Effect of anions on the phase stability of \gamma-FeOOH nanoparticlesand the magnetic properties of gamma-ferric oxide derived from lepidocrocite

The effect of anions such as $Cl^-$, $SO_4 ^{2-}$, and $HPO_4 ^{2-}$ on the phase stability of FeOOH (\alpha or \gamma) during precipitation is investigated. Oxidation of $Fe(OH)_2·xH_2O$ from $FeCl_2$ solution with high $Cl^-$ concentration $([Cl^-]/[Fe]=R_{Cl}\geq 8)$ or $(NH_4)_2Fe(SO_4)_2$ (FAS) with $[HPO_4 ^{2-}]/[Fe]=RP \geq 0.02$ 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 $HPO_4 ^{2-}$ ions in the matrix of the intermediate phase $(Fe_x ^{2+}Fe_y ^{3+}(OH)_{2x+2y - nz} \cdot xH_2O(A)_z ^{n-}$, where A is anions such as $Cl^-$, $SO_4^ {2-}$, $HPO_4 ^{2-}$, 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 $Pt^ {4+}$, $Pd ^{2+}$ or $Rh ^{3+}$ are present in $FeCl_2 (R_{Cl} \geq 8)$ 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 \gamma-FeOOH contains randomly distributed micropores bringing in the relaxation effects of spins on the surface atoms as deciphered from Mössbauer spectroscopy. This leads to the low $\sigma_s$ (44–48 emu/g) and $H_c$ (120–130 Oe) for $\gamma-Fe_2O_{3- \delta}$ 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 $\gamma-Fe_2O_{3- \delta}$ renders the particles pore-free