Morphological evolution in nanostructured secondary phases in entropy stabilized oxides

Abstract

High entropy oxides (HEO), containing equimolar ratios of five or more oxide components, have emerged recently as an important class of materials with a variety of interesting properties. Some, but not all, HEO systems can be classified as entropy stabilized oxide (ESO) materials, which means they have a phase state that is stabilized by entropy. In these materials, the competition between the metastable entropy stabilized and equilibrium enthalpy stabilized phase states leads to a reversible entropic phase transformation and the formation of secondary phases after heat treatment. The morphologies of the secondary phases (i.e., Cu-rich tenorite and Co-rich spinel) in bulk (Co,Cu,Mg,Ni,Zn)O were visualized in three dimensions for the first time by using focused ion beam (FIB) tomography. The Cu-rich tenorite secondary phase is observed to manifest as several highly complex particle morphologies with features in the nanometric regime. These morphologies are influenced by heat treatment conditions and the interaction with the Co-rich spinel secondary phase. Additionally, the morphology is dependent on the size of the secondary phase particles themselves, with the morphology evolving from spheres, to needles, to plates as the particles get larger. These results indicate that several strategies can be implemented to manipulate the morphologies of the secondary phases, making it possible to engineer bulk ESO materials with specific microstructural features. The ability to form complex nanostructured architectures will provide a powerful strategy for controlling behavior and expanding the possible applications of ESO materials. • Cu-rich tenorite and Co-rich spinel phases segregate in ESOs during heat treatment. • FIB tomography was used to examine the secondary phase particles in 3D. • Particles have complex geometries with sphere, needle, and plate morphologies. • Most particles have a nanostructured dimension, regardless of overall size. • Cu-rich tenorite phase particles have a size dependent morphological evolution.