High‐entropy oxides: Harnessing crystalline disorder for emergent functionality

Abstract

High-entropy materials defy historical materials design paradigms by leveraging chemical disorder to kinetically stabilize novel crystalline solid solutions comprised of many end-members. Formulational diversity results in local crystal structures that are seldom found in conventional materials and can strongly influence macroscopic physical properties. Thermodynamically prescribed chemical flexibility provides a means to tune such properties. Additionally, kinetic metastability results in many possible atomic arrangements, including both solid-solution configurations and heterogeneous phase assemblies, depending on synthesis conditions. Local disorder induced by metastability, and extensive cation solubilities allowed by thermodynamics combine to give many high-entropy oxide systems utility as electrochemical, magnetic, thermal, dielectric, and optical materials. Though high-entropy materials research is maturing rapidly, much remains to be understood and many compositions still await discovery, exploration, and implementation.

Spin-Cluster Glassy and Long-Range Ordered Magnetic States in Honeycomb-Layered Compositionally Complex Oxides Na3–xLixT2SbO6 (T = Cu1/3Ni1/3Co1/3)

“…1−5 HEOs are often considered as entropy-stabilized oxides (ESOs), but actually, these two classes overlap only partially. 5 The seminal work by R o s t e t a l . 6 i n t r o d u c e d r o c k s a l t -t y p e Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 O that is both HEO and ESO because individual CuO and ZnO do not belong to the rock salt type.…”

“…A rapidly growing class of high-entropy oxides (HEOs) attracts much attention because many of them display interesting and useful properties (catalytic, dielectric, optical, magnetic, and electrode behavior, ionic conductivity, low thermal conductivity, etc.) that usually cannot be predicted based on the properties of corresponding constituents. − HEOs are often considered as entropy-stabilized oxides (ESOs), but actually, these two classes overlap only partially . The seminal work by Rost et al introduced rock salt-type Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 O that is both HEO and ESO because individual CuO and ZnO do not belong to the rock salt type.…”

“…Therefore, their five- or six-component solid solutions will be HEO but not ESO. It is now commonly adopted that HEOs are solid solutions combining on a single site at least four or five , different cations in (approximately) equal fractions, irrespective of the entropy stabilization.…”

“…From the cited recent reviews, − it is evident that magnetic studies of HEOs are now concentrated on a rather short list of structure types: those of spinel, perovskite, and rock salt, with rare or solitary works on magnetoplumbite-, bixbyite-, and pyrochlore-type phases. We only can add three reports on magnetism in high-entropy REBa 2 Cu 3 O 7– y , and wolframite- and Zn 2 Mo 3 O 8 -type materials …”

Phase Stability and Magnetic Properties of Compositionally Complex n = 2 Ruddlesden–Popper Perovskites

“…High-entropy alloys have attracted a significant amount of research interest in recent years, the materials incorporate multiple principle elements and were first reported by Yeh et al The high-entropy alloys were found to have improved hydrogen storage and tensile strength. , The approach has more recently been expanded to include ceramics and oxides. − As a result, there have now been several studies regarding compositionally complex and high-entropy perovskites, particularly those with the ABO 3 type structure. − The compositionally complex n = 1 Ruddlesden–Popper perovskites have only more recently been reported with research largely focused on the cuprate and thin film-based materials. − However, a more recent study investigated the local ordering of the cations in a compositionally complex n = 1 Ruddlesden–Popper perovskite for the first time . These types of materials display a wide range of functional properties, including ionic conductivity, magnetism, and ferroelectricity. ,, The magnetic properties of ABO 3 type compositionally complex perovskites have generally been antiferromagnetic in nature with a small ferrimagnetic moment as observed in both the RE­(M5)­O 3 and (RE5)­MO 3 , where RE are rare earth elements (Gd, La, Nd, Sm, and Y) and M are transition metals (Co, Cr, Fe, Mn, and Ni). ,, A similar effect was recently reported in a study focusing on the Sm­(M4)­O 3 compounds where M = Co, Cr, Fe, and Mn. The difference in zero-field and field-cooled magnetization data is often attributed to glassy behavior without further investigation but is more likely due to the inhomogeneous nature of the samples …”