A Comparison of Cation Distribution and Valence State in Spinel Crystal Structure of Zinc and Nickel Ferrites Using the Synchrotron X-ray Absorption Spectroscopy (XAS) Analysis

Abstract

In this work, the physical structure, magnetism and local structure of zinc and nickel ferrites (ZnFe₂O₄ and NiFe₂O₄) synthesized by typical sol-gel combustion method, were investigated by X-ray diffraction (XRD), vibrating sample magnetometer (VSM), X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). The formation of the single phase cubic spinel crystal structure and the different values of crystallite size (D), interplanar distance (d) and lattice constant (a) for all ferrite samples were evaluated by the XRD data. The VSM measurement provides the characteristic magnetic hysteresis loop (M-H) for each sample which was found to be significantly different from each other. The chemical shifts in Zn, Ni and Fe K-edges XANES spectra indicate the existence of Zn²⁺, Ni²⁺ and Fe³⁺ ions in these ferrites. The EXAFS spectra analyses applied to track Zn, Ni and Fe cation distribution indicate the distinct character of spinel crystal structure of both ferrites. The results exhibit that zinc ferrite is a normal spinel, while the nickel ferrite is an inverse spinel. Moreover, these EXAFS spectra analyses reveal that the distances between metal ion (Zn²⁺ or Ni²⁺) to central oxygen ion and to Fe³⁺ ions in the opposite lattice site for each ferrite sample are unequal which highly affect its magnetism. The overall simulated results are one of the important evidence encouraging the explanation on the paramagnetism for ZnFe₂O₄ and the ferrimagnetism for NiFe₂O₄.

Keywords:

• Ferrites
• spinel
• sol-gel combustion
• cation distribution
• XANES
• EXAFS