57Fe Mössbauer effect study of Al-substituted lepidocrocites.

Abstract

Seven A|-containing lepidocrocite samples, ~/-Fe,_xAlxOOH, prepared from FeC12/AI(NO3)3 solutions with initial A1/(A1 + Fe) mole ratios C~ of 0.0025, 0.01, 0.025, 0.05, 0.075, 0.10 and 0.15 tool/ mol, were examined by means of M6ssbauer spectroscopy at room temperature (RT) and at various temperatures in the range of 8 to 80 K. The spectra at RT and 80~ 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 A1 cations are substituting into the structure. The temperature at which the onset of magnetic ordering is reflected in the spectra, was measured by the therrnoscan method with zero source velocity. A gradual shift from 50 K for Ci = 0.0025 mol/mol to 44 K for Ci = 0.10 mol/mol was observed for that temperature. As compared to earlier studies of Al-free ~-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 A1 cations into the lepidocrocite structure. A similar conclusion is inferred from the variation with C~ 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 Ci = 0.15 mol/mol, not all of the initially present A1 has been incorporated into the structure.