Local coordination of Fe3+ in ZnO nanoparticles: multi-frequency electron paramagnetic resonance (EPR) and Newman superposition model analysis

Iron-doped ZnO nanoparticles have been synthesized through high-energy ball milling of powders produced by the co-precipitation method. Fine particles with an average size down to 40 nm have been obtained after 15 min of milling. Fe3+ cations have been incorporated into the ZnO lattice within the limits of the solubility. By using multi-frequency and high-field electron paramagnetic resonance (EPR) we have resolved all electronic transitions for the S = 5/2 high-spin system and have accurately determined the EPR spin-Hamiltonian parameters. By combining data from crystallographic x-ray diffraction and EPR with the semi-empirical Newman superposition model we have found the local configurational position of Fe3+ and have confirmed the symmetry of the lattice. Results presented in this paper indicate that Fe cations most probably substitute at Zn-sites in ZnO. At nanosizes the effect of Fe3+ cations on the surface becomes remarkable: additional size effects can be observed in the EPR spectrum, which are different from the spectrum of bulk.