Formation, composition and stability of Mg-Co compounds

The existence and stability of Mg2Co and MgCo is still a controversial issue. Even though none of these compounds are included in the last assessed Mg-Co phase diagrams, some authors have claimed their existence. In this paper we propose that only one compound with more than 40 at.% Mg exists, and determine its composition, formation and stability using mainly XRD and DSC. We explore different synthesis methods: β-Mg2CoH5 dehydriding (prepared both by reactive mechanical alloying and by sintering), mechanical alloying of Mg and Co under Ar, mechanical alloying followed by a heat treatment, and Mg-Co solid-state reaction. We conclude that MgCo is a stable compound, whose formation process can be greatly influenced by kinetic factors that may suppress its presence in favor of MgCo2, the only well-known stable Mg-Co compound.