Stability Landscape and Charge Compensation Mechanism for Isovalent and Aliovalent Substitution in High Entropy Oxides

Abstract

The emergence of the entropic stabilization concept has opened up new opportunities in material design. The discovery of a new class of complex oxide systems with more than four component was possible due to the high configurational entropy of the system such that the TΔS term (in the expression ΔG = ΔH – TΔS) dominates the positive enthalpy of mixing term (ΔH), leading to the stabilization (ΔG < 0) of single-phase solid solution. In the case of the five-component system (Co0.2Cu0.2Mg0.2Ni0.2Zn0.2)O, the TΔS term dominates the enthalpy term at temperatures >850°C. Accommodation of cations with varying sizes and maintaining electrical charge neutrality in the oxide system is an essential factor that minimizes the enthalpy term. However, exploring a single phase with varying compositions would give more freedom in tailoring the properties of these materials. Therefore, exploring the stability landscape of high entropy oxides with varying ionic sizes and valency of the component cations is important. In this chapter, we will discuss HEOs’ stability landscape when isovalent cations of varying sizes are incorporated. This chapter discusses the effects of the incorporation of aliovalent cations and the mechanism of charge balancing in these materials. © 2024 Anuj Kumar and Ram K. Gupta.