Transition of cationic local structures in Mg1-Ni_xAl₂O₄

In this study, cationic local structures in the series of MgAl2O4-NiAl2O4 solid solutions (Mg1-xNixAl2O4) were examined using Mg, Al, and Ni Kedge X-ray absorption near-edge structure (XANES). Theoretical calculations of XANES using the all-electron full-potential augmented plane-wave in addition to the local-orbital method were performed to investigate the transition of the cationic local structures depending on their compositions. For the calculations, the atomic coordination was studied using crystal structures of normal and ordered inverse spinels. The experimental spectra of the end members MgAl2O4 and NiAl2O4 were successfully reproduced from the theoretical spectra. Furthermore, the inversion degrees of the end members, analyzed using the calculated spectra, were consistent with previous experimental reports. The Ni2+ ions show three kinds of local structures, and their volume fractions change depending on the composition of Mg1-xNixAl2O4, whereas the Mg2+ ions show two kinds of local structures. The Al3+ ions change the local structure between tetrahedral and octahedral sites to balance the behaviors of Mg2+ and Ni2+ in the compositions. Although the analysis using XANES provides rich information, a theoretical approach is desirable for reliable and quantitative analysis of multication oxides and their disordering.