Effect of cationic substituents on particle morphology of goethite and the magnetic properties of maghemite derived from substituted goethite

Preparation of nanoparticles with the desired shape and size by the wet-chemical precipitation process is a challenging task. Thus, the effects of different substitutional impurities such as Al, Cr, Co or Ni on the particle morphology and phase stability of goethite have been investigated. Goethite is prepared by air oxidation of Fe(OH)₂ · xH₂O gel under near neutral conditions. Below certain concentration levels of dopants (Al³⁺ ≤ 10; Cr³⁺ ≤ 5; Co²⁺ ≤ 10 and Ni²⁺ ≤ 5 at.%) the samples remained monophasic as revealed by XRD, TEM, and IR studies. Above these levels, the substituents produce traces of secondary phases such as lepidocrocite, spinel ferrite and Mⁿ(OH)ₙ. The goethite structure is stable in spite of the iso- or aliovalent substitutions. The individual additives have divergent influence on the particle morphology; Al³⁺ and Cr³⁺ decreases the particle size to <50 nm and aspect ratio (AR) <2. Co-substitution produces slender particles with AR as high as 25. Whereas, Ni²⁺ does not have any influence on the particle morphology. The attributable factors in morphology control are the increased nucleation rate, restricted growth along needle axes, and the strain induced in the goethite lattice as a result of difference in ionic radii. Maghemite, γ-Fe₂O₃₋δ, particles are obtained from goethite wherein the topotactic conversion renders the retention of the particle morphology of the precursor. Maghemite with substituted impurities showed substantial differences in magnetic properties. Saturation magnetization (σₛ) and coercivity (H c) go down to very low values due to relaxation of spins on the surface atoms as revealed by Mössbauer spectroscopy. Decrease in coercivity is by way of the presence of diamagnetic ion (Al³⁺). Whereas, Co-substituted maghemite has enhanced H c as a result of high magnetocrystalline anisotropy accompanied by the shape anisotropy.