Non-stoichiometric zinc-ferrite spinel nanoparticles

When prepared as a bulk material, ZnFe₂O₄ has a normal spinel structure with Zn²⁺ incorporated almost exclusively at the tetrahedral lattice sites and Fe³⁺ at the octahedral sites ((Zn)[Fe₂]O₄). Due to its “rigid” structure, its composition is also closely defined at Zn²⁺/Fe³⁺ ≅ 0.5. However, when prepared as nanoparticles, a significant proportion of Zn can enter the zinc-ferrite structure at the octahedral sites, resulting in a flexibility of the composition. In this work, the non-stoichiometry of zinc-ferrite nanoparticles was studied as a function of the particle size. Nanoparticles with a narrow size distribution were synthesized using co-precipitation in water-in-oil microemulsions. The zinc-ferrite nanoparticles with a size of ~9 nm retained the single-phase spinel structure when Zn/Fe was between 0 and ~0.8. With a decrease in the particle size the compositional range of the zinc-ferrite spinel broadened. The process of structural rearrangement during the heating of the nanoparticles was also studied.