How to get ternary solid solutions Fe1-xM′xO (M = Co, Ni)?: A thermodynamic concept

The existence ranges of ternary solid solutions Fe1-xM'O-x (M = Co, Ni) have been estimated using thermodynamic modelling by CALPHAD methods: a complete series of mixed crystals Fe1-xCoxO is stable only at higher temperatures >= 900 degrees C. Below the solid solution shows a miscibility gap due to the different redox potentials [p(O-2) = f(T)] of the binary phases "FeO and CoO. Hence CoO becomes reduced into metallic cobalt while "FeO" is oxidised into Fe3O4. The pseudo binary system FeO/NiO shows solubility ranges only at the border of the binary oxides, the phases built are alpha(Fe1-xNixO) (x <= 0.13) and alpha'(FeyNi1-yO) (y <= 0.60, theta <= 1400 degrees C). Due to the broader difference of redox potentials [p(O-2) = f(T)] of "FeO" and NiO a complete series of mixed crystals cannot exist. In application of the results of thermodynamic modelling the syntheses of mixed crystals Fe1-xCoxO and their characterisation by X-ray diffraction and thermal analysis succeeds. The mixed phases crystallise in the halite structure type (space group Fm (3) over barm) with a progress of the lattice constants corresponding to the Vegard rule [alpha(Fe1-xCoxO) = 430.0(2) pm (x = 0), 427.8(2) pm (x = 0.5), 425.0(2) pm (x = 1)]. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).