X-ray differential line broadening on tabular haematites

Abstract

The haematites obtained by dehydration of three synthetic goethites of different sizes have been characterized by X-ray diffraction, Mössbauer 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°C 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 X-ray 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 wihl=3n (n=0,1,2,...). Increasing the treatment temperature made all lines sharp. The relative integrated intensities,l(1 1 0)/l(1 0 4), of each series remained unaltered, but the relative intensities did not. The above results lead to the conclusion that the differential X-ray 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.