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

Abstract

AbstractThe 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 Fm$\bar {3}$m) 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)