Effect of anions on the phase stability of γ-FeOOH nanoparticles and the magnetic properties of gamma-ferric oxide derived from lepidocrocite

The effect of anions such as Cl-, SO42- and HPO42- It the phase stability of FeOOH (alpha or gamma) during precipitation is investigated. Oxidation of Fe(OH)(2).xH(2)O from FeCl2 solution with high Cl- concentration ([Cl-]/[Fe] = R-Cl greater than or equal to 8) or (NH4)(2)Fe(SO4)(2) (FAS) with [HPO42-]/[Fe] = R-p greater than or equal to 0.02 yields phase-pure gamma-FeOOH. In the medium ranges of R-Cl and R-p, mixed phases of alpha-FeOOH and gamma-FeOOH are obtained. Replacement of OH- by Cl- with the bridging cations or strongly bonded HPO42- ions in the matrix of the intermediate phase (Fex2+Fey3+(OH)(2x+2y - nz).xH(2)O(A)(z)(n-), where A is anions such as Cl-, SO42-, HPO42-, etc.), promoted the lower density gamma-FeOOH. However, the particles are less developed and have poor crystallinity as evidenced from transmission electron microscope and thermogravimetry-differential thermal analysis of the precipitates. Whereas, monophasic, uniformly sized, nano-lath shaped particles with high aspect ratio > 10 are obtained when morphology-controlling cation additives such as Pt4+, Pd2+ or Rh3+ are present in FeCl2 (R-Cl greater than or equal to 8) solution. Preferential adsorption of additives on (0k0) and (h00) planes limits the growth in the perpendicular directions leading to high aspect ratios. The effect of these additives are suppressed by the phosphate ion, a strong complexing ligand, giving rise to fibrous aggregate with the length of individual particles as small as 10-30 urn. While most of the Cl- ion is removed from the final precipitates on washing, phosphate remained as HPO42- as evidenced from IR absorption spectra. Maghemite obtained by dehydroxylating gamma-FeOOH contains randomly distributed micropores bringing in the relaxation effects of spins on the surface atoms as deciphered from Mossbauer spectroscopy. This leads to the low sigma(s) (44-48 emu/g) and H-c(120-130 Oe) for gamma-Fe2O3-delta particles. Whereas nearly pore-free single crystalline particles obtained by reduction followed by reoxidation has high value of sigma(s) (73 emu/g) and H-c (320 Oe), which decreases to 30 emu/g and 75 Oe, respectively, for nanoparticles obtained from phosphate stabilized lepidocrocite. The mobility of iron ions and counter mobility of vacancies during the topotactic transformation of gamma-FeOOH to magnetite to gamma-Fe2O3-delta renders the particles pore-free. (C) 2003 Elsevier Ltd. All rights reserved.