X-ray absorption spectroscopy studies of the off-center Ni2+ ions in NicMg1-cO solid solutions

In this work we present the x-ray absorption spectroscopy study of Ni_cMg_1-cO (0.01 ≤ c ≤ 1) solid solutions. The extended x-ray absorption fine structure (EXAFS) above the Ni K-edge was analyzed using the multi-shell fitting procedure. It was found that nickel ions are located in a distorted environment and shift upon dilution to the off-center positions. This fact follows from two main results: (1) in the first shell, the average Ni-O distance increases linearly upon dilution; (2) in the second shell, the average Ni-Ni distance remains unchanged and decreases slightly at low nickel concentration while the dependence of the Ni-Mg distance has a break at c ≈ 0.6 and is nearly constant below and above this concentration. The obtained results are discussed in comparison with the ones from x-ray diffraction, optical spectroscopies and magnetic measurements.