Colossal dielectric constant in high entropy oxides

TLDR

High‑entropy oxides exploit configurational disorder to stabilize simple solid solutions that would otherwise not form, extending the concept of entropy‑stabilized materials beyond alloys to oxides with rock‑salt structures. This study aims to demonstrate that high‑entropy oxides can accommodate aliovalent substitutions through a charge‑compensation mechanism. The authors introduced aliovalent cations into the high‑entropy oxide lattice and applied charge‑compensation strategies to maintain overall neutrality. The aliovalent substitution expands the compositional phase space and produces at least one high‑entropy oxide with colossal dielectric constants, indicating potential for large‑k dielectric applications.

Abstract

Entropic contributions to the stability of solids are very well understood and the mixing entropy has been used for forming various solids, for instance such as inverse spinels. A particular development was related to high entropy alloys in which the configurational disorder is responsible for forming simple solid solutions and which are thoroughly studied for various applications especially due to their mechanical properties but also electrical properties, hydrogen storage, magnetic properties. Many unexplored compositions and properties still remain for this class of materials due to their large phase space. In a recent report it has been shown that the configurational disorder can be used for stabilizing simple solid solutions of oxides, which should normally not form solid solutions, these new materials were called "entropy-stabilized oxides". In this pioneering report, it was shown that mixing five equimolar binary oxides yielded, after heating at high temperature and quenching, an unexpected rock salt structure compound with statistical distribution of the cations in a face centered cubic lattice. Following this seminal study, we show here that these high entropy oxides (named HEOx hereafter) can be substituted by aliovalent elements with a charge compensation mechanism. This possibility largely increases the potential development of new materials by widening their (already complex) phase space. As a first example, we report here that at least one HEOx composition exhibits colossal dielectric constants, which could make it very promising for applications as large-k dielectric materials.