Real Space Observation of Reaction Pathway in the Formation of Entropy-Stabilized Oxide

Configurational entropy has enabled a new synthesis pathway to previously unrealized, multi-component, single-phase oxides [1]. Such high entropy and entropy-stabilized oxides are a novel class of materials that offer a wide range of utility as well as an opportunity to understand the role of entropy in the solubility, phase behavior, and stability for the design of disordered materials [2][3]. There has been a large disparity between high entropy compounds that have been predicted to exist as single-phase materials and those that have been generated experimentally, with some high entropy oxides (HEOs) only being realized through high kinetic energy processing such as pulsed laser deposition rather than solid state processing [4]. This suggests that complex reaction pathways may be hindering the ability to discover stable HEOs [5]. Here we show that varying precursor choices can affect the ability to dissolve metal cations into a high entropy structure and in some cases can result in differing phase states at identical conditions. Dissolution of spinel Fe3O4 and corundum Fe2O3 into known entropy stabilized oxide, (Mg0.22Ni0.2Cu0.2Co0.2Zn0.2)O, individually is found to produce different products: Single phase and spinel rocksalt mixture respectively. This can be done reversibly. Using Co3O4 instead of CoO in the synthesis of (Mg0.22Ni0.2Cu0.2Co0.2Zn0.2)O hinders the formation of a single-phase material with spinel as a secondary. Notably, the dissolution of Co3O4 as a sixth component to (Mg0.22Ni0.2Cu0.2Co0.2Zn0.2)O precursor leads to a single phase. Our results demonstrate that precursor choice affects reaction intermediates and final structure under a fixed set of conditions. Further, synthesis from pre-formed (Mg0.22Ni0.2Cu0.2Co0.2Zn0.2)O instead of binary constituents also appears to affect the ability to dissolve metals into a high entropy complex. Examining the synthetic pathways and thermodynamic processes that result in these variances is crucial to understanding solubility and the rational design of disordered systems.