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) a nd at various temperatures in the range of 8 to 80 K. The spectra at R T and 80 degrees K consist of broadened quadrupole doublets and were a nalyzed in terms of a single doublet and of a model-independent quadru pole-splitting, distribution, the latter yielding the best fit. The ob served variations of the quadrupole-splitting parameters with increasi ng C-i are inconclusive as to whether the Al cations are substituting into the structure. The temperature at which the onset of magnetic ord ering is reflected in the spectra, was measured by the thermoscan meth od with zero source velocity. A gradual shift from 50 K for C-i = 0.00 25 mol/mol to 44 K for C-i = 0.10 mol/mol was observed for that temper ature. As compared to earlier studies of Al-free gamma-FeOOH samples w ith similar morphological characteristics, the fractional doublet area in the mixed sextet-doublet spectra at 35 K is significantly higher f or the present lepidocrocites. This observation is ascribed to the sub stitution of Al cations into the lepidocrocite structure. A similar co nclusion is inferred from the variation with C-i of the maximum-probab ility hyperfine field derived from the spectra recorded at 8 K and fit ted with a model-independent hyperfine-field distribution. The magneti c 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 th e structure.