Untangling individual cation roles in rock salt high-entropy oxides

TL;DR

This study investigates the individual roles of cations in high-entropy oxides, revealing how each influences phase stability and properties through combined experimental, thermodynamic, and computational methods.

Contribution

It introduces a systematic approach to dissecting cation-specific effects on stability and properties in high-entropy oxides, guiding tailored material design.

Findings

Mg, Ni, and Co stabilize the rock salt phase in bulk.

Mg and Ni promote single-phase stability in thin films.

Synthesis conditions influence Co's role as stabilizer or destabilizer.

Abstract

We unravel the distinct roles each cation plays in phase evolution, stability, and properties within Mg1/5Co1/5Ni1/5Cu1/5Zn1/5O high-entropy oxide (HEO) by integrating experimental findings, thermodynamic analyses, and first-principles predictions. Our approach is through sequentially removing one cation at a time from the five-component high-entropy oxide to create five four-component derivatives. Bulk synthesis experiments indicate that Mg, Ni, and Co act as rock salt phase stabilizers whereas only Mg and Ni enthalpically enhance single-phase rock salt stability in thin film growth; synthesis conditions dictate whether Co is a rock salt phase stabilizer or destabilizer. By examining the competing phases and oxidation state preferences using pseudo-binary phase diagrams and first-principles calculations, we resolve the stability differences between bulk and thin film for all compositions. We systematically explore HEO macroscopic property sensitivity to cation selection employing both predicted and measured optical spectra. This study establishes a framework for understanding high-entropy oxide synthesizability and properties on a per-cation basis that is broadly applicable to tailoring functional property design in other high-entropy materials.