filho, M. F. de
Several synthetic hematites have been analysed by X-ray diffraction and M~Sssbauer spectroscopy. They have tabular shape and their Morin transition at 85 K show a dependence on the ratio (R) of the sizes in c and a directions. The Morin transition has been observed for samples with R = 0.43 _+ 0.05.
Mössbauer study of rock paintings from Minas Gerais, Brazil.
(1991) Costa, Geraldo Magela da; Souza, Luiz Antonio Cruz; Jesus Filho, M. F. de
Four samples of a wall containing rock paintings have been studied by M(:,ssbauer spectroscopy in combination with optical microscopy analysis and X-ray diffraction. Hematite and goethite were identified as the pigments responsible for the colors and the mineral tinsleyite, as the principal component of a light pink layer that is present in some parts of the wall.
X-ray differential line broadening on tabular haematites.
(1992) Costa, Geraldo Magela da; Jesus Filho, M. F. de
The haematites obtained by dehydration of three synthetic goethites of different sizes have been characterized by X-ray diffraction, M6ssbauer spectroscopy and transmission electron microscopy. The transformation itself has been studied by thermogravimetric analysis and differential scanning calorimetry. Samples (24) obtained by annealing the goethites for 30 min at several temperatures up to 900 ~ were analysed. All samples had a tabular shape. The variation of the internal magnetic field, as well as of the line widths of the spectra at room temperature, indicate growth of the particle size. The constant value for the quadrupole splitting and the absence of a distribution of hyperfine interactions of the spectra at 80 K excluded the presence of iron ions out of their normal positions. No strain was detected by Xray diffraction. The results also showed a differential line broadening in the haematite pattern, but there was no shift in the position of the lines. This broadening was present in all lines, including those wih /= 3n (n = 0,1,2 .... ). Increasing the treatment temperature made all lines sharp. The relative integrated intensities, /(1 1 0)//(1 04), of each series remained unaltered, but the relative intensities did not. The above results lead to the conclusion that the differential Xray line broadening of haematites derived from goethite occurs only in tabular (or acicular) haematites with small size. For small spherical particles, the broadening should be uniform for all lines.