Synthesis and characterization of nanophasic goethite

X-ray diffraction, Mossbauer spectroscopy surface area and positron annihilation lifetime measurements were carried out to characterize alpha-FeOOH crystals as produced from the oxidation of Fe(OH)(2) using different oxygen flow rates at 298 K. From the resulting oxidation rates, average sized a-FeOOH crystals that ranged from 7 to 48 nm were obtained. Of these, crystals from 7 to 13 nm mainly showed superparamagnetic behavior and a surface area of about 200 m(2)/g; those from 23 to 48 nm were characterized by their hyperfine magnetic Mossbauer spectrum and an intermediate surface area of about 100 m(2)/g. Positron annihilation data showed that these nanometer-sized alpha-FeOOH crystals contained a high concentration of crystal defects. Three lifetime components were needed to fit the data, and the relative intensity of the defect-lifetime (tau(2)) increased as the average crystal size of the goethite decreased. Depending on the crystal size of the goethite, the crystal defects may or may not contribute to the surface area. By correlating the crystal size, the surface area, and the concentration of crystal defects, one may be able to assess the crystallinity of these goethites. (C) 1998 Acta Metallurgica Inc.