Fe-57 Mossbauer effect study of Al-substituted lepidocrocites

Seven Al-containing lepidocrocite samples, gamma-Fe1-xAlXOOH, prepared from FeCl2/Al(NO3)(3) solutions with initial Al/(Al + Fe) mole ratios C-i of 0.0025, 0.01, 0.025, 0.05, 0.075, 0.10 and 0.15 mol/mol, were examined by means of Mossbauer spectroscopy at room temperature (RT) and at various temperatures in the range of 8 to 80 K. The spectra at RT and 80 degrees K consist of broadened quadrupole doublets and were analyzed in terms of a single doublet and of a model-independent quadrupole-splitting, distribution, the latter yielding the best fit. The observed variations of the quadrupole-splitting parameters with increasing C-i are inconclusive as to whether the Al cations are substituting into the structure. The temperature at which the onset of magnetic ordering is reflected in the spectra, was measured by the thermoscan method with zero source velocity. A gradual shift from 50 K for C-i = 0.0025 mol/mol to 44 K for C-i = 0.10 mol/mol was observed for that temperature. As compared to earlier studies of Al-free gamma-FeOOH samples with similar morphological characteristics, the fractional doublet area in the mixed sextet-doublet spectra at 35 K is significantly higher for the present lepidocrocites. This observation is ascribed to the substitution of Al cations into the lepidocrocite structure. A similar conclusion is inferred from the variation with C-i of the maximum-probability hyperfine field derived from the spectra recorded at 8 K and fitted with a model-independent hyperfine-field distribution. The magnetic results suggest that for the sample corresponding to C-i = 0.15 mol/mol, not all of the initially present Al has been incorporated into the structure.