MOSSBAUER STUDY OF THE THERMAL-DECOMPOSITION OF LEPIDOCROCITE AND CHARACTERIZATION OF THE DECOMPOSITION PRODUCTS

The lepidocrocite (gamma-FeOOH) to maghemite (gamma-Fe2O3), and the maghemite to hematite (alpha-Fe2O3) transition temperatures have been monitored by TGA and DSC measurements for four initial gamma-FeOOH samples with different particle sizes. The transition temperature of gamma-FeOOH to gamma-Fe2O3 and the size of the resulting particles were not affected by the particle size of the parent lepidocrocite. In contrast, the gamma-Fe2O3 to alpha-Fe2O3 transition temperature seems to depend on the amount of excess water molecules present in the parent lepidocrocite. Thirteen products obtained by heating for one hour at selected temperatures, were considered. Powder X-ray diffraction was used to qualify their composition and to determine their mean crystallite diameters. Transmission electron micrographs revealed the particle morphology. The Mossbauer spectra at 80 K and room temperature of the mixed and pure decomposition products generally had to be analyzed with a distribution of hyperfine fields and, where appropriate, with an additional quadrupole-splitting distribution. The Mossbauer spectra at variable temperature between 4.2 and 400 K of two single-phase gamma-Fe2O3 samples with extremely small particles show the effect of superparamagnetism over a very broad temperature range. Only at the lowest temperatures (T less-than-or-equal-to 55 K), two distributed components were resolved from the magnetically split spectra. In the external-field spectra the DELTA-m(I) = 0 transitions have not vanished. This effect is an intrinsic property of the maghemite particles, indicating a strong spin canting with respect to the applied-field direction. The spectra are successfully reproduced using a bidimensional-distribution approach in which both the canting angle and the magnetic hyperfine field vary within certain intervals. The observed distributions are ascribed to the defect structure of the maghemites (unordered vacancy distribution on B-sites, large surface-to-bulk ratio, presence of OH- groups). An important new finding is the correlation between the magnitude of the hyperfine field and the average canting angle for A-site ferric ions, whereas the B-site spins show a more uniform canting. The Mossbauer parameters of the two hematite samples with MCD104 values of respectively 61.0 and 26.5 nm display a temperature variation which is very similar to that of small-particle hematites obtained from thermal decomposition of goethite. However, for a given MCD the Morin transition temperature for the latter samples is about 30 K lower. This has tentatively been ascribed to the different mechanisms of formation, presumably resulting in slightly larger lattice parameters for the hematite particles formed from goethite, thus shifting the Morin transition to lower temperatures.