Composition dependence of the lattice parameter in NicMg1-cO solid solutions

We present an accurate x-ray powder diffraction study of stoichiometric Ni_cMg_1-cO solid solutions performed using an internal-standard method. The results obtained allowed us to establish for the first time that the composition dependence of the lattice parameter a(c) is not linear, as had been asserted, and interpreted within Vegard's model, in previous works. A new model, which is able to explain both the a(c) behaviour and the variation of the local structure around nickel ions, obtained previously by x-ray absorption spectroscopy (Kuzmin A, Mironova N, Purans J and Rodionov A 1995 J. Phys.: Condens. Matter 7 9357), is suggested and discussed.