How to Get Ternary Solid Solutions Fe1–xM′xO (M = Co, Ni)? A Thermodynamic Concept

TLDR: In this article, the existence ranges of ternary solid solutions Fe1−xM′xO (M = Co, Ni) have been estimated using thermodynamic modelling by CALPHAD methods.

Abstract: The existence ranges of ternary solid solutions Fe1–xM′xO (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 °C. Below the solid solution shows a miscibility gap due to the different redox potentials [p(O2) = 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 areα(Fe1–xNixO) (x ≤ 0.13) and α′(FeyNi1–yO) (y ≤ 0.60,  ≤ 1400 °C). Due to the broader difference of redox potentials [p(O2) = 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 Fmm) with a progress of the lattice constants corresponding to the Vegard rule [a(Fe1–xCoxO) = 430.0(2) pm (x = 0), 427.8(2) pm (x = 0.5), 425.0(2) pm (x = 1)].(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)