Investigation of the chemical disorder of LiNi0.5Mn1.5O4 lattice by means of extended X-ray absorption fine structure spectroscopy

This work reports on a characterization methodology for investigating the transition-metal site disorder in the LiNi0.5Mn1.5O4 spinel structure (LNMO, Fd3̅m space group, cF56 lattice). This approach, previously adopted on simple intermetallic ordered solid solutions, is here applied to a complex lattice. The LNMO material synthesized by us has been investigated by advanced techniques such as X-ray absorption spectroscopy (XAS), X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy. High-quality XAS spectra measured at both the Ni K-edge and the Mn K-edge have been analyzed using double-edge multiple-scattering data analysis. Computer simulations, based on structural lattice models describing substitutional disorder between Ni and Mn in the 16d site of the LNMO lattice, have been carried out by varying the order parameter s of the site occupancy from 0 (completely disordered) to 1 (completely ordered). These simulations allow us to predict the evolution of the two- and three-body structural parameters versus the order parameter s. The chemical ordering in the transition-metal 16d atomic site of the primitive LNMO cell has been analyzed by coupling computer simulations and extended X-ray absorption fine structure (EXAFS) data analysis results. The EXAFS signals are sensitive to the surrounding substitutional disorder, and their intensities, especially those of the collinear three-atom configurations, can be used to evaluate the ordering level of the transition metals in the lattice. The final refined order parameter value (s = 0.0) suggests that only negligible ordering occurs in this lattice between Ni and Mn, and the structure is therefore completely disordered.