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The effect of anions such as Cl − , SO 4 2− , and HPO 4 2− on the phase stability of FeOOH (α or γ) during precipitation is investigated. Oxidation of Fe(OH) 2 · x H 2 O from FeCl 2 solution with high Cl − concentration ([Cl − ]/[Fe]= R Cl ≥8) or (NH 4 ) 2 Fe(SO 4 ) 2 (FAS) with [HPO 4 2− ]/[Fe]= R P ≥0.02 yields phase-pure γ-FeOOH. In the medium ranges of R Cl and R P , mixed phases of α-FeOOH and γ-FeOOH are obtained. Replacement of OH − by Cl − with the bridging cations or strongly bonded HPO 4 2− ions in the matrix of the intermediate phase (Fe x 2+ Fe y 3+ (OH) 2 x +2 y − nz · x H 2 O(A) z n − , where A is anions such as Cl − , SO 4 2− , HPO 4 2− , etc.), promoted the lower density γ-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 Pt 4+ , Pd 2+ or Rh 3+ are present in FeCl 2 ( R Cl ≥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 nm. While most of the Cl − ion is removed from the final precipitates on washing, phosphate remained as HPO 4 2− as evidenced from IR absorption spectra. Maghemite obtained by dehydroxylating γ-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 σ s (44–48 emu/g) and H c (120–130 Oe) for γ-Fe 2 O 3− δ particles. Whereas nearly pore-free single crystalline particles obtained by reduction followed by reoxidation has high value of σ 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 γ-FeOOH to magnetite to γ-Fe 2 O 3− δ renders the particles pore-free. |
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