X-ray absorption spectroscopy study of Ni_cMg_1-cO solid solutions on the Ni K edge

We present an X-ray absorption spectroscopy study of Ni_cMg _1-cO (0.01<or=c<or=1) solid solutions on the Ni K edge. The analysis of XANES and EXAFS regions allowed us to obtain the following results. The nickel ions are located in a distorted environment and shift upon dilution to the off-centre positions. In the first shell, the average Ni-O distance increases linearly upon dilution. In the second shell, the average Ni-Ni distance remains unchanged and decreases slightly at low nickel concentration while the dependence of the average Ni-Mg distance has a break at c approximately=0.6 and is nearly constant below and above this concentration which corresponds to the point of antiferromagnetic-paramagnetic phase transition. The number of nickel and magnesium ions in the second shell of nickel ions varies in agreement with the random distribution model. The results obtained are discussed in comparison with the ones from X-ray diffraction, optical spectroscopy and magnetic measurements.