Structural and microstructural evolution due to increasing Co substitution in Ni1−xCoxFe2O4: An X-ray diffraction study using the Rietveld method

X-ray diffraction studies employing the Rietveld analysis is reported on the influence of increasing Co substitution on the structural and microstructural evolution in AB2O4 type spinel ferrites: Ni1−xCoxFe2O4 (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0). The specimens were synthesized by the organic precursor method. Structure refinement reveals that the pure NiFe2O4 is not an exact inverse spinel and about 11% of Ni2+ ions occupy the tetrahedral (A) sites. Increasing Co concentrations had the effect of increasing the ratio of B[Fe3+]/A[Fe3+][Fe3+]B/[Fe3+]A and gradual expansion of the ferrite unit cell. The microstructure refinement estimates that the particle size values are in the order of ∼nm, ranging from 31 to 61 nm, which gradually increase with increasing Co doping accompanied by almost negligible lattice micro strains (∼10−4). The corresponding particle size distribution for each specimen was obtained from the X-ray diffraction data from the basic assumption that the spherical nanoparticles follow the log-normal distribution. The size distribution for the pure NiFe2O4 was also estimated from transmission electron microscope and agreed well with that obtained by the diffraction data analysis.

► Rietveld analysis performed during increasing Co substitution in spinel Ni1−xCoxFe2O4 ferrites.
► Increasing Co concentrations increases the ratio of B[Fe3+]/A[Fe3+][Fe3+]B/[Fe3+]A and expands the ferrite unit cell.
► Nano-sized ferrite particle are almost spherical and the size increases with increasing Co concentration.
► The particle size distributions reveal a bimodal nature.
► Size distribution shifts to the high particle size region, with increasing Co concentration.