Novel electronic and magnetic properties of high Entropy Oxides – NEO

This project is devoted to explore novel high-entropy oxides (HEOx) which display exotic electronic and magnetic properties. HEOx constitute a new class of materials that has been discovered recently (C. M. Rost et al, Nature Communications, 2015), extending to oxides the concept of high-entropy materials that is well-known for metallic alloys. They comprise at least five different cations and are obtained by heating at high temperature followed by quenching. When the temperature is large enough, the entropy of configuration becomes the dominant contribution to total Gibbs energy. Thus, instead of forming phases being driven by the enthalpy of formation, the system crystallizes in a metastable solid-solution at high temperature that can be frozen at room temperature by quenching, leading to entropy-stabilized oxides.

In the case of the first synthesized HEOx, (MgCoNiCuZn)O, a simple rocksalt structure where the cations are distributed randomly on the cationic sublattice is formed when a mixture of the binary constituents is heated at temperatures (> 875 °C) then quenched. We note that some of the constituting binary oxides do not crystallize in a rocksalt structure, or even in a cubic system and they do not form solid solutions. As the structure of HEOx is determined by the configuration entropy, it may become possible to accommodate cations in the crystal structure which are generally difficult to dope, such as Zn2+ ion in an octahedral site and not in the usual tetrahedral symmetry. Besides the rocksalt, several other HEOx materials such as perovskite and spinel have been reported, suggesting a huge variety of materials to be realized.

HEOx is therefore an interesting system to develop new oxide materials. Although the synthesis of such novel oxides has an impact in the field of solid-state chemistry, the resultant materials are in fact not only new but also have been found to provide unexpected functions possibly related to the local lattice distortions with unconventional cation arrangement. In the rocksalt-type HEOx, we have discovered unexpected functions and properties such as
-colossal dielectric properties in a wide frequency range
-alkali ions mobility with ionic conductivities larger than 1 mS/cm for lithium
-long-range antiferromagnetic magnetic ordering

The novel properties and functions should not be limited in the rocksalt HEOx, and further intriguing properties should be expected. Particularly, by applying the concept of high-entropy stabilization to so-called correlated electron systems exemplified by complex transition-metal oxides, we may have a chance to realize unprecedented electronic and magnetic properties. The correlated transition-metal oxides often display competing electronic ground states, which are coupled with subtle lattice distortion. By realizing HEOx-type correlated oxides where the lattice structure is strongly randomized with “entropy-stabilized” multiple cations, the system may not be able to find a unique ground state among the competing phases, and thus may form an unexpected electronic phase and display surprising properties and functions. We therefore explore novel HEOX materials in complex transition-metal oxides such as perovskite, pyrochlore and spinel structures. The expected properties include giant magnetoelectric effect, high-performance thermoelectrics and exotic magnets such as quantum spin-liquid.

We would like to stress that HEOx are not simple “classical” solid solutions but represent a new class of materials produced by “entropy-stabilization”. Their properties are thus not just a “cocktail” of the starting materials, and novel properties are expected to emerge. Through the collaboration of two teams, the pioneers of entropy-stabilized materials (the French partner) and the leading group in the field of correlated electron physics (the German partner), we hope to open a new arena for the exploration of novel electronic and functional oxide materials.