Cation exchange in Ni-Cu-Zn aluminate spinels revealed by EXAFS

Nanocrystalline Ni-Cu-Zn aluminate powders (Ni1-xCuxAl2O4, Cu1-xZnxAl2O4, and Zn1-xNixAl2O4) with different divalent ion concentrations (0 <= x <= 1) were successfully synthesized by the sol-gel auto combustion method using diethanolamine (DEA) as a novel fuel. The structural, morphological, and the local atomic structures of each calcined sample were investigated by XRD, TEM, and synchrotron EXAFS measurements. The XRD results confirmed the formation of single-phase spinel structure with Fd-3m space group of all samples. When XRD Rietveld refinement and Williamson-Hall analysis were applied to the XRD data, the exact crystallite size and structural parameters were obtained. The EXAFS spectra showed the main translocation trends in the trivalent (Al3+) and divalent (Ni2+, Cu2+, Zn2+) ions between the tetrahedral (A) and octahedral (B) sublattices. Furthermore, the degree of inversion in these spinel aluminates is completely dissimilar and identified as 0.493-0.499 for Ni1-xCuxAl2O4, 0.254-0.498 for Cu1-xZnxAl2O4 and 0.093-0.453 for Zn1-xNixAl2O4 series, as indicated by the multiple-edge curve-fitting analysis of Ni, Cu, and Zn K-edge EXAFS spectra. These values comprise essential data for comprehension of numerous relevant properties.