Cation distribution in nanocrystalline cobalt substituted nickel ferrites: X-ray diffraction and Raman spectroscopic investigations

Nanocrystalline Ni1-xCo8Fe2O4 (x = 0.0, 0.4, 0.5, 0.6, 1.0) samples were synthesized using sol-gel technique. X-ray diffraction (XRD) and vibrational spectroscopy techniques were employed to study the structure, cation distribution and ordering in the Co2+ substituted nickel ferrite (NCFO) samples. The XRD analysis of the specimen confirmed the formation of single cubic spinel phase. The substitution of Co2+ for Ni2+ cations resulted in the increase in lattice parameter. The distribution of cations in tetrahedral (A) and octahedral (B) sites in the unit cell was estimated using X-ray diffraction and Raman data. Infrared and Raman spectroscopies too confirmed the spinel phase formation in the samples. An observed systematic shift and asymmetric broadening in Raman spectra with a change in Co2+ concentration were correlated with different ligand parameters such as effective mass, bond length and force constant, etc. A Raman peak (A(1g)) was employed to estimate the Co2+/Ni2+ and Fe3+ distribution between the two interstitial sites. The estimation of cation distribution made by the two techniques (Rietveld refinement of X-ray diffraction data and Raman spectroscopy) is in very good agreement. It is revealed that nickel ferrite (NFO) is nearly inverse (96% Ni2+ on B-sites). The substitution of Co2+ in NFO results into a partially inverse mixed ferrite phase with Co occupying nearly 18% A-sites in Ni0.5Co0.5Fe2O4 and 25% in CoFe2O4.