Ellingham diagrams of binary oxides

Abstract

Controlling the oxidation state of constituents by tuning the oxidizing environment and materials chemistry is vital to the successful synthesis of targeted binary or multicomponent oxides. We have conducted a comprehensive thermodynamic analysis of 137 binary oxides to calculate their Ellingham diagrams. It is found that the “reactive” elements that oxidize easily are the f-block elements (lanthanides and actinides), elements in groups II, III, and IV (alkaline earth, Sc, Y, Ti, Zr, and Hf), and Al and Li. In contrast, the “noble” elements are easily reduced. These are coinage metals (Cu, Ag, and especially Au), Pt-group elements, and Hg and Se. Machine learning-based sequential feature selection indicates that the ease of oxidation can be represented by the electronic structures of pure elements, for example, their d- and s-valence electrons, Mendeleev numbers, and groups, making the Periodic Table a useful tool for qualitatively assessing the ease of oxidation. The other elemental features that weakly correlate with the ease of oxidation are thermochemical properties such as melting points and the standard entropy at 298 K of pure elements. Applying Ellingham diagrams enables the oxidation of multicomponent materials to be predicted, such as the Fe–20Cr–20Ni alloy (in wt. %) and the equimolar high entropy alloy of AlCoCrFeNi. These Ellingham diagram-based predictions are in accordance with thermodynamic calculations using the CALPHAD approach and experimental observations in the literature.