Composition dependence of the lattice parameter in solid solutions

We present an accurate x-ray powder diffraction study of stoichiometric 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.