Interpretation of the Ni K-edge EXAFS in nanocrystalline nickel oxide using molecular dynamics simulations

Analysis of atomic structure at the nanoscale is a challenging task, complicated by relaxation phenomena and thermal disorder. In this work, the x-ray absorption spectroscopy at the Ni K-edge was used to address this problem in nanocrystalline NiO (nano-NiO) at 300 K. The analysis of the first two coordination shells using conventional two-shell single-scattering approximation allowed us to determine the expansion of the average lattice but contraction of the Ni - O bonds in the first coordination shell in nano-NiO in comparison with the bulk nickel oxide. The EXAFS signal generated within the first six coordination shells (up to ∼ 6.5 Å) was successfully interpreted using classical molecular dynamics and ab initio multiple-scattering EXAFS theory. We found that simple rigid-ion force-field model is able to describe the structure relaxation and dynamics in both bulk and nano-NiO. Such approach requires less parameters than conventional EXAFS analysis and allows accounting explicitly for thermal effects and many-atom distribution functions. Finally, we showed that the EXAFS signal is rather sensitive to small variations of the force-field model and, thus, the agreement between the experimental and calculated EXAFS signals can be used for the force-field model optimization.